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In rice plants, large traces of the panicle or each leaf run down through two internodes and turn into diffuse bundles in the second lower node (figs. l, 2 and 3). The diffuse bundles take oblique course surrounding the swelling elliptical leaf traces, and join with them only at the basal points of the node (○ mark in fig. 2). It may be supposed that assimilates from a leaf go down through the phloem of swelling leaf traces, and rise up through the diffuse bundles via the 〇 mark points by the metabolic activity of their numerous phloem parenchyma (figs. 7 and 10). In the xylem of swelling leaf traces, tracheids, and xylem parenchyma containing xylem transfer cells (figs. 17, 18, 19, 20, 21 and 22), display a mosaic structure (figs, 14, 16, 17 and 18). But, no phloem transfer cell has been found in the vegetative nodes. When the cell wall ingrowth of xylem transfer cells is formed in a vegetative node, the leaf of this node is developing out, and the second upper leaf primordium, about I mm long, begins to elongate, whose leaf traces do not function as conducting tissues yet. In the swelling leaf traces, solutes of transpiration stream are absorbed by the xylem parenchyma, and are raised to the upper internode by two pathways as follows. A part of the solutes moves horizontally into the phloem of the swelling leaf traces, and travels with assimilates. The other part moves horizontally across the bundle sheath and parenchyma bridges lying between the swelling leaf traces and the diffuse bundles (figs. 7, 23 and 24), and travels by the diffuse bundles. This supposition was able to be verified by the distribution in the node of barium absorbed the roots (fig. 25).

Excised immature radish cotyledons were allowed to take up [3 H] 6 -benzyl am i ri op uri n e for up to 5 h and were then transferred to cytokinin-free medium for various intervals, after which they were extracted. The cytokinin was rapidly metabolized by the tissue to two main products which accounted for 20 per cent of the extracted radioactivity after 1 h of uptake, and for 95 per cent after transfer to the cytokinin-free medium. These metabolites were coincident with cytokinin activity on chromatograms of the cotyledon extract and were also formed when 6-benzylaminopurine was supplied to 9-d-old derooted seedlings through the transpiration stream. The metabolites were identified by mass spectrometry, u.v. spectro scopy and chemical degradation as 6-benzylamino-7-glucosylpurine and 6-benzylamino-9 glucosylpurine. Several minor metabolites were detected in the derooted seedlings ; one was 6-benzylaminopurine riboside while another appeared to possess high cytokinin activity.

The significance of root pressure in the transport of xylem sap has been investigated in Cocos nucifera L. and a few other palms. Despite the fact that excised palm roots can generate considerable pressures in situ, the quantity of water transported is only a small fraction of the demand resulting from transpiration. Most water transport is induced by negative pressure gradients, as in other higher plants. The development of considerable negative pressures has been demonstrated both directly and indirectly. Acoustic detection was used for the first time to monitor cavitation in water-stressed Cocos leaves. Its detection implies the ready disruption of xylem sap under these tensions. We suggest that root pressure might serve to refill cavitated xylem conduits when water is abundantly available and transpiration practically zero. However, little or no positive pressure could be demonstrated in intact palms subjected to low water stress: experimentally.

The uptake and movement of a chemical into a wheat leaf has been simulated in a 5-compartment model. The compound is considered to be applied to the leaf as discrete droplets of solution, from which water evaporates at a uniform rate. Solute diffuses from the droplets through the plant cuticle into the epidermal cell wall, and from there either into and out of the cytoplasm and vacuole of the mesophyll cells or along cell walls to the xylem. Once in the xylem it is carried in the transpiration stream towards the tip of the leaf. Accurate values for the parameters required by the model have not yet been measured, but the model simulates in a qualitative manner the movement pattern observed for 1-methyl-2-pyridone. The results presented focus attention on the possible importance of membrane permeability in determining the phytotoxicity of xylem mobile pesticides.

Summary According to the evidence that the transport of the transpiration water in the endodermis of the needles mainly occurs in the cytoplasma the process of the water transport is examined by excluding the influence of the stomata. With increasing vapour pressure deficit it shows a saturation kinetic and with constant pressure deficit a Q10-value of 2,5. Inhibitors of the production of energy slow down the water transport and increase the permeability at the same time. The water transport takes place against a gradient and can cause an exudation of liquid water. Several results contradict the explanation by osmosis. Its importance for the non-stomatical regulation of the transpiration with regard to the biologic amplitude of the water economy, resistance against drought, productivity of the transpiration and the use of antitranspirantia is discussed.

The experiments were carried out during 1962-1965, to clarify the daily and seasonal changes of the water saturation deficit (W. S. D.) of leaves, as an index of the degree of water deficit in Satsuma orange trees. And further observations were made on the relationship between W. S. D. of leaves, climatic factors and soil moisture. The results obtained were summarized as follows:1. Daily changes of W.S.D. of leaves in summer were rose in a steady curve with sunrise, and maximum value was found to occur around noon, thereafter, values were gradually descended, and minimum value was observed around 6.00P.M. each day. On the other hand, its changes in winter were smaller than that in summer, although, the tendency was almost similar to that observed in summer.2. On the seasonal changes of W.S.D. of leaves, during the period from early to late February, its values were definitely ascended, and it might be mainly due to the low temperature and cold wind. And its values in early February were found to have the highest in winter period, but thereafter to late March, W.S.D. of leaves were gradually descended. In early May, however, W.S.D. of leaves were ascended rapidly and formed a peak, it seemed to be closely related to the drought of spring and sprouting a current shoots, but thereafter to late June, its values were slightly descended. During the period from early July to late August, under the condition of high temperature and drought, W.S.D. of leaves were notably ascended, and the trend was most remarkable in early August, its values in this period were the highest one. W.S.D. of leaves in September were considerably high compared with that in October, thereafter from early October its values were descended.3. In the winter half (Nov.-Apr.), W.S.D. of leaves had the high negative correlations with the air temperature, soil temperature, precipitation and saturation deficit of water vapour pressure. On the contrary, W.S.D. of leaves in the summer half (May-Oct.) had the high positive correlations with the air temperature, soil temperature and saturation deficit of water vapour pressure.4. W.S.D. of leaves in autumn and winter were ascended with the increasing of cold wind velocity, also, its values were ascended with the increasing of hours of exposed to the cold wind. Further, velocity of transpiration stream in shoot was increased by the exposed to the cold wind, and its velocity was diminished with the decreasing of soil moisture.5. W.S.D. of leaves in summer had a very high negative correlation with the soil moisture, therefore, by the use of curved regression equation, it was possible that the soil moisture is presumed with the W.S.D. of leaves. When the soil moisture was between field capacity and moisture equivalent, changes of W.S.D. of leaves were very small, and then W.S.D. of leaves were slowly ascended with the decreasing of soil moisture in less than moisture equivalent. Thereafter, its values were extremely ascended, being accompanied with a marked decreased of soil moisture to the wilting point. In addition, when the W.S.D. of leaves were reached 8.0 per cent, the trees showed visible symptoms of wilting in leaves, and its values reached 10.0 per cent, showed visible symptoms of wilting in fruits.In the winter period, however, no correlation could be found between W.S.D. of leaves and soil moisture, accordingly, the ascension of W.S.D. of leaves in winter were might be mainly due to the climatic factors.

THE SUBJECT of phloem transport has been quite adequately reviewed in recent years. Curtis (1935), Mason and Phillis (1937) and Crafts (1938) have summarized existing data and hypotheses, each from his respective point of view. The opinions expressed by the many workers in this field are diverse both as to tissues involved and forces responsible for movement. Hence, it is anticipated that further reviews will contribute less than presentation of experimental results. EXPERIMENTAL.-Since the problem of phloem transport is an old one, many different experimental methods have been employed in its attempted solution. The present study utilizes the advantages gained by the use of tracer elements and presents data which heretofore have not been available by means of the application of older methods. This method is characterized by extreme convenience in manipulation, and it allows detection of extremely minute quantities of migratory material. It is rapid, direct and precise to about seven per cent under the conditions of these experiments. The object of the study is to follow the translocation of phosphorus from the leaf to other parts of the plant. It is known to accumulate in the leaves following delivery there by the transpiration stream (Biddulph, 1940). In order to demonstrate its redistribution, an "injection technique" was employed to localize an amount of radiophosphorus in the leaf to be studied; then the radiophosphorus was followed to other parts of the plant by means of its radioactivity. The path of its movement can be adequately traced by means of the presence of the radioactive material in the tissues traversed. The investigation consists of two parts, as follows: first, experiments were conducted in order to learn how closely the migratory phosphate follows the vascular traces of the petiole while moving from the leaf into the stem; second, experiments were conducted in order to learn whether the radiophosphorus, after leaving the leaf, moved through the xylem or the phloem of the stem. Both parts yielded evidence on the direction of movement in the stem. Vascular anatomy and translocation.-Cotton seed (Gossypium sp.), obtained from the Agronomy Department of the Alabama Experiment Station, was delinted by immersion in concentrated sulfuric acid for three minutes, washed free from acid and dried. The seeds were then dusted with Semesan and placed in moist paper toweling (Scott Tissue Towels No. 5a, Folded) to germinate, after which they were transferred to sand culture or to solution culture.

Fluid extracted from grapevine shoots by suction contained up to o-2 per cent (w/v) of sugar. The total sugar concentration and the relative concentrations of sucrose, glucose, and fructose changed along the length of a shoot. Whereas sucrose is absent from or present only in traces in fluid bleeding from cut stumps, it was found to be a major constituent in fluid extracted from sections of wood by suction. 14C-glucose administered to the wood of an excised shoot moved up in the transpiration stream, moved laterally, into the bark, and was partly converted to sucrose and fructose. When 14C02 was supplied to a leaf of an intact vine, radioactive sugars were found in the fluid afterwards extracted from the wood by suction. When a ring of bark was removed from the stem at a point above the 14C02-treated leaf most of the 14C in the bark, wood, and extracted fluid below the ring was in the form of sugar but 14C in tissues above the ring was mainly in organic acids and amino acids. It is suggested that a barrier within the wood prevents diffusion of sugars into the transpiration stream, and that the fluid extracted from the wood by suction cannot be regarded as ascending sap since it contains considerable amounts of non-moving material.

SUMMARY Branches of Chenopodium album, including leaves and young inflorescences, were put into a solution of 15N-L-proline in order to absorb the latter with the transpiration stream. After 48 hrs of exposure a number of nitrogen fractions were separated. 15N was found in free and bound amino acids, especially in arginine, alanine, glutamic and aspartic acid as well as in amino-lipids and chlorophyll fraction. Proline is considered to be an important source of nitrogen.

The transport of ions from a bathing solution to the top of a plant has been described as a passive (7, 8, 18, 19, 22, 23, 28) and active (2, 3, 17, 26, 27) process. Brouwer (3) states that at least 70 to 85 % of the ions appearing in the tops of plants go through an active process, i.e., "owing to a mechanism controlled or started by metabolism lands into the transpiration stream". On the other hand, Epstein (7) concludes that "the cells of the roots, as well as the shoots, derive their ions from the solution in the 'outer' space of the root tissue". He defined outer space as the "space to which inorganic ions have free and reversible access by diffusion". The purpose of this study was to investigate these suggested concepts of ion passage from outside solutions to the top of the plant, and to formalize the observations with equations consistent with available data. The technique is kinetic analysis of ion uptake from solutions of radioactive rubidium ranging in concentration from 10-5 to 10-2 molar, in the presence and absence of inhibitors.

Summary. The inhibition of photosynthesis of bean plants (Phaseolus vulgaris L.) by root-applied herbicides was determined at different rates of transpiration, obtained by varying air humidity, light intensity and temperature. The plants were treated with 5×10−6M of simazine, simeton, lenacil, isocil, diuron and fenuron in the nutrient solution. CO2 uptake and transpiration of intact bean plants were measured continuously. The decrease in photosynthesis was more rapid at higher transpiration rates. For each herbicide the total transpiration to the 50% inhibition level of photosynthesis was almost constant at different rates of transpiration, indicating that the degree of inhibition is determined by the transpiration stream in the plants. Treatments with lenacil and diuron resulted in a lower degree of photosynthesis inhibition than did the other herbicides. The results are discussed in relation to uptake studies, herbicidal effects and adsorption phenomena of these and similar herbicides. They are also related to the field performance of such herbicides. Effet de la uitesse de la transpiration des haricots sur l'inhibition de la Photosynthese par quelques herbicides administeres par les racines Resume. L'inhibition de la Photosynthese des haricots (Phaseolus vulgaris L.) par des herbicides administeres par les racines a ete determinee a des taux differents de transpiration, obtenus en faisant varier l'humidite de I'air, l'intensite de la lumiere et la temperature. Les plantes ont ete traitees avec une concentration de 5 × 10−6 M de simazine, de simetone, de lenacile, d'isocile, de diuron et de fenuron dans la solution nutritive. L'absorption du CO2 et la transpiration des plantes intactes ont ete mesurees en continu. La diminution de la Photosynthese fut plus rapide pour un taux de transpiration eleve. Pour chaque herbicide, la transpiration totale jusqu'au niveau de 50% d'inhibition dc la Photosynthese fut assez constante aux differents taux de transpiration, ce qui indique que le degre d'inhibition est determine par le courant de la transpiration dans les plantes. Le lenacile et le diuron ont moins influencee la Photosynthese que les autres herbicides. Les resultats ont ete discutes par rapport a I'assimilation, aux effets phytotoxiques et aux phenomines de l'adsorption de ces herbicides et d'autres produits similaires, Les resultats ont ete consideres aussi en relation avec l'effet de ces herbicides au champ. Einfluss der Transpirationsintensitat von Bohnen Pflanzen auf die Photosynthesekemmung nach Application einiger Herbizide uber die Wurzel Zusammenfassung. Die Photosynthesekemmung von Bohnenpflanzen (Phaseolus vulgaris L.) durch einige uer die Wurzeln applizierte Herbizide wurde unter verschiedenen Transpirationsbedingungen, die durch Anderung der Luftfeuchtigkeit, Lichtintensitat und Temperatur erhalten wurden, bestimmt. Die Pflanzen wurden mit 5 × 10−6 M Simazin, Simeton, Lenacil, Isocil, Diuron und Fenuron in der Nahrlosung behandelt. CO2-Aufnahme und Transpiration intakter Pflanzen wurden kontinuierlich gemessen. Die Abnahme der Photosynthese war bei hoherer Transpirationsintensitat starker. Fur jedes Herbizid war die Gesamt transpiration bis zur Stufe der 50%-Hemmung bei verschiedenen Transpirationsintensitaten weitgehend konstant. Dieses Verhaiten zeigt, dass der Grad der Hemmung durch den Transpirationsstrom in der Pflanze bestimmt wird. Die Behandlung mit Lenacil und Diuron ergaben einen geringeren Hemmungsgrad als bei den ubrigen Herbiziden. Die Ergebnisse werden im Hinblick auf Aufnahme, Herbizidwirkung und Adsorphanosphanomene dieser Herbizide besprochen. Sie werden auch auf die Wirkung dieser Herbizide im Freiland bezogen.

SUMMARY Transpiration rates of single leaves of Pelargonium and wheat were measured under constant conditions of light, temperature, and air flow. Concurrently, stomatal movement was followed with the resistance porometer during cycles of changing water content of the leaf and changes induced by light and darkness. Stomatal movement was found to exert a large controlling influence on the transpiration rate, whereas water content had an extremely small or negligible effect. An approximately inverse linear relation between transpiration rate and logarithm of resistance to viscous flow through the leaf is believed to be the resultant of an inverse curvilinear relationship between the diffusive conductance of the stomata and log. leaf resistance and the decreasing difference of vapour pressure arising from the higher transpiration rates with increasing stomatal conductances. Nevertheless, the relation demonstrates that the transpiration rate is influenced by the degree of stomatal opening throughout its entire range. There was some evidence of lower transpiration rates during and after recovery from wilting than before wilting. This is attributed to a decrease in a cell-wall conductance, the evaporating surface being located within the cell wall. During wilting partially irreversible contraction of the cell wall occurs. There was also evidence of slow changes in cell volume at full turgidity attributable to plastic flow. These occurred when the leaf was transferred from environments of a high to low potential for evaporation. Extensive movement of the stomata followed changes in leaf water, passive opening resulting from decrease and closure from increase of leaf water. It is suggested that the direction and extent of stomatal changes induced by water deficits is a consequence of the rate of change of leaf water content and not of the absolute values. The stomata also showed an enhanced tendency to close in dry moving air following a period of wilting even after the leaf had regained turgidity.

In the field of plant nutrition there are many problems which require, for their solution, an understanding of the distribution of accumulated mineral salts between various plant parts. In their discussion of interrelations of salt accumulation with growth and development of the plant body, Steward and Sutcliffe (14) have provided a background for understanding distribution problems. But, they have concluded that despite the fact that the pattern of distribution is controlled and integrated, the method by which it is accomplished is totally unknown. In a discussion of upward movement of solutes, Biddulph (2) suggests that adsorptive forces residing in the biocolloids constituting the walls of the conducting channels might operate differentially to regulate the ascent of various ions towards their receiving tissues or organs. This suggestion could be justified from an experiment of Hewett and Gardner (8) showing an adsorption of zinc ions passing through sections of grape canes. Pursuing the matter further, Biddulph et al. (5) showed that labeled calcium did not ascend the stem of bean plants with the labeled water (THO) of the transpiration stream as would be expected in a mass flow system. Instead, the ascending calcium was shown to participate in exchange and deposition reactions. In short, the data indicated that calcium ascended the stem by an exchange mechanism. In recent years studies of solute uptake by roots and other tissues have been much emphasized, and the active phase of the process, as it relates to mineral accumulation in tissues, has been quite well described (12). However, agreement concerning the mechanics of uptake by roots of those ions destined for translocation to the shoots has not been attained. One of the questions which remains unanswered is the role of the ascending transpiration stream on both the uptake and release of nutrient ions into the vascular tissues (6,10, 13, and others). Since knowledge of the mechanisms of entry into the roots of translocatable ions is limited, the present experiments on the mechanism of ascent were designed in such a way that their interpretation would be independent of the mode of entrance. This was accomplished by studying the ascent of calcium, and the arresting of its ascent, w: thout interfering with the transpiration stream. The experiments were considered to be conclusive when, after arresting the ascent of calcium, it could be started again, at will, by introducing replacement ions of Ca, Sr, and MIg, but not with K. In this respect, the results displayed features which were consistent with exchange reactions, but which were wholly incompatible with mass flow.

By making a tongue-slit in the shaft of the inflorescence ofArabidopsis thaliana 7 different urethanes (1/200 mol+1/200 mol KCl) were forced into the buds through the transpiration stream. After this treatment with 6 of these substances the frequency of the monohybrid segregating mutations was significantly raised 5- to 10times over the control as determined by seedling tests. The mutation frequency was positively correlated with the size of the urethan molecule and seemed to be little affected by its chemical substitution. The mutants were of the same type than those obtained after irradiation with X-rays. These results are in agreement with the conception that the mutagenic effect of the urethanes is indirect and probably due to general disturbances of different metabolic processes in the cell.

