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The localization of antigenic proteins on ultrathin sections of pollen grains represents an interesting approach to understanding the release mechanisms of these antigens when the pollen grains come in contact with various physiological fluids. Using different rabbit antibodies we have demonstrated the locations of these antigens in the various structures of pollen grains. We further demonstrated the cross-reactivities between alder (Alnus incana), birch (Betula verrucosa) and hazel (Corylus avellana) pollen allergens. Ultrathin sections of the pollen grains were prepared and allowed to react with two individually raised rabbit antibodies, (Ab-BV and Ab-ALK), against birch pollen. The sites of the Ag/Ab complex on the sections were labelled by protein A/gold, and identified in a transmission electron microscope. The two birch antibodies showed either quantitative or qualitative differences regarding their binding to various structures on the pollen sections. Using Ab-BV, the antigen-binding sites were located in the apertural region of the pollen grain and in the cytoplasm, while almost no gold labelling could be seen on the pollen surface. With the other antibodies (Ab-ALK), we could visualize the antigen-binding locations on the surface material of the pollen grains, particularly in the exine part of the wall and in the cytoplasm. A few gold particles could also be seen in the apertural region of the pollen. In hazel and alder pollen the exine part of the wall was the most densely labelled, whereas the cytoplasm and the aperture bound smaller numbers of gold particles. Cross-incubations: birch pollen incubated with antibodies against hazel (Ab-CA), or alder (Ab-AI), showed various intensities of gold labelling for each of the three species. Statistically, the differences in the number of gold particles bound per micron 2 grain section between birch, hazel and alder, were highly significant. The cross-reactivities between these antigens from the three pollen species were further tested using house-produced rabbit antisera against antigens of the three species by means of electrophoretic and autoradiographic techniques (CIE and CRIE). The three antibodies could precipitate the major IgE-binding antigen from all three pollen species.

Detailed observations on the pollen morphology which includes size and shape of grains, nature of aperture, type of sculpturing and exine thickness have been made on seven taxa of Flacourtiaceae. These observations reveal the underlying similarity among the various genera considered here.

The stamens of most of the world's flowering plants are longitudinally dehiscent, releasing their pollen passively, whereupon floral visitors may collect it. In nearly 400 genera in 65 plant families, the anthers dehisce by means of short apical slits or true pores. In these forms, the small light pollen can only be efficiently released by native bees capable of vibrating these stamens. This intrafloral behavior propels pollen out of the pores striking the bees on their venters. It is then collected for use in larval cell provisions. Aspects of the historical development of this novel pollination syndrome, known as “buzz” or vibratile (equals vibrational) pollination, are presented including a discussion and figures of a poricidal anther, a buzzing bee and the model system. A biophysical model for the pollen/locule wall interactions resulting in pollen expulsion upon bee or artificial vibration is developed. The model was created with the morphology of anthers of Solanum (Solanaceae) in mind, but the results obtained are generally applicable to any apically dehiscent flower which is vibrated by bees to release pollen. The anthers were modeled as a tall rectangular box with an apical pore and containing numerous small particles. As the box vibrates, particles striking the walls rebound elastically. If a pollen grain strikes a receding wall, it loses energy. If a grain strikes an advancing wall, it gains energy in the collision. In each oscillation, there is a net gain in the energy of the particles. As the anther (box) is shaken, vibrational energy is transmitted from the pterothorax of the bees to the flower, the pollen grains gaining significant energy. As the energy increases and the particles begin to move about more and more vigorously, they will begin to escape through the hole in the box (or stamina] pore). The rate at which particles leave the box and time required to empty the box are calculated as functions of the geometry of the model system and the frequency of vibration. In order to test the influence of air currents, Bernolli effects and viscous drag, the flowers were mecahnically vibrated in vacuum. The pollen cloud thus produced was virtually unchanged ans so it seems unlikely that air plays any significant role in the phenomenon of vibrational pollen release. Finally, variables such as: inelastic interactions, electrostatic forces, slightly sticky pollen due to presence of “pollenkitt”, duration and types of bee buzzes are discussed in relation to the mathematical model presented.

