644 



GROWTH PERIODICITY 



ing its maximum during the afternoon hours (Chap, XVIII). The gradual 

 diminution in elongation rate during the daylight hours parallels more or less 

 closely this decrease in the water content of the plant. Conversely the 

 gradual acceleration in elongation rate which takes place during the hours 

 of darkness reflects the increase in the hydration of the tissues which occurs 

 during this period. Actual growth rates are influenced primarily by the 

 variations in the hydration of the meristematic tissues but, in general, changes 

 in the hydration of meristcms parallel variations in the hydration of the plant 

 as a whole. Since both cell division and cell enlargement are soon retarded 

 by a deficiency of water in a plant, this close correlation between daily varia- 

 tions in rate of elongation and the inten- 

 sity of the internal water deficit indicates 

 clearly that the latter is an important 

 factor in determining the daily periodicity 

 of growth. 



Light also influences the daily peri- 

 odicity of elongation of plant organs, al- 

 though it is uncertain whether its direct 

 effects (Chap. XXXII) or its indirect 

 heating effects are the more important. 

 As previous discussion has shown, because 

 of the higher temperature of the leaves 

 the rate of water loss of plants exposed 

 to a high light intensity is usually greater than that of plants exposed to a 

 low light intensity under otherwise similar conditions. Consequently more 

 severe internal water deficits usually develop under high light intensities 

 than under low ones. Light therefore affects growth rates at least in part 

 through its effect upon the internal water relations of the plant. The usual 

 retardation in the rate of elongation of plant organs which occurs during the 

 daylight hours of clear days is at least partly a result of either direct or 

 indirect effects of light, and most probably of both. 



During the night hours temperature is undoubtedly very often the factor 

 limiting the rate of increase in length of plant organs. Conversely daytime 

 temperatures may be higher than the optimum for elongation or enlargement 

 under the other prevailing environmental conditions and may thus contribute 

 to the retardation in rate of increase in length of plant organs which is of 

 frequent occurrence during the daylight hours. 



Innumerable other patterns of daily elongation periodicity are possible, 

 depending upon the cyclical behavior of environmental and internal factors. 

 Only a few of these will be mentioned. Other extreme factors being equal, 



MX NOON MX NOON MX NOON MX 



Fig. 142. Daily periodicity in 

 rate of elongation of hyacinth leaves. 

 Data of MacDougal (1901). 