Hydrostatic pressures varying from 0 to 6.0 bar were applied to roots of intact Capsicum annuum L. cv. California Wonder plants growing in nutrient solution and the rates of transpiration, and net CO2 assimilation, apparent compensation point and leaf water potential measured. Increasing the pressure on the roots of plants with roots in solution with either -0.5 or -5.0 bar osmotic potential with 1 bar increments resulted in a decrease in transpiration. With the application of 1 or 2 bar pressure the rate of transpiration returned to near or above the original rate. An application of 3 or 4 bar pressure reduced the rate of transpiration of all plants. The transpiration of plants with roots in solution with -0.5 bar osmotic potential remained at the reduced rate for as long as these pressures were maintained. The transpiration of plants with roots in solution with -5.0 bar was only temporarily suppressed at these pressures. Changing the applied pressure from 3 or 4 bar to 0 resulted in a rapid increase in transpiration which lasted approximately 15 minutes. This was followed by a decrease in transpiration to a rate lower than before the pressure was applied. The pattern of response was similar for plants at low or high light intensity or at normal or low CO2 concentrations. When leaf diffusive resistance was 6.0 s cm−1 or greater, changes in net CO2 assimilation were similar to those of transpiration. The apparent CO2 compensation point increased as pressure was applied and decreased with a release in pressure. Leaf water potential increased with an increase in pressure and decreased with a decrease in pressure. The changes in leaf water potential were frequently but not always proportional to changes in pressure. It is postulated that the respouses noted were due to changes in resistance to flow of water from xylem terminals through the mesophyll cells and stomatal cavities to the atmosphere.

Summary An experimental procedure is described by which contemporaneous assessments of leaf resistance, rates of transpiration and of carbon dioxide exchange can be made on attached leaves. The relative humidity and carbon dioxide concentration of an air stream flowing into a leaf chamber are controlled, and of the outflowing stream monitored. The effects of light intensity and of water relations of the leaf were investigated with reference to leaf resistance and rates of transpiration and photosynthesis. In Pelargonium × hortorum quick stomatal opening in response to illuminatioxi was observed in plants which were not under water stress. On illumination of darkened leaves the rates of transpiration and photosynthesis were shown to rise rapidly, bearing a relation to leaf resistance, and indicating stomatal control of these processes. Over long periods of illumination water deficits develop, leaf resistance increases and rates of transpiration and photosynthesis decline. Typical findings of the levels of leaf resistance at a range of light intensities, and the corresponding values of transpiration rate and photosynthetic rate, are given. The overriding importance of water deficit in influencing stomatal behaviour in leaves under substantial water stress is shown in a series of experiments. When water deficits are fairly small, the stomata of leaves open in response to illumination, but more slowly than those of leaves of well-watered plants; in leaves under water stress stomatal closure is also much accelerated on darkening. Under moderate water deficits, the stomata of leaves may open in response to light, but close again fairly quickly as the stress increases. Changes in rates of transpiration and photosynthesis parallel closely those of leaf resistance under these conditions. The effect of watering plants under water stress on leaf resistance and rates of gaseous exchange is shown in two experiments. The results indicate that watering leads to stomatal closure at first, with decreased rates of photosynthesis and transpiration, but subsequently leaf resistance diminishes and the rates increase as normal turgor relations in the leaf re-establish. Stomatal behaviour and control of rates of transpiration and photosynthesis are discussed, and the water relations of the leaf shown to be of critical importance in this connection.

Summary The aim was to vary the transpiration rate of water culture plants and to measure corresponding values of ΔW, the leaf water potential depression (= DPD = suction force), as well as relative water content (relative turgidity) and stomatal aperture. On the Ohm-law analogy ΔW and transpiration rate should be linearly related. Ricinus communis plants were placed in a wind tunnel and the transpiration rate was varied by changing the level of the relative humidity of the air stream. With increasing rate of transpiration, ΔW rose steeply to a value of about 6 atmospheres at which it remained virtually constant (Fig. 1). Thus from very low transpiration rates to the highest obtainable, far from ΔW rising linearly with transpiration, there was no response in ΔW and similarly no response in relative water content and stomatal resistance. When transpiration was reduced to zero the leaves still maintained a ΔW of about 3 atmospheres instead of zero, but reasons are put forward for considering this value to present water saturation. The main resistance to water movennent in the plant was shown to reside in the roots. If a leaf was detached from a transpiring plant and allowed to continue transpiration at a similar rate, water being freely supplied to the cut end of the petiole, its ΔW fell from 6 to 3.5 atmospheres (Fig. 3). Thus the resistance must lie‘below’the petiole. Similarly it was shown to lie below the stem. The constancy of leaf ΔW in the face of large changes in flux of water through the roots, implies that the root resistance declines in step with the rise in flux. A simple hydraulic model is presented which would behave in this way.

Sodium is excluded from the tops of many plants (1-4, 7, 8), and hence accumulation of this ion in the leaves of;such plants which apparently are rather sensitive to it (2) is avoided. The barrier to sodium transport is effective at moderate external sodium concentrations. At high external concentrations (2, 7, 8), and as a result of prolonged growth in a saline medium (1), sodium contents are found to increase in the stems and sometimes also in the leaves of so-called Na nonaccumulator plants. Gauch and Wadleigh (4) assumed that sodium movement might be restricted by the membranes of certain extrastelar tissue, excluding it from the vascular system. Huffaker and Wallace (8) suggested that the term Na nonaccumulator plant should be considered as a quantitative rather than a qualitative expression. Bernstein et al. (1) found comparable levels of sodium accumulation in the roots, wood, and bark of young apricot trees grown for one year in saline plots; only the leaves and twigs of these plants exhibited restricted sodium accumulation. The authors supposed that sodium had entered the vascular tissue of the trees and that its translocation into the leaves via the transpiration stream had been restricted by retention in the living wood parenchyma cells. During the third year of these experiments sodium levels increased also in the leaves of various stone-fruit trees. Bernstein et al. (1) consider this increase to be due to the development of heart wood during the third year of growth, release of sodium by the senescent parenchyma and its subsequent translocation into the leaves via the transpiration stream. No direct proof has been brought forward for any of the hypotheses regarding the location of the barrier to sodium transport and the mechanism of its rupture at high external concentrations. The interpretation proposed by Bernstein et al. (1) for sodium leakage to the leaves during the third year of their experiment would not be adequate to explain the gradual advance of sodium in the stem with increasing external concentrations as found in brief tracer experiments (7, 8). The present work is an attempt to supply more data about sodium transport and distribution in bean plants-a Na nonaccumulating species-in order to provide evidence for the elucidation of these phenomena.