The difficulty, not to say impossibility, of presenting groups of orders as variously shaped figures in a two-dimensional diagram to indicate relationships is first emphasized. However, the method can be used to demonstrate the distribution of character states. The distributional patterns of a number of character states are presented with brief comments, viz. sympetaly, epigyny and perigyny, centrifugal succession in multi-staminate androecia, pollen grains with one aperture, the successive type of microsporogenesis, apocarpy, three-nucleate pollen grains (the “dry” stigmas and the C4 pathways being largely correlated with this attribute), unitegmic and tenuinucellate ovules and cellular and helobial types of endosperm formation. In addition, the distribution in dicotyledons is outlined for the synthesis of ellagic acid, iridoids, benzylisoquinoline alkaloids, glucosinolates and polyacetylenes derived from fatty acids. In monocotyledons other characters prove useful in gross taxonomy, for instance the presence of vessels in the stem, the presence of starch in the endosperm and the presence of oxalate raphides and silica bodies, as well as more conventional characters such as floral symmetry, epigyny versus hypogyny, stamen number and fruit type. Comments on the possible taxonomic significance of the characters shown are included. An apparent connection between certain constellations of orders (superorders) is pointed out. Finally, an appeal is made for further cooperation in presenting the distribution of other character states which may be of taxonomic importance at the higher levels.

Measurements of stomatal aperture in epidermal peels are used in a variety of physiological studies, but variability between stomates often hinders experimentation. We mount epidermal peels of Vicia faba over a microscopic grid that enables us to map the positions of individual stomates and thus measure the same individual stomates repeatedly, using a simple digitizing system. Repeated measurements of the same population of 15 stomates show much lower variability than measurements of different populations either on the same peel or on different peels.

The various terms employed to designate a particular structure in the pollen grain aperture or pore are reviewed along with the equivocal use of such terms to describe two apparently identical structures which actually differ in moment and place of origin. The factors which may have led to this confusion are analysed as well as the problems encountered in the search for an appropriate term which would avoid misinterpretation of results, given the need to adopt an international terminology to be used by all authors. The term “oncus”, originally proposed by Hyde in 1955, is considered the most appropriate in this sense, as it has the advantage of reflecting the origin of this structure in the form of a swelling or tumor, as long as it is clearly indicated whether the swelling is located in the intine or the exine., This work was supported by the "Comisión Interministerial de Ciencia y Tecnología" and CSIC. Project No. 181/906.

Bundles of actin filaments were observed in in vitro cultured pollen of Lilium longiflorum and Nicotiana tabacum which had been permeabilized in a buffered medium containing 5% dimethylsulphoxide (DMSO), EGTA, rhodaminephalloidin for F-actin staining, and sucrose as an osmoticum. In imbibed pollen grains, especially those of lily, numerous short bundles and small foci of F-actin were clearly visible. In germinated pollen grains, fine bundles of F-actin could be seen to converge at the aperture of the pollen grain. Along the entire length of the pollen tube, including the extreme tip, a dense three-dimensional netaxial distribution of actin filaments was observed. The F-actin patterns visualized after permeabilization with DMSO are much finer and more detailed than those observed after conventional fixation with formaldehyde.

The pre-fertilization behavior of the sperm of the thecate hydroids Campanularia flexuosa and Campanularia calceolifera has been observed and recorded by means of dark-field cinephotomicrography. The sperm of both species activate and aggregate homotypically around the aperture of the female gonangium. Plots of the paths of sperm approaching the aperture show that the aggregations are the result of directed turning movements and not of so-called trap-actions or an increased random turning of the spermatozoa. An aperture-associated tissue of ectoderm origin has been found to produce the attractive substances. Active sea water and alcohol extracts of female gonangia of C. calceolifera have been prepared. The substance affects sperm in the same ways as the material released by the female gonangium. The active material is heat stable, non-volatile, dialyzable, polar, and appears to be a single molecular species of less than 5,000 mw. Injection of these extracts into sperm suspensions by means of a micropipette produces activation and aggregation of only the sperm of C. calceolifera about the pipette tip. If agar impregnated with the chemotactant is used as a source, the sperm are seen to orient with considerable accuracy. Extraction of C. flexuosa female gonangia has not yielded active extracts. These results demonstrate that chemotaxis does occur during fertilization in an animal. The mechanism of the reaction remains to be determined.

Pollen mother cells at early meiotic prophase fromFritillaria lanceolata, F. mutica, Tulbaghia violacea, the lily “Formobel”,Triticum aegilopoides, T. dicoccoides, T. aestivum and synaptic and asynaptic forms ofT. durum were studied in thin sections with the electron microscope (a) in relation to distribution of nuclear pores (b) in respect of fine structure of the pore complex in those of the first four. The pores were distributed in random clusters during leptotene to pachytene in all plants, except in the two forms ofT. durum where there were either no pores or so few that they were not detectable. Probably correlated with this, the two membranes of the nuclear envelope were often widely separated and frequently sacculated. No pores were seen at leptotene in the part of the envelope to which, in theFritillarias and lily, the nucleolus was adpressed at this time. Evidence supporting a recent model which proposes that annuli are composed of three rings of eight granular subunits was obtained. These subunits as well as a dense central element, observed in most pores, were composed of filaments about 3 nm in diameter and evidently protein in character. There was evidence of a continuity between filaments in the central element and those in the rings of subunits which encircle the pore aperture at both the nuclear and cytoplasmic sides of the pore. In profiles of pores knobbed filaments were sometimes seen extending laterally from the pore wall into the perinuclear space at two sides. Questions concerning the role of the annulus are discussed.

The structure or granules associated with the pellicle strips ofEuglena granulata (Klebs) Schmitz has been studied using light and electron microscopy. Two types of bodies can be distinguished in this association on the basis of their staining reactions, distribution, ultrastructure, and other characteristics. The first type, called pellicle pores, is distributed with regularity along certain pellicle strips; the pores are very uniform in size (about 300 mΜ in diameter) and in content. They consist of an aperture and a compartment, the latter lined by the plasmalemma; in each compartment a dense, osmiophilic body and a series of tubules occur. The structure of the pellicle strips is modified at the points where the pellicle pores are present; the aperture extends to the cell surface in the groove between two pellicle strips. The second type of body, which undergoes vital staining with neutral red, is called a pellicle vacuole. These are variable in size (ca. 1 to 2 microns in diameter) and contain a variety of components including myelin figures, proliferated membranes, and sheets of electrondense material. The structure of these elements as well as their possible functional significance are discussed in some detail.

Rapateaceae is a family of monocots restricted to End_Page 133------------------------------ northern South America and Liberia. Although pollen is basically of a monsulcate type common in monocots, specializations occur: (1) lengthening of the furrow into an aperture widened at both ends, providing two germ pores instead of one; (2) lengthening of the furrow to form a continuous aperture around the equator of the grain; (3) presence of two identical furrows; and (4) variations in coarseness of exine patterning. Evolutionary and taxonomic correlations of these features are discussed. Pollen of Rapateaceae is an excellent example of the fact that no one method of preparation is sufficient, and reasons are given for use of several methods to provide (1) complete descriptions and (2) accurate descriptions. Current literature on pollen of the family is deficient in these respects End_of_Article - Last_Page 134

Microtubules have been shown to run around the perimeter of the pit aperture of developing bordered pits of Salix fragilis, L. These microtubules are parallel to the microfibrils being produced and do not converge with them. Although microtubules may be parallel to microfibrils in differentiating vessel elements as well as in fibres, many cases have been found where microtubules are not so orientated. There is no direct evidence for the involvement of microtubules in the orientation of microfibrils in the secondary wall, although their position during wall synthesis suggests some ancillary role.

