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From an analysis of the length-time relation for tiller growth in wheat it was concluded that tillers which emerged traced a discontinuous curve, and that the discontinuity marked a critical event in tiller growth. Tiller buds which did not emerge as tillers continued to grow even more slowly, at least until anthesis of the primary shoot. The critical event was identified as that of escape from the cavity in which the bud was tightly contained throughout early development. The timing of vascular differentiation in tiller buds was shown to be size-dependent and there was no evidence for any correlation with the critical event of escape. A strong negative correlation between the lengths of the 3rd tiller buds and their subtending internodes was established, and the presence or absence of the 4th tiller bud was also correlated with its internode length. Current hypotheses relating to apical dominance are reviewed, and it is suggested that the dynamic physical constraints implicit in tiller bud growth add a further complication. It was concluded that hormonal, nutritional and constraint mechanisms need to be integrated as partial mechanisms which interacted at the interfaces between successive levels within apical systems.

With the aid of three-dimensional, scale drawings the development of the shoot apex of bulrush millet is described from germination through its transition to the reproductive condition. The apex is remarkable in that the branch primordia of the inflorescence are arranged in regular spirals, and that this involves a transition from the distichous condition of the vegetative apex. A number of phenomena relating to this transition support the view that physical constraint is a significant element in the determination of form, especially during early stages of development. Nevertheless, such constraint is seen mainly as a modifier of events at lower levels within the system. The new information is consistent with the mechanico-chemical field theory of phyllotaxis.

The initiation and growth of the tiller bud is described in the context of the developing shoot in wheat. The experimental plants were subjected to two levels of nitrogen supply and to two light intensities so as to diversify the pattern of tiller growth. The descriptions cover properties of tiller bud placement within the shoot, the external developmental morphology, and some aspects of the developmental anatomy of the bud, and of the nodal plexus in relation to growth of the leaf primordium and tiller bud. The results have been integrated in terms of linear relative growth rates in both radial and axial directions. In spite of the mutual constraints imposed within the continuous system, severe gradients in axial growth rates were found to be consistent with the maintenance of the integrity of the system.

Young wheat seedlings were subjected to mild physical constraint by placing short plastic collars with internal diameters of 1.5, 1.7 and 2.0 mm around the coleoptile close to the grain. In others the natural constraint of the coleoptile was reduced experimentally by making a vertical slit just above the site of the coleoptile tiller. The narrowest of the plastic collars reduced the growth of the primary shoot, but the others did not. Collars of all sizes prevented the emergence of all tillers sited immediately within them, though the coleoptile tiller, and occasionally the first tiller 'escaped' through the coleoptile or leaf sheath tissues below the collar. The coleoptile tillers of about half of the surgically treated seedlings grew vigorously, but those of the control plants did not. The results are interpreted as supporting the hypothesis that physical constraint can be an impor- tant determinant of developmental events within shoot apical systems.

The results of pot culture experiments, laboratory incubations, and soil chemical studies on phosphate availability are reported for a neutral loam from Griffith, N.S.W. Phosphorus-free organic matter (glucose) was incubated aerobically and anaerobically in soil to which known, relatively unavailable phosphorus sources had been added. These included rock phosphate, strengite (FePO4.2H20), and variscite (AlPO4. 2H2O). Native soil phosphate and added strengite became more available following anaerobic incubation. This effect was sometimes but not always increased when glucose had been incubated with the soil. Added rock phosphate and variscite were rendered less available following anaerobic incubation. These effects were prevented when glucose had been present. The organic acids produced during the anaerobic incubation of glucose in the soil seem to have played little part in determining the availability of phosphate in these experiments. Anaerobic conditions as such appear to have played a major role.

Vegetative shoots from the spring flush of an experimental citrus grove tagged and sampled on three occasions at intervals of 6 months. The effects of four cultural treatments, four levels of nitrogen supply, four combinations of stock and scion, and of time on leaf area and dry weight, and on relative and absolute contents of water, nitrogen, and phosphorus are presented and discussed. While nitrogen nutrition is still the dominant factor, the evidence strongly suggests that phosphorus nutrition has become important as a determinant of treatment effects within the experimental grove. The possible relevance of soil temperature and soil organic matter for some of the cultural treatment effects is discussed.

Experiments were conducted in controlled environments to determine the effects of high temperatures on grain development and yield in wheat. Two Australian and three Indian cultivars of wheat were exposed, from a week after anthesis until maturity, to "day" temperatures of 25, 28, and 3l°C, and "night" temperatures of 9 and 12°C. There was a mean reduction in yield of 16%' for the 6° rise in day temperature, but the cultivars did not differ significantly in their response to these temperatures. There were no significant effects of night temperature on grain weight, but stem weight was less at 12°C. Senescence was hastened only slightly by high day temperature, and there were no differential effects between cultivars in this respect.In a subsidiary experiment one Indian and five Australian cultivars were subjected to three day-night temperature regimes (24/19°, 27/22°, and 30/25°C). Highly significant but complex interactions were established between temperature regime and cultivar. A growth analysis for the Australian cultivars Ridley and Diadem indicated that the developing grain of Ridley had a greater capacity for growth than that of Diadem from the earliest stage. This, together with the confirmation of grain size as a very stable characteristic for all the varieties, points to the developmental and synthetic activity of the grain as an important determinant of grain yield. The relevance of this study to the production of wheat in India is briefly discussed.