A mechanism is postulated to explain the immobility of boron in plants, i.e., the absence of any significant retranslocation of this element from one leaf to another. It is shown that boron readily enters the bark and is translocated within the bark. Since it has also been shown that boron remains water-soluble in plants, the immobility cannot be explained through a chemical fixation, lack of entry into phloem, or absence of phloem transport. Rather, boron enters the phloem in leaf margins where concentrations are high, is transported in these conduits, is lost therefrom where the xylem concentration is low, i.e. in basal areas of the leaf and in petiole, is transported back in the xylem and accumulates in terminal place of the transpiration stream. A high local mobility of boron, together with the essentially unidirectional flow of the transpiration stream, thus cause a cyclic movement of B and prevent the efflux of this nutrient; this explains the immobility over long distances.

SummaryA dynamic model is presented in which the problem of predicting P response is broken down into various components, such as:(a) Weight and P content of emerging seedling.(b) Normal growth curve of the fully nourished plant.(c) A ‘deficiency-tolerance’ factor relating depression of relative growth rate to plant P concentration.(d) An ‘affinity’ term relating sink concentration to P status of plant.(e) A perirhizal resistance term for diffusive transport to roots.(f) Capacity and intensity of P supply from the soil. Mass flow supply via the transpiration stream is also included.By changing parameter values one may attempt to simulate the effect of any of these factors on the shape of the P response curve and any other part of the system throughout crop life. At present the model over-estimates growth at low levels of P supply, but predicted plant P concentrations agree reasonably well with observed data.

There is some scattered evidence which indicates that an upward current of water normally exists in aquatic as in terrestrial plants. Following Arber's (1920) usage it is convenient to refer to this as a transpiration stream, though, clearly, transpiration in the ordinary sense (that is, escape of water vapour) is not a factor in its maintenance, and other mechanisms must be operative. The physiological evidence relating to this flow has recently been reviewed by Wilson (1947). He emphasizes that the direct measurement of water movement in aquatic plants presents peculiar difficulties, hitherto not fully appreciated, and some of the earlier experimental work cannot be accepted at its face value. Nevertheless, it seems likely that such an upward flow is not uncommonly present, probably maintained by root pressure. Thus, von Minden (I899) and Weinrowsky (I899) have observed the outflow of water from the tips of the leaves of some aquatic plants, e.g. Potamogeton spp., Callitriche autumnalis, the submerged form of Littorella, under-water leaves of Alisma natans and A. ranunculoides, when these are lifted above the water. The culture experiments of Pond (1905) and Brown (1913) may also be cited as having an indirect bearing on the problem. They grew various aquatic plants (a) rooted in the soil, (b) rooted in sand and (c) anchored in water over soil. In general, the plants rooted in the soil showed better growth than the others, and this was attributed to the effectiveness of the root system in the absorption of solutes and in providing an upward current in the xylem, in which the solutes are transported to the region of growth. This rather supports von Minden's conclusions. Aside from the physiological evidence, however, we have another indication of the probability of an upward water movement as a general phenomenon in aquatics; this is the very widespread occurrence, in submerged plants, of hydathodes or apical openings of various kinds in the leaves. These are frequently very similar in structure to corresponding organs in aerial plants where their behaviour is not in doubt, so that it appears highly likely that they may play some part in the physiology of aquatic plants. The term hydathode is a very general one, and includes at least two types of fundamentally different structures, as Haberlandt (1914) has pointed out. From a physiological point of view he distinguishes between passive and active hydathodes. Aquatic plants show, very generally, the presence of the first type-structures which are, in effect, openings at the vein endings which may passively allow the escape of water. Such structures have been described by many authors, including Borodin (I870), Askenasy

Growing navel orange fruits (Citrus sinensis) 5.4 to 5.7 centimeters in diameter were used as a model system to determine the effects of transpiration and carbohydrate translocation on water and osmotic potentials in fruit tissues. Evidence supported the hypothesis that osmotic potential in the vesicles would be affected little by changes in transpiration or carbohydrate translocation because the vesicles are anatomically isolated from the transpiration stream and are at the end of the carbohydrate translocation pathway. In the mesocarp tissue, which contains a vascular network, osmotic potential decreased during the daytime when environmental conditions favored transpiration and increased at night. Exocarp water potential followed a similar pattern. Girdling of the stem above the fruits 5 days before sampling caused an increase of osmotic potential in the mesocarp but had no effect on exocarp water potential. Neither diurnal changes in transpiration nor girdling of the stem affected the osmotic potential of the vesicles.Osmotic potentials in all tissues of the fruit were in the range of -10 to -15 bars. Measurements of osmotic potential at 16 locations along a longitudinal plant through the fruit axis showed that osmotic potential increased from the stem to the stylar end, but it decreased from the pericarp tissues to the vesicles. As exocarp water potential decreased during a 20-day period after watering, osmotic potential decreased in the vesicles and exocarp. Turgor pressure, calculated as the difference between water and osmotic potentials, decreased with water potential in the vesicles but not in the exocarp. The lack of decrease of turgor pressure in the exocarp may result from a measurement error caused by pectins or from osmotic adjustment related to carbohydrate accumulation at low water potentials.

A study of the fine structure of minor veins of mature leaves of 975 species and 242 families of Angiosperms shows that transfer cells are widespread amongst herbaceous Dicotyledons, are much rarer in woody Dicotyledons, and are virtually absent from the Monocotyledons. The evolutionary significance of the distribution of the cells amongst and within orders, families and minor groupings is discussed. Four types of transfer cell are recognized in minor veins, all possessing irregular ingrowths of wall material protruding into their protoplasts, and all being regarded as modified parenchyma of the minor vein. Two types occur in phloem. One (the A-cell), with ingrowths distributed right round its periphery, is associated specifically with the sieve elements. The other (the B-cell) occurs more generally throughout the phloem and has zones of wall ingrowths oriented towards sieve elements and their associated companion cells or A-cells. Two other types (C- and D-cells) occur in xylem parenchyma and bundle sheath respectively, and have ingrowths only on walls in contact with or in close proximity to vessels or tracheids. Each species has a characteristic combination of types of transfer cell. The variations encountered in the survey are classified. Consistent differences in the frequency and form of ingrowths are to be found between the different types of transfer cell of a single species, and between different species in respect to a particular type of transfer cell. The functional significance of transfer cells in minor veins is discussed in relation to the loading and unloading of the conducting elements and to the retrieval of extra-cytoplasmic solutes from the mesophyll and the transpiration stream.

