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

No abstract

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.

Plants were grown to the stage of anthesis at constant temperatures from 10 to 30 °C and under continuous illumination at intensities ranging from 200 to 2500 ft-c. The absolute growth rate was maximal at the time of ear emergence. The relative growth rate declined continuously with time; the rate of decline was greatest at the higher temperatures and light intensities. The optimal temperature for the mean absolute growth rate over the whole growth period was 20–25 °C. The optimum for the mean relative growth rate was 15–20 °C. This difference is attributed to the faster rate of floral development at high temperatures. Both absolute and relative growth rates increased with increasing light intensity up to 2500 ft-c.

The increase in the leaf area ratio during early growth and the later decrease were primarily caused by an initial increase in the growth of leaves relative to stems and roots and a later rapid increase in stem growth associated with flowering. Changes in leaf thickness with time had little effect on the leaf area ratio.Increasing temperature from 10 to 25 °C increased the leaf area ratio because the growth of leaves was promoted to a greater extent than the growth of stems and roots, and because leaves were thinner. A further increase in temperature to 30 °C lowered the leaf area ratio because of an increased proportion of stem and thicker leaves.Increasing light intensity from 200 to 2500 ft-c decreased the leaf area ratio because of an increase in the relative proportion of stem and root and increased leaf thickness.These effects of the environment were mostly independent of changes in the rate of development of the plant.