This paper reports a study in crop physiology in which an analysis of plant growth response and nutrient intake under irrigation in the field is used to interpret the effects of rice hulls as an organic manure. An exploratory experiment showed that relatively speaking the main effects of treatment took place quite early in the growth of the crop. For this reason the main experiment was virtually restricted to these early growth stages. The test plant in both experiments was the tomato. It was found that rice hulls can influence the growth of the crop in at least three ways. Seedling emergence may be delayed, early growth may be retarded by temporary shortage of available nitrogen, and growth may be stimulated by increase in available phosphorus. All three effects are modified by the addition of sulphate of ammonia, but in all cases the result is an increase in yield with rice-hull treatment. Mean seedling emergence for the six experimental treatments had a range of 2.8 days, and it is shown that the mean weights of 18-day-old seedlings could have been accounted for by these differences alone. The pattern of response at 18 days remains for at least five weeks and is not without its effect on the final yields. In spite of their low nitrogen content, rice hulls alone had only a transitory adverse effect on the nitrogen status of the soil. It is suggested that this may not have had any direct effect on the rate of dry-matter production, but that there was a temporary deflection of assimilates to favour root growth at the expense of shoot growth. Such an effect would, of course, have an adverse effect on fruit yield. Plant analyses suggest very considerable increases in available soil phosphorus due to rice-hull treatment. Much of this phosphorus appears to have been fixed during the course of the experiment, but Truog values obtained after the experiment indicated that significant effects of treatment were still present. The increases in Available phosphorus were less when sulphate of ammonia was added with the rice hulls.

The results of two field experiments and a pot-culture experiment to assess the residual effects of organic manures of the plant-residue type are reported. In the first field experiment, to which superphosphate was added at seeding, no residual effects of the organic manures were observed. In the second field experiment, with cabbage as the test crop, rice-hull pretreatment gave a highly significant yield increase of seven tons per acre. This is attributed to increased phosphorus supply, and the interpretation is supported by analytical data for the cabbage heads. A parallel pot-culture experiment with tomatoes confirmed the findings of the field experiment and indicated that the nitrogen status of the soil had also been slightly improved by rice-hull pretreatment. It is shown that at the beginning of the cabbage and pot-culture experiments, rice-hull pretreatment had increased the inorganic but not the organic soil-phosphorus fractions. The development of the interaction of current phosphorus treatment with rice-hull pretreatment is illustrated by seedling-harvest data for the cabbage experiment, and weekly leaf-area measurements are used in a detailed study of treatment interactions in the pot-culture experiment.

A method based on leaf photographs is described for the estimation of leaf area. The method of preparing the standards is given, and a set of standards covering the full range of leaf form in young plants of the Pearson variety of tomato is presented. The method is rapid and does not damage the experimental plants. With five observers, the method was tested for the presence of bias resulting from fatigue or from variation in leaf size or form. Some of the observers showed personal idiosyncrasies of judgment which did not, however, invalidate the method for comparative studies. Estimates by all observers of leaf area per plant were very highly correlated with estimates based on an independent reference method.

Procedures for the preparation of perspective drawings from serial sections of shoot apices and similar small structures are described, and the properties of the projection are critically examined. The procedures have potential value for morphogenetic studies, and examples are given for shoot apices of flax and lupin. For lupin it is shown that the folioles arise in an oblique circle at the top of the emerging petiole. After the median member, they appear in pairs at unit plastochron intervals.

This study extends the account of the development of the fourth leaf of subterranean clover. In particular, the four leaf constituents deoxyribonucleic acid, ribonucleic acid, proteins, and cell wall materials are expressed on whole leaf and cell bases. Form change in the leaf is illustrated with perspective elevations for some early stages. Cell number increases exponentially until day 13, with a mean generation time of 18 hr. The final number per leaf is c. 6.5 million, and deoxyribonucleic acid phosphorus is estimated as 0.197 pg per cell. The leaf is highly succulent, with a maximum water content of 478 % dry weight. Soluble constituents are also high, with a maximum of 39 % of the dry matter. The absolute amounts of ribonucleic acid phosphorus and protein nitrogen are maximal on days 19 and 25 respectively, with losses of about 60% by day 36. Cell wall materials increase from 8 to 57 % of the residual dry weight during a period of 4 days prior to emergence. This is shown to be due to the concurrent differentiation of the vascular system and the growth of 3000 leaf hairs. Changes on a per cell basis are recorded, and the concept of relative growth rate is used to extend the analysis of the data. Rates of production of one constituent per unit of another are also presented and discussed.

The shoot apex of Trifolium subterraneum L. is spirodistichous, with a divergence angle of 160° and a phyllotaxis index of 2. The vegetative plant is defined quantitatively in terms of lengths of successive leaves, leaf areas, and dry weights of the leaves and all major plant parts for a 29-day growth period. The selected environment gave very high values of net assimilation rate (3 .0 mg cm-2 day-1for early growth). The properties of the shoot apical system are further defined in terms of volumes of meristematic and very young parts. The picture is extended by linking it with fresh weight data for the leaves. To a remarkable extent the data, transformed to logarithms, can be accounted for by arrays of linear regressions on time, the members of which show decreasing slopes with increasing leaf number. However, the most instructive description is obtained from an array of relative growth rate curves for successive leaves. While the existence of one general pattern of leaf growth seems unlikely, there is evidence for a marked increase in relative growth rate during leaf initiation. Exponential growth in length seems to be fairly common in young leaf primordia, but exponential growth in volume occurs in a variety of patterns in different plants. Attempts to link these occurrences with events in vascular differentiation and thus with the supply of substrate for growth have not been successful. It is suggested that properties of the system as a whole may be relevant.