Indole-3-acetic acid-1'-(14)C (IAA-(14)C) and tryptophan-1-(14)C injected in small amounts into cotyledons of Phaseolus coccineus L. seedlings were found to be translocated acropetally into the epicotyls and young shoots. Similarly IAA-(14)C was translocated acropetally into coleoptiles of Zea mays following injection into the endosperms. Labeled metabolites of the injected compounds were also extractable from shoot tissue. However, evidence that IAA-(14)C itself was translocated acropetally was obtained by collection in agar blocks applied to cut surfaces of coleoptiles of injected seedlings. The acropetal translocation in Phaseolus was shown not to occur in the transpiration stream but in living tissue. Cotyledons of Phaseolus coccineus and Phaseolus vulgaris contain extensive vascular tissue.Tryptophan-(14)C was not actively translocated through excised segments of Phaseolus coccineus epicotyl and Zea mays coleoptile when supplied from donor agar blocks in concentrations as high as 100 mum. The small amount of tryptophan-(14)C which did reach receiver blocks when high concentrations were used may be accounted for by passive diffusion through the fluid-filled xylem vessels. Translocation of a non-toxic dye, Light Green SF Yellowish, through xylem vessels was found to occur when supplied from donor blocks placed acropetally or basipetally. Metabolism of the supplied tryptophan-(14)C by the tissue segments was shown to occur during the 3 to 6 hour translocation experiments. IAA-(14)C was transported in a strictly basipetal manner in both tissues. Only 1 labeled compound with an R(F) value of IAA was found in receiver blocks. Composition of a simple green safelight suitable for work in plant physiology is described.

Soil sterilization by methyl bromide greatly increases the uptake by pine trees of I/sup 131/ which had been introduced into the soil. This effect seems to be due to destruction of differentially permeable cytoplasmic membranes of pine roots in treated soil, destruction of soil microorganisms which biologically absorb minerals in the rhizosphere, and prior occupation of the small but significant anion adsorption capacity of soil by bromine compounds. Under these conditions, iodine remains mobile in the soil solution and is swept into the transpiration stream. For other isotopes, soil sterilants with different characteristics might be more effective. (auth) Dynamics of spontaneous supraweak luminescence (5 x 10/sup 4/ quant/sec cm/sup 3/) of tissues was observed during acute radiation sickness. Tissue homogenization as functions of thermal peroxide decay and temperature was analyzed. The concept of an autolytic mechanism in the terminal stages was in agreement with the tissue luminescence. (R.V.J.)

Summary Spore discharge from Epichloe typhina can be observed with the unaided eye. A method is described for studying the rate of spore liberation. Discharge tends to be periodic with a minimum in the morning and a maximum in the late afternoon or evening. In fairly dry air spore discharge continues for several days provided that the water supply to the host tissue is maintained, but when the transpiration stream is stopped by severing the stem, discharge soon ceases.

The relationship between the rate at which water and the rubidium and phosphate ions are absorbed by intact plants, and transferred to their shoots has been investigated in water culture under varying conditions of transpiration and nutrient supply. When the external concentration and the nutrient status of the plants are sufficient low, wide variations in the rate of transpiration have little effect on the transfer of nutrients to shoots; when little water is being lost by transpiration the concentration in the transpiration stream may exceed that in the external medium by factors exceeding 100. In contrast when the external concenration and the nutrient status of the plants are high the rate of transfer of ions to shoots may vary closely with the rate of transpiration and the concentration in the transpiration stream may be similar to, or less then, that in the external medium. The occurrence of concentrations of ions in the roots is transpiration stream which greatly exceed those in the medium external to the roots is regarded as evidence that ions not transferred passively across the roots of intact plants to a significant extent.

We started from the supposition that the exudation is brought about by two processes : (i) an active transport of salt to the xylem, the 'salt secretion', a process that depends on metabolism and is sensitive to inhibitors, and (2) a water trans port effected by the osmotic suction of the sap in the xylem. It was demonstrated that by enhancing the concentration of the medium the exudation rate rapidly decreases. This is followed by a slower rise to a new level which is the consequence of the salt secretion that continues and enhances the osmotic value of the exuda tion sap. After some time secondary changes of the exudation rate appear. In lowering the concentration of the medium identical changes occur, but now in the reverse direction. By tracing the influence of addition of substances different in mol. weight and lipoid solubility it could be established that the gradual change was the result of salt secretion and not of permeation of the added substance. From this simple supposition about the causes of the exudation process we derived formulae for calculating the salt secretion, the osmotic value of the exudation sap, and the conductivity for water of the root system. Though most observations are in harmony with this concept, there is a difficulty that too low a value is mostly found for the osmotic value of the exudation sap. The possible causes of this deviation are discussed. There is no necessity to accept an active water secretion as well as active salt secretion since salt and water transport cannot be separated. Important factors in exudation are water conductivity of the protoplasm and salt secretion into the xylem. Both factors are influenced by the osmotic value of the outer solution (D.P.D. of the medium) and by the presence of ions in the outer solution. The two factors must have an identical influence on the transport of water into the transpiration stream.

Kinetin has generally been thought to be immobile in plants. This was confirmed in the case of laminar applications in this study, but not in regard to petiole, vein, or root applications. Radioactivity from kinetin-8- 14 C (Kn * ) moved freely in the vascular system of several types of leaves. This movement was usually distal to the point of application and seemed to occur with the transpiration stream. Basipetal as well as acropetal translocation of radioactive kinetin was achieved in tobacco leaves. The translocated material was extracted from veinal tissue, shown to be radioactive, and to be able to retard senescence. Similar but less decisive results were obtained from agar blocks inserted into the vascular system of leaves receiving Kn * by petiole uptake. A bioassay employing disks from primary bean leaves was developed for the qualitative determination of substances like kinetin which possess the ability to retard chlorophyll breakdown and plant senescence. The use of radioactive kinetin provided a refinement in this bioassay because treated non-senescent areas could be correlated with exposed areas on radioautographs made from dried leaf disks. Root treatments showed that cotton seedlings did not take up Kn * but that similarly treated tobacco seedlings both absorbed and translocated the isotope readily.