The objections to the use of sealed tubes are known to every practical chemist, and are a serious drawback to their employment. The chief of these are the time expended in the manufacture of the tubes, the amount of skill in glass-blowing required, the danger experienced in opening them, and above all, the fact that only a small quantity of material can be heated at one operation. Moreover, the same tube can seldom be used for more than three or four experiments, as each time it is sealed up its neck must be drawn out, and its length thus considerably decreased. These disadvantages were especially felt by me whilst preparing bromacetic acid, which was required in considerable quantities, and where as many as half a dozen tubes of bromine and acetic acid had to be heated before 100 grammes of the acid could be obtained. To obviate these objections I have had an apparatus constructed, which consists of a cylinder of glass, the walls of which are about half an inch thick. Its length is fifteen inches, its external diameter three inches, and its capacity about 600 cubic centimetres. At one end it is drawn out to a tube, whose aperture is only about one-sixth of an inch in diameter, though its walls are as thick as the rest of the apparatus. Originally this tube was provided with a stopcock, but at Professor Brown's suggestion, I have substituted a glass plate, which is ground fiat, and accurately adapted to the top of the tube.

Publisher Summary This chapter discusses surface membrane specializations and the pollen wall. After meiosis, the four microspores of Nelumbo become separated by a callose-containing wall continuous with the special cell wall. Thiocarbohydrazide (TCH), a multidentate ligand, binds to certain cations found in thin sections of biological material. When microspores of Nelumbo in the tetrad stage are treated with PA-TCH-Pro, the fibrous wall gives a positive response, as do restricted areas on the plasma membrane, small vesicles in the cytoplasm, and distal cisternae of dictyosomes. The microtubules of Aegiceras have a similar diameter to the wall microtubules in Nuphar, but they are found on the exine surface and the locular surface of tapetal cells. A plug of polysaccharide is found in the aperture of Impatiens after the microspores are released from the special cell wall.

Two types of potometers which have been found very useful in studies on roots are shown in figure 1. Type I is used for studies of exudation only, type II for absorption and exudation. The dimensions of a potometer of either type are based upon such factors as : the diameter and length of the root selected for study ; capacity of the vessel, A, which should be sufficient to provide an adequate oxygen supply when gases are not passed through the solution surrounding the root ; and by the type of apparatus in which the potometer is placed. Each potometer is filled or drained through the tube, N, by using a small pipette with a rubber bulb and a short piece of rubber tubing to fit over the external orifice of N. This pipette is used to stir the solution when desired. In type I, tube ? is also used to pass gases or solutions continuously into the vessel and out through the side arm, P. The external orifices of ? and ? are closed with minute glass stoppers or by clamping rubber tubing passed over the projecting side arms. A root with no lateral branches is threaded through the respective holes, R, S, and ? of each potometer. In type I the hole, S, extends through the glass cover (which in some potometers is attached with vaseline to permit easy removal) ; in type II, S is the external aperture of the neck. The diameters of R and S correspond in general to the diameters of the selected roots ; they are a fraction greater to permit threading without injury. Threading a root through the aperture is conveniently done by holding the plant and excising the root at the upper level of R after threading. Excision is always made with a sharp razor blade through a drop of water surrounding the root at R. The excised root is held in place by the walls of the hole, T, bored through a small rod attached to the vessel wall. The root is placed so that its cut surface is just below the top edge of the opening R. A small drop of water over the cut edge provides a meniscus as a reference point at the beginning of an experiment. Ex?date flows from the cut surface into the " exudation ' ' tube, E, attached to the bent glass rod, B, which is the potometer support. When desirable the opening R is covered with a minute piece of cover glass. An "absorption'' tube, D, attached to the neck of a potometer vessel of type II is the essential difference between the two potometer types. This is filled or drained by passing fluid through tube N. Determination of the rates of absorption or exudation is based upon observation of the excursions of the terminal menisci in the ''absorption'' or

The primitive antgiosperm family Annonaceae possesses a remarkable type of pollen that appears unique in its size (diameter up to 350 micrometers), lack of exine over nearly 50 percent of the grain surface at maturity, and proximalipolar aperture. This unique pollent is further distinguished by being in polyads which are compartmentalized individually within septate stamens.

