170 PLANT PHYSIOLOGY 



growth is slower than during the night, when food is sup- 

 phed in abundance to the growing parts. * This periodicity 

 is far less marked in seedlings, with an abundant food- 

 supply in the cotyledons or in the endosperm, and in young 

 plants growing up from bulbs, tubers, and other parts in 

 which food is stored. The growth of parasites and sapro- 

 phytes may also vary periodically if they are subjected to 

 periodically varying conditions. Light affects growth quan- 

 titatively, as well as directing it in the ways to be described 

 in the next chapter ( p. 208 et seq. ) . If plants furnished with 

 a constant food-supply are subjected to otherwise constant 

 conditions, their growth rate will be constant for a time. 

 For reasons not wholly understood, but certainly including 

 other factors than food-supply, the growth-rate of any part 

 or organism will rise to a maximum and afterward fall 

 again. For each cell and for each individual there is what 

 has been rather pompously termed "the grand period of 

 growth." This means simply that from its formation by 

 the division of its mother-cell until the time when it ceases 

 to increase in volume, each cell passes through a period dur- 

 ing which it can grow, and during which its rate of growth 

 gradually rises from nothing and falls again to nothing. 

 After growth ceases, differentiation may still go on as a 

 separate process. The maximum growth-rate is not neces- 

 sarily coincident with the maximum food-supply or with the 

 maximum of any other tangible factor. 



Sachs and other physiologists f ha^-e called attention to 

 the fact, without fully explaining it, that the growth-rate 



* Sachs, J. von. Physiology of Plants. Oxford, 1887. Miss Gardner 

 (Trans, and Proceed.. Bot. Soe. Pennsylvania. Vol. I, No. 2, 1901) 

 claims that the growth of roots is faster by day than by night. This re- 

 result is probably due to the favorable action of light on processes upon 

 which growth depends rather than upon growth itself. The question de- 

 serves critical investigation. 



t Sachs, J. von. Uber den Einfluss der Lufttemperatur und des Tages- 

 lichts auf die stiindlichen und taglichen Anderungen des Langenwachs- 

 thums (Streckung) der Internodien. Arbeiten des bot. Instituts Wiirzburg, 

 Bd. II., 1872. Gesammelte Abhandlungen, Bd. II. Lectures on the Physi- 

 ology of Plants, English transl., p. 552. Kraus, Gregor. Physiologisches 

 aus den Tropen, