BRIEF OUTLINE OF VEGETABLE PHYSIOLOGY 239 



supposed that one phase ceases altogether before another begins. We 

 distinguish the phases in a general way. 



574. Grand period of growth. — If the elongation of a short section 

 of a very young growing part, as for instance a section very near the 

 tip of a growing root, is marked off and measured from time to time 

 through several days, it will be found that at first the rate of elon- 

 gation in the given section is low, then gradually increases to a 

 grand maximum, and finally declines until growth disappears. The 

 whole time of growth of an organ, characterized by such a general 

 rise and ultimate fall of the rate of growth, is termed the grand pejiod 

 of growth. Within this there are minor variations, chief among which 

 are the daily fluctuations. 



575. Daily fluctuations. — If the length of a growing stem were to 

 be measured at frequent intervals during the twenty-four hours, it 

 would be found that elongation does not go on uniformly. It is 

 periodic, being less rapid in the daytime than at night. The diur- 

 nal minimum is usually reached sometime in the afternoon ; the 

 maximum, commonly after midnight. This is due to the nature of 

 the plants themselves, not directly to the working of external causes. 

 For if a well-nourished growing plant is kept for several days in the 

 dark, the periodic changes in growth rate still continue. All this has, 

 however, been induced in plant nature, in the past, by alternation of 

 day and night. 



576. The chief external influences affecting growth are temperature 

 and light. 



577. Temperature. — Favorable temperatures vary greatly, accord- 

 ing to the plant in question. Thus, in northern latitudes and on high 

 mountains certain species are found growing vigorously in early spring, 

 even through a covering of snow, at a temperature very slightly above 

 freezing ; while most plants of warm climates altogether cease to grow 

 at a temperature several degrees higher. For many common plants 

 the most favorable (^optimum) temperature is between 70° and 85° F. 



578. Light. — In general, light acts against growth. Too great 

 light may quite prevent growth. In nature, accordingly, the rate of 

 elongation increases during the night, especially after midnight, and 

 decreases during most of the day. 



579. Movement. — Transfer of substances in the plant, as of water 

 or food substances, and circulation of living protoplasm in cells have 

 been mentioned. In the descriptive chapters movements of particular 

 organs have been noted in detail, as the movements of roots of seed- 

 lings, stems, leaves, tendrils, tentacles, and floral organs. These activi- 

 ties have now to be briefly considered together. 



580. Most movements of bending are due to unequal growth on 

 different sides of the organs in question. Curvatures of mature 

 organs, like bending of pulvini of leaves, and sudden movements like 



