200 PERIODICITY OF GROWTH 



of the division and rejuvenescence of embryonic cells, and these would 

 probably ultimately die if their growth was mechanically prevented. 



A preliminary explanation of the phenomena of rhythm is obtained when we 

 can ascribe them to definite powers of reaction possessed by the organism, although 

 such explanation gives us no deeper insight into the vital mechanism. Nor do we 

 exhaust all the possible combinations when we are able to explain certain phenomena 

 as being the result of direct stimulation, or as being due to a change of tone, and 

 others again to correlative reactions. It must further be remembered that an 

 exclusive formation of vegetative organs may result from a deficiency as well as from 

 a superfluity of food, and that restricted vegetative growth need not necessarily 

 induce a formation of reproductive organs. 



SECTION 58. The Daily Periodicity of Growth. 



The daily changes of illumination induce a periodicity in all functional 

 activities influenced by light. Growth is accelerated by darkness and 

 retarded by light, so that if the opposite sides of a growing shoot respond 

 differently, photonastic movements of nutation are produced daily, whereas 

 the daily movements of adult pulvini are due to the fact that a decrease of 

 illumination produces unequal tendencies to expansion in the upper and 

 lower halves of the pulvinus. The rigidity and tissue-strains in the pulvini 

 of Pliaseohis and Mimosa increase during the night, and G. Kraus * has 

 shown that the tissue-strains attain a maximum in growing organs at 

 sunrise, and fall to a minimum at sunset. Further, transpiration, the 

 exudation of water, the opening and closing of the stomata 2 , the movements 

 of leaves and of chloroplastids, all exhibit a daily periodicity. Owing to the 

 dependence of photosynthesis on light, green plants only produce organic 

 food during the daytime, whereas metabolism, to judge from the increased 

 rate of growth and respiration, is usually slightly more active at night than 

 during the day. There is, however, no general tendency to restrict growth 

 and formative activity to the night-time, and in fact the cell-division of 

 Spirogyra^ the formation of the sporangia of Piloboh/s, and the elongation 

 of the stipe of Coprimts tend to cease at night. In nature other factors 

 modify more or less the periodicity induced by the alternation of light 

 and darkness. In most cases the fall of temperature in the evening retards 

 growth, whereas the increased transpiration during the daytime may cause 

 so pronounced a fall of turgor as to diminish the activity of growth, or even 

 cause it to completely cease. According to the combined effect, growth 

 will either be more active at night or during the day. The former seems to 

 be the case during fine summer days in our climate, for the influence of the 

 rise of turgidity and of darkness on growth then surpasses the retarding 



1 G. Kraus, Bot. Ztg., 1867, p. 122 ; 1871, p. 371. 



2 Fr. Darwin, Phil. Trans., 189^, Vol. cxc, pp. 587, 619. 