The uptake of monosilicic acid by crimson clover (Trifolium incarnatum L.) was investigated using solution cultures in which the level ranged from 0.4 to 60 ppm SiO2, and soils in which the level in solution ranged from 7 to 67 ppm SiO2. With increasing levels of silica in the external solution there were systematic increases in uptake, but the quantities of silica in the tops were always less than those which were theoretically carried to the roots in the mass flow of water. The silica content of the roots was higher than in the corresponding tops and seemed to be largely associated with the epidermis. These findings and the observation that the concentration of monosilicic acid in the xylem sap is lower than that in the external solution, are regarded as evidence that the plant excludes a proportion of the monosilicic acid from the transpiration stream. This exclusion is attributed to a barrier in the root through which monosilicic acid passes at a slower rate than water.

The factors affecting the absorption and translocation of simazine by young barley plants in short-term experiments in water culture have been investigated. Chromatographic examination of the xylem sap indicated no extensive breakdown of the herbicide in the transpiration stream. Under varying conditions of humidity, light intensity, temperature, and in the presence of metabolic inhibitors, the concentration of simazine in the trans piration stream relative to that in the uptake medium was always less than unity. This could, in part, be attributed to retention of simazine at a higher concentration on a fresh weight basis in the root tissues than in the ambient medium. There was little evidence that the absorption and translocation of simazine was influenced by metabolism except in so far as this affected movement of water, and micro-organisms present on the plant roots at ambient laboratory levels had no effect on the uptake and transport of the herbicide. Simazine brings about a decrease in the rate of transpiration and a reduction in the total uptake of rubidium and phosphate at relatively high concentrations. However, when the phosphate concentration in the ambient medium was below the level at which translocation of this ion is affected by the rate of transpiration, the herbicide did not reduce uptake of phosphate. There is therefore no evidence that in short-term experiments simazine has an effect on active transport processes.

In Pisum arvense , the amides and amino-acids normally supplied to the shoot in the transpiration stream transfer carbon to protein largely through the aminoacids, aspartic acid ( + asparagine), glutamic acid (+glutamine), threonine, lysine, arginine, and proline. Carbon from carbon dioxide enters the protein of photosynthesizing tissues through an essentially complementary set of aminoacids including glycine, alanine, serine, valine, and the aromatic amino-acids tyrosine, phenylalanine, and histidine. Young tissues of the shoot synthesize certain amino-acids de novo by metabolism of sugars supplied from photosynthesizing leaves. Each mature leaf on a shoot contributes carbon to current synthesis of protein at the shoot apex. Sucrose accounts for more than 90 per cent of the labelled carbon leaving any age of leaf which has been fed with 14C02. Upper leaves supply labelled assimilates directly to the shoot apex, and the radiocarbon from these assimilates is subsequently incorporated into a wide range of amino-acid units of protein. The majority of the labelled assimilates exported from a lower leaf move downwards to the root and nodules and, in consequence, the aminoacids and amides associated with root metabolism are strongly represented among the compounds eventually labelled in the apical region of the shoot.

1. These studies are concerned with the ultimate cause of the transpiration stream in the xylem tissue of vascular plants. 2. The current theories of sap rise involve active secretion of water by living cells of the leaves, excepting at times when the evaporation rate exceeds the secretion rate of these cells. 3. Repetitions of Dixon's experiments on sap rise with submerged shoots, and shoots transpiring into a saturated atmosphere without first satisfying the saturation deficit of the shoots, gave results in agreement with his findings. But in shoots in which the saturation deficit was first relieved there was no evidence of secretory activity of the leaf cells, and no significant rise of sap. The phenomena observed by Dixon are therefore believed to have been caused by saturation deficits in the twigs and shoots existing at the time the experiments were performed. 4. Although no experimental evidence is presented, it is believed that downward translocation of dye in the xylem, when leaves on wilty plant...

The temporary transpiration increase which normally occurs when a shoot or a part of a shoot is cut off in the air was studied qnantitatively in young wheat plants by the aid of the corona-hygrometer. The temporary transpiration increase can be characterized by the maximum increase in transpiration rate after the cutting, or by the total time of the temporary transpiration increase, or by the quantity of water given off by the shoot due to the temporary transpiration increase. The influences of the water vapour pressure, the speed of the air stream, and the light intensity on the temporary transpiration increase were determined. It is important to pay attention to the climate in the chamber where the shoot transpires. The maximum temporary transpiration increase was reduced more or less lineary with increasing water vaponr pressure of the air surrounding the shoot and increased with increasing speed of the air stream through the transpiration chamber. The reduction of the maximum temporary transpiration increase at higher light intensities was mainly due to the higher water vapour pressure in the chamber. The total time of the temporary transpiration increase was very little influenced by the water vapour pressure but was reduced more or less lineary with the logarithm of increasing light intensity. When the shoot was cut off in the water, there was normally no temporary transpiration increase. Only at low light intensities there could occur temporary transpiration increases similar to those found when the shoot was cut off in the air. Some hypotheses which could explain the temporary transpiration increase are discussed. The results in this investigation seem to favour the hypothesis that the temporary transpiration increase is due to a sudden reduced water transport up into the leaf, which can bring about a passive opening of the stomata.

The plant component of the soil-plant-atmosphere system is an organized body or structure permeated by water. When we speak of a plant as an organization we refer to evidence of the integrated unity that is apparent throughout its entire phenologic cycle ( 186) . In the view of Needham (1 12) the plant (as a micro-universe) can be con­ sidered as having a number of levels of organization and complexity from the atomic to the whole organism. Laws applying to one level may not apply to other levels. Each level requires its own concepts. Any study of more than one level at a time must involve an attempt to determine the intrinsic rela­ tionships between one level of complexity and the others, and, finally, the organizations perceived at each level of complexity must be integrated. These same considerations should govern our concepts of the relation between the plant and the soil and atmospheric environmental factors affect­ ing its life, with water recognized as a vital factor that is both intrinsic and extrinsic. Water is essential to the life and growth of plants, being a major constituent of living green tissues and often comprising as much as 90 to 95 per cent of the fresh weight. Water provides plants with their mechanical strength through the mechanism of cell turgor, since most vegetative tissues are composed of cells with non-lignified walls. Such plastic bodies become mechanically stable under the influence of the hydrostatic pressure developed by water inside the cell. Water is a raw material in metabolism and in the synthesis reactions that occur in living plants, even though the amounts involved here are rather small compared to the total amount of water that passes through the plant. Large amounts of water move into the roots, through the stem, into the leaves, and out into the atmosphere in the form of vapor, in what is known as the transpiration stream. Growth itself is an expression of cell enlargement brought about by the action of water. Many of the metabolic reactions of plants are conditioned by the degree of hydra­ tion of certain of the reactants or enzymes (23, 185). To understand this complex system we must approach it with the realiza­ tion that it is highly dynamic and composed of many interlocking and inter­ dependent phenomena. One of the most difficult problems encountered in understanding the behavior of water in the soil-plant system arises because observed reactions involving water have to be integrated both over time and through volume (141) .