Pollen metabarcoding has received much attention recently for its potential to increase taxonomic resolution of the identifications of pollen grains necessary for various public health, ecological and environmental inquiry. However, methodologies implemented are widely varied across studies confounding comparisons and casting uncertainty on the reliability of results. In this study, we investigated part of the methodology, the effects of level of exine rupture and lysis incubation time, on the performance of DNA extraction and Illumina sequencing. We examined 15 species of plants from 12 families with pollen that varies in size, shape, and aperture number to evaluate effort necessary for exine rupture. Then created mock communities of 14 of the species from DNA extractions at 4 levels of exine rupture (0, 33, 67, and 100%) and two levels of increased lysis incubation time without exine rupture (2 or 24 hours). Quantities of these DNA extractions displayed a positive correlation between increased rupture and DNA yield, however increasing time of lysis incubation was associated with decreased DNA yield. Illumina sequencing was performed with these artificial community treatments with three common plant DNA barcode regions (rbcL, ITS1, ITS2) with two different primer pairings for ITS2 andrbcL. We found decreased performance in treatments with 0% or 100% exine rupture compared to 33% and 67% rupture, based on deviation from expected proportions and species retrieval, and increased lysis incubation was found to be detrimental to results.

Pollination induced dissolution of nucellar cells at the micropylar end forming a passage for pollen tube entry even when the pollen tubes are in the stylar region.

Estimates of water passing through an oyster based on: (1) amount of sediment deposited; (2) number of food organisms collected; and, (3) direct measurement of flow through a tube in the cloacal chamber, vary from 333 cc. per hour to 200 quarts per day. Galtsoff1 with method (3) obtains maximum pumpage of 7 liters per hour. This method is defective since tube in the cloacal chamber prevents normal operation of the first and second zones of control; the shell and the reflected mantle borders, and interferes with blood supply to the gills from the accessory hearts, Hopkins.2 In Ostrea virginica much of the water entering the right gill anterior to the adductor muscle leaves through an asymmetric excurrent aperture on the right side hence never enters the cloacal chamber. The small tube, 6 mm. creates high ratio of friction to hydraulic head developed.A superior method, Nelson,3 measuring all water passed while the oyster functions normally, is attachment of the rubber apron of Moore4 to a large (23 mm.) tub...

The decrease in stylar p O 2 encountered by pollen as it approaches the ovary can induce bursting of pollen tube tips. Anaerobic conditions induced a high percentage of tube tip bursting in most pollen germinated in vitro. Changes in tube tip metabolism with decreased oxygen probably sets up cell wall stress resulting in pollen tube rupture.

Pollen of Eupomatia laurina R. Br. and of Eupomatia bennertii F. Muell. are described. The pollen of both species is subspheroidal with a zonizonasulculus (a band-like, encircling aperture at the equator). E. laurina has a less dense and more vacuolated pollen containing lipids. E. bennettii has a dense pollen with many starch granules. Differences occur in their wall layers, especially in the aperture, but basically both species have an atectate, psilate pollen of medium size.

