The pattern of tillering developed during vegetative growth of well-spaced wheat plants agreed closely with calculation, if one assumes constant rates of leaf and tiller emergence. The average number of leaves formed between the appearance of successive tillers on an axis can be calculated from the ratio between the total number of leaves and tillers, and provides an estimate of apical dominance.The increase in total number of leaves per plant obtained by increases in light intensity over the range 200 to 2500 ft-c was brought about by an increase in both the rate of leaf emergence on individual axes and the rate of tillering. As the rate of tillering increased to a greater extent than the rate of leaf emergence, apical dominance was reduced. An increase in temperature over the range 10–25 °C also increased the total number of leaves and tillers, but the rate of leaf emergence was stimulated more than the rate of tillering so that apical dominance was increased.These effects of the environment on the relationship between leaf and tiller production are attributed to the close dependence of tillering on the supply of assimilates.

The area of a fully-grown leaf lamina varied according to its position on the stem, and the temperature, light intensity, and photoperiod under which the plant was grown.I n continuous illumination, raising the temperature by 5' intervals between 10 and 25" C, resulted in progressively higher rates of leaf initiation, emergence, and expansion. The length of the lamina increased with each increase in temperature, but the breadth and thickness decreased. The greatest area of individual leaves was formed a t 20°C. Each increase in light intensity over the range 200, 500, 1000, 1750, and 2500 ft-c resulted in higher rates of leaf initiation, emergence, and expansion, and increases in breadth and thickness, but a decrease in length. The greatest area was formed a t 1000-1750 ft-c.An increase in daylength from 8 to 24 hours increased leaf length, breadth, and area. This was a photoperiodic effect, unlike the increase in thicl;ness with increased daylength, which was related t o the total light energy received.Changes i n leaf shape under different elivironmental conditions were not directly related to changes in leaf dry weight. H control of leaf growth by hormonal mechanisms sensitive to photoperiod seems more probable than does control by internal co~npetition for assimilates among developil~g leaves.'Manuscript received May 3, 1962. Contribution No. 223 from the

The number of spikelets on the differentiating inflorescence and the ear at anthesis was highest at high light intensities and at low temperatures. The length of the developing inflorescence and the ear, the height of the main stem, and the total plant dry weight at the time of anthesis were also greatest under these conditions.These results are related to differential effects of temperature and light intensity on the rates and duration of apical elongation, morphological development of the ear, and spikelet formation.

Under continuous illumination, floral initiation was earlier with each increase in light intensity from 200 to 2500 ft-c, and with each increase in temperature between 10 and 30 °C. This effect of light intensity is attributed to promotion of floral initiation by energy in the far-red (730 mμ).The rate of formation of leaf primordia was accelerated by increases in light intensity to a greater extent than floral initiation, so that the final leaf number on the main shoot was greatest for the plants grown at high light intensities. Between 10 and 25 °C an increase in temperature had similar effects on the rate of formation of leaf primordia and floral initiation, so that the final leaf number was not altered. The final leaf number was lower at 30 °C than at 25 °C because leaf primordium formation was retarded.After floral initiation, the growth of the apical meristem was most rapid at 30 °C and 2500 ft-c, resulting in the earliest heading and anthesis (33 and 38 days). Low temperatures strongly retarded the later stages of ear development and emergence.

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