Experiments with small plants of Populus tremula L. growing in solution culture indicate that polarly transported auxin is an important factor in the control of axillary bud growth. If the auxin supply from the growing apex is eliminated, the number of buds released is influenced by factors translocated in the transpiration stream from the roots. Suckers may be induced to develop from aspen roots, the age of which is six weeks or more. Removal of the growing apex and the axillary buds or stoppage of shoot growth by short day treatment were effective in inducing abundant suckering in small aspen plants. Some mature leaves had to be maintained, indicating the dependence of sucker formation on carbohydrate supply. These treatments are known to decrease auxin production in the shoots. Extraction and biological assay showed a decrease in the content of auxin in the roots as a consequence of removal of growing shoot parts. The results indicate that suckering in roots of intact aspen plants is prevented by auxin transported into the roots from growing shoot parts.

THERE are, roughly speaking, two sets of experiments by means of which we can investigate the presence of the transpiration stream in small plants. One of these is concerned with the root of the plant and with the demonstration of the phenomenon known as root pressure. The other set deals with the shoot and the suction exerted by it on the water which is being absorbed by the root. We all know what happens when we fix some kind of root pressure apparatus to the stump of a small plant, like a fuchsia for example. Water will soon be pressed out of the stump with sufficient force to raise a column of mercury to a considerable height. We can at the same time attach to the shoot of the very plant used for the root pressure experiment some form of potometer. We will observe suction and a column of mercury can be raised to a considerable height thereby. Do these two separate experiments really-even approximately-show what was going on in the plant at the time the experiment was set up? I think the answer must be no. When the plant in question was cut across, the stump exhibited the phenomenon of pressure, but the shoot that of suction at the same level. It is obviously impossible that pressure and suction should be exhibited by an intact plant at the same level. It must be mentioned here, however, that if the two experiments are set up very quickly, we may find the stump exhibiting suction for a short time. This changes to pressure when the stump is saturated with water. The root may of course become saturated in a few minutes by simply being exposed to water during the setting up of the experiment. We do not therefore get a clear idea of what is going on in the plant, when we isolate the -shoot from the root, by attaching to each one a separate apparatus. In order to learn more about the relation between shoot suction and root pressure it is obviously necessary not to separate the two 356 [MAY

Numerous data are available regarding the amount of water nmoving through land planlts, but very little is known concerning the movement of water through submerged plants. The current of water moving through the water-conducting system of a plant, regardless of the force or forces causing the movement, has been termed the " transpiration stream." Using the term in this broad sense, rooted submerged hydrophytes as well as ordinary land plants have a "transpiration stream." In land plants this stream is associated primarily with the evaporation of water from the leaves. If other forces are involved, it must be admitted that little is known about their relative significance. In submerged plants under normal conditions the factor of evaporation from the leaves is absent. In these plants, then, it is possible to measure the amount of the "transpiration stream " in the absence of evaporation. The object of the present study was (i) to obtain data as to the amount of water moving through submerged plants and (2) to locate, if not determine, the force or forces active in causing the water movement.

Developments in the past decade are reviewed. All the compounds considered possess direct fungitoxicity and none appear to depend on modifying the host plant, chemically or physically, to control diseases. Water solubility is low to very low. Yet these compounds, when sprayed, enter aerial tissues, and, on reaching plant roots, enter via non-living tissue and pass into the xylem. They are swept upwards in the transpiration stream, to an extent depending on the properties of the compound, and may accumulate in the margins of leaves. They do not appear to enter organs, such as petals, that do not transpire, nor are they re-exported from leaves to new growth. Any downward translocation in the phloem, if it occurs with any of these compounds, is of no practical significance. The range of plant diseases controlled by some compounds is confined to powdery mildews; other compounds control several types of disease, and some, especially among the benzimidazoles, control a very wide range indeed. None offe...

SUMMARY 2,6-Dichlorobenzamide (BAM) induced leaf margin chlorosis (LMC) on the leaves of kale seedlings and apple trees when applied to the roots. The leaf symptoms were similar to those sometimes seen after use of the herbicides chlorthiamid and dichlobenil. BAM was deposited mainly in the margin of the leaf to which BAM was transported via the transpiration stream. BAM appeared to be the causative agent of chlorosis although hydroxy derivatives of BAM were also present in the leaf. Factors possibly responsible for the variations in the occurrence and in the intensity of LMC are discussed.

Hydraulic conductivity in woody-stemmed plants is reviewed and published determinations collated and statistically analysed. Confusion between the expressions for conductivity and specific or relative conductivity are clarified, and their relationship established. Generalizations from the data are examined and attention drawn to systematic errors present in relative conductivity determinations. Estimates of maximum possible stress in water columns of trees are made from published values of relative conductivity and transpiration stream velocity.

Water in the transpiration stream is distributed throughout the leaves in the vascular bundles. In wheat, water appears to be confined to the main veins by the mestome sheath and to enter the mesophyll through the walls of the smaller veins. Within the mesophyll the water in the transpiration stream moves in the free space of the cell walls to the evaporating surfaces of the leaf. The lead chelate, which is used to trace the transpiration stream, accumulates at the final points of evaporation at the margin of the leaf. Lead chelate accumulates beneath and on the surface of the cuticle, being partly associated with the anticlinal walls of the epidermal cells, the walls of the stomatal guard cells and specialized epidermal cells. Chelate does not accumulate at the base of substomatal cavities, indicating that the cuticle of the epidermis is the main evaporating surface of the leaf. The behaviour in broad bean, laurel, and plantain is essentially the same. The rate of peristomatal and cuticular transpiration is closely related to the size of the stomatal aperture. Conditions which control stomatal aperture also cause changes in the dimensions of the epidermal cells.