Ultrastructure, histochemistry and sequence of germination phases in mosses have been studied in three isosporous species, viz. Polytrichum commune Hedw., Ceratodon purpureus (Hedw.) Brid. and Funaria hygrometrica Hedw. and in one anisosporous species, Macromitrium sulcatum (Hook.) Brid. In each the aperture region is characterized by: 1. drastic thickening of the intine, 2. decrease in thickness of the exine, 3. lamellate structure present subjacent to the thin part of the exine and 4. accumulation of electrondense material into a thin separating layer between the intine and the exine. Further, in P. commune, a knob-like structure associated with the thickened part of the intine is present. When mature air-dried spores were soaked in tap water in daylight, water was absorbed through the aperture region. The subsequent sequence of stages was observed to be: swelling, spore wall rupturing, protrusion and recovery followed by spore distension. Spore swelling consists of two basically different kinds of swelling: symmetrical and asymmetrical. The symmetrical swelling is a process of remoistening rather than a stage in the germination. The asymmetrical swelling is caused by swelling of the asymmetrical intine which protrudes beyond the exine and perine in the aperture region. The recovery phase, not previously reported for mosses, is characterized by almost no structural changes. During this the protruded protoplast of the spore is protected from the surrounding environment only by the intine wall complex of the aperture region. The "opaque zone," previously reported to be present between the intine and exine, originates from the lamellate structure present in the aperture region. As in pollen grains and fern and liverwort spores, we assume that exine formation in mosses occurs by apposition of lamellae of the lamellate

Late Triassic and mid-Cretaceous appear to have been times of evolutionary innovations of seed plant pollen. Between these times dispersed pollen indicates relatively little change, although megafossil studies record the appearance of several new plant groups in the late Jurassic. This incongruity suggests that knowledge of pollination arrangements of Mesozoic seed plants is far from complete; some plants have been described with stigmatic surfaces, and investigation of their morphological intermediate position needs much fuller attention. The search for Mesozoic ancestors of angiosperms is particularly difficult because of the lack of a satisfactory definition of an angiosperm in Cretaceous time. It is suggested that all Mesozoic seed plants should be placed in a Mesosperm Group of fossils and that no living angiosperm taxon should be used for fossil plant remains before the Cenozoic unless at least two separately preserved plant organs in the same beds can be shown to be associated in supporting that taxon. All Mesozoic seed plant taxa should be based solely on evidence from fossils. In this short paper my object is to attempt to clear some of the obscuring fog around the problem of angiosperm ancestors. Hitherto, on the basis of the current understanding of the terms 'gymnosperm' and 'angiosperm,' I have supported (Hughes, 1976) the view that certain Cretaceous Barremian pollen represented the earliest angiosperms and that any ancestor should properly be sought among gymnosperms in earlier Cretaceous rocks. Progress, however, has been relatively slow because only a small proportion of those interested in the problem work with newly discovered fossil evidence. At the Ninth Botanical Congress (Montreal 1959), the struggle for understanding against obscure diversion (cf. Scott et al., 1960; Hughes, 1961) centered on supposed cryptogenic upland plants; more recently diversions take the form of lightly documented curiosities of comparative morphology of pollen aperture and exine structure (e.g., Klaus, 1979; Comet, 1980). In this age of advanced techniques of study of the available abundance of microfossils, a solution to the whole problem actually seems to be attainable by painstaking production of good records of fossils alone. The dangers to such continuous progress by hard work appear to lie in poor communications resulting from ill-defined terms and in the impatience commonly expressed through unnecessary neobotanical association theories. After examining briefly the current progress with fossil gymnosperms, I shall turn to definitions of terms and to those classification procedures that appear to obstruct un

The pollen morphology of all 24 species of the genus Macrotyloma is very distinctive and is unique in the subfamily Papilionoideae. The tectum is finely rugulate with supratectal tubercles, spines, bacula or clavae. There is a very thick endexine, no foot layer, very short dense columellae and very thin tectum. There are two aperture types, tricolporate and triporate. The taxonomically least specialised species M. ciliatum, M. decipiens and M. maranguense have tricolporate pollen while the remaining more derived species have triporate pollen.

Although discounted in a monograph recognizingStylisma as distinct fromBonamia, pollen morphology is useful in distinguishing these genera, Light and scanning electron microscopy show that the pollen ofStylisma is characterized by an advanced 3-aggrecolpate aperture totaling 12 or 15 colpi, while pollen ofBonamia is typically of a primitive 3-colpate type.

