CHAPTER XXXVI 

 GROWTH PERIODICITY 



The growth of a plant or plant organ never proceeds steadily hour after 

 hour or day after day, but is subject to more or less regularly recurring, often 

 rhythmical, daily and seasonal variations in rate. Seasonal variations in 

 growth phenomena involve qualitative as well as quantitative differences in 

 development during different stages of the growth cycle. IVlost plants, for 

 example, produce flowers only at certain stages in their life history and either 

 grow only vegetatively or not at all at other time^. The more obvious ex- 

 amples of growth periodicity often correlate very closely with cyclical daily 

 or seasonal variations in environmental conditions, but internal factors also 

 play an important role in many periodic growth phenomena. 



Daily Periodicity of Growth. — All actively growing plant organs char- 

 acteristically exhibit a daily periodicity in growth rate. A number of studies 

 have been made of daily variations in the rate of increase in the length of 

 stems or monocot leaves. Elongation of either of these types of organs in- 

 volves both the cell division and cell enlargement phases of growth. Under en- 

 vironmental conditions approximating those of a "standard day" (Chap. XIII) 

 curves for the daily periodicity of elongation of plant organs will often ap- 

 proximate those shown in Fig. 142. The maximum rate of elongation of 

 hyacinth leaves occurred in this experiment between midnight and 4 A. M. The 

 rate of elongation then gradually diminished until a minimum value was 

 reached a little after noon. Subsequently there was usually a consistent rise 

 until the maximum value was again attained. 



Cyclical variations in the rate of elongation of plant organs during the 

 course of a day can be interpreted in terms of the principle of limiting factors. 

 During the progress of the day first one factor and then another is limiting. 

 The rate of growth at any particular moment will be largely limited by the 

 factor in relative minimum at that time. The three principal factors influenc- 

 ing the daily periodicity in the rate of elongation of plant organs are the 

 internal water relations of the plant, light, and temperature. 



Under conditions which favor a daily periodicity of elongation of the 

 type just described a water deficit usually develops within the plant, attain- 



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