A simple apparatus has been developed whereby the extent of stomatal opening on isolated epidermal strips can be monitored by measuring the flow rate of bathing solution through the stomata. This apparatus has been used to study the opening and closing response of stomata to various treatments. A supply of ions, particularly K+, was found necessary to initiate and maintain the opening of stomata in the light. In the presence of K+ stomata could be opened in the light and closed in the dark. This cycle could be repeated. The pattern of stomatal opening in the light and closing in the dark showed a similar shape with time to that shown for stomata on leaves. Similar results could be obtained in a bathing medium consisting of KH2P04-Na2HP04 buffer. The best pH found for opening was 8� O. No specificity of stomatal opening was found for the anion associated with K +. The addition of Ca2+ or Mg2+ to the bathing medium caused reductions in aperture. The extent of stomatal opening in response to the concentration of K+ in the range 0-10 mM gave a saturation curve with a concentration for half maximal opening of O� 32 mM. Increasing the concentration of K+ past 10 mM resulted in reductions of aperture.

Stepwise decreases in the stomatal aperture of tobacco leaf epidermal strips followed stepwise increases in the concentration of KHCOa added to bathing solutions. Removal of KHCOa from the bathing solution resulted in a rapid increase in aperture. The reduction in aperture caused by KHCO., both in the light and dark, can be reversed by the addition of ATP or phosphoenol pyruvate to the bathing solution. The stomatal opening, supported by a NaCl bathing medium, is reduced by the addition of NaHCOa? From the results it is suggested that HCO;/C02t increases the permeability of guard cell membranes causing a net efflux of water or ions or both from the guard cells.

Three species of Compositae-Heliantheae have a uniquely complex lamellated structure in the foot layer of the ektexine, as revealed by electron microscopy. The possible importance of this is pointed out. Pollen exine morphology has been found to be constant for a given species and even throughout certain genera; within and among the higher taxonomic categories patterns of variations in exine morphology occur, and are useful in suggesting taxonomic and phylogenetic relationships as well as in distinguishing among unrelated groups (Carlquist, 1961, 1964; Raj, 1961). To date the characters used for taxonomic purposes have been primarily those dealing with exine size, shape, ornamentation, and aperture characteristics. However, Faegri (1956) and Erdtman (1960a, 1960b) have demonstrated with light microscopy that exine stratification based upon morphological and chemical differences exists and can be of systematic importance; they have established two competing terminological systems for designating exine strata. The system developed by Faegri which recognizes the outer, sculptured and basicfuchsin-staining layer as the ektexine and the inner, non-basicfuchsin-staining layer as the endexine has proven to be the most useful in our electron microscopic studies in that chemical staining properties have been found to be equated with morphological organization (Larson and Lewis, 1961; Larson et al., 1962; Skvarla, 1965). The ektexine, as we recognize it (following Faegri and Iversen, 1964), is subdivided into three morphological units: a basally positioned foot layer supporting columellae which in turn support a tectum (Fig. 1) [it is also recognized that any of the three ektexine units may be absent from particular exines]. With the increased resolution provided by electron microscopy these specific units have come under more comprehensive investigation. Of particular significance are the observations made on the foot layer. While the foot layer was shown to be variable in thickness among plant taxa, and even absent in some species (Larson et al., 1962) it was found to be consistently homogeneous in detailed structure and spatially distinct from the under1 This work is part of a Ph.D. dissertation completed at The University of Texas.

IF a capillary tube, A, Fig. 1, with a wedge-shaped cut in it at a distance of about 10 mm. from its upper end, at right angles to, and deep enough to make a breach in one side of its aperture, is lowered slowly into water the rising column of water pauses for an instant when it reaches the gap, which is necessarily higher than the surface of the water into which the tube is being dipped, and then leaps past it and rises to the top of the aperture.

WHEN these metals are heated in a vacuous tube in such a way as to cause an extremely thin sublimate of the metal to condense upon the glass, the film so obtained will be found to possess a beautiful and strongly-marked colour. That this colour is not in any way due to the combination of the metal with any lingering minute traces of oxygen, is evident from the fact that vacuous tubes which have contained the clean and bright metal for years, and in which the metal has been frequently melted and rolled about, and even vapourized in places, and in which, therefore, it is impossible to conceive of any oxygen remaining, will continue to show the phenomenon whenever a portion of the contained metal is heated. The experiment may readily be made by introducing a freshly-cut fragment of the metal into a glass tube sealed at one end and drawn down to a narrow and thickened constriction near the middle. The tube is then drawn out at the open end and connected to a Sprengel pump. As soon as a good vacuum is obtained the tube is warmed throughout its entire length, the pump being still in operation, and the metal heated sufficiently high to cause it to melt and run out of the crust of oxide. When the exhaustion is again as complete as possible the tube is sealed off. The metal is once more melted, the whole tube being at the same time gently heated, and the molten mass allowed to filter through the constriction into the other portion of the tube. The vacuous condition of the tube allows of the metal freely running through an extremely fine aperture, and in this way it becomes perfectly separated from all dross. The tube is then sealed off at the constriction. On gently heating a minute fragment of the bright metal so obtained, by means of a small pointed gas flame, the coloured film of sublimed metal will at once be seen. Viewed by transmitted light, the colour of the film of sodium thus obtained is greenish-blue, inclining to green. Potassium gives a sublimate which is of a magnificent rich purple colour, while rubidium, on the other hand, forms a film which is a pure indigo blue.

The evolutionary sequence from homospory through heterospory to the seed habit is reviewed in terms of the “small spore” (miospore) part of the life cycle. In the early stages of this progression, as seen in the Devonian and Carboniferous, the miospore shows little outward modification. The earliest miospores to function as pollen grains still retained proximal germination via a triradiate suture. The term “prepollen” is redefined so as to cover all such cases in which the miospore behaved as a pollen grain (in that its corresponding megaspore was retained in a ovule) but in which the germinal aperture was still proximal. The change from proximal to distal germination, which evidently lagged behind the retention of the megaspore to form an ovule, occurred independently in the cordaites and pteridosperms. The behavior of the pollen in living cycads shows an intermediate state between that of prepollen and the true pollen of the living conifers; for although the germination of cycad pollen is dista...

In most flowering plants, pollen is dispersed as monads. However, aggregated pollen shedding in groups of four or more pollen grains has arisen independently several times during angiosperm evolution. The reasons behind this phenomenon are largely unknown. In this study, we followed pollen development in Annona cherimola, a basal angiosperm species that releases pollen in groups of four, to investigate how pollen ontogeny may explain the rise and establishment of this character. We followed pollen development using immunolocalization and cytochemical characterization of changes occurring from anther differentiation to pollen dehiscence. Our results show that, following tetrad formation, a delay in the dissolution of the pollen mother cell wall and tapetal chamber is a key event that holds the four microspores together in a confined tapetal chamber, allowing them to rotate and then bind through the aperture sites through small pectin bridges, followed by joint sporopollenin deposition. Pollen grouping could be the result of relatively minor ontogenetic changes beneficial for pollen transfer or/and protection from desiccation. Comparison of these events with those recorded in the recent pollen developmental mutants in Arabidopsis indicates that several failures during tetrad dissolution may convert to a common recurring phenotype that has evolved independently several times, whenever this grouping conferred advantages for pollen transfer., Financial support for this work was provided by the Spanish Ministry of Science and Innovation (Project Grants AGL2004-02290/AGR, AGL2006-13529-C01, AGL2007-60130/AGR, AGL2008-04255 and BFU2008-00203), GIC-Aragón 43, Junta de Andalucía (AGR2742) and the European Union under the INCO-DEV program (Contract 015100). JL. was supported by a grant of the Junta de Andalucía. The authors thank K. Pinney for helpful suggestions to improve the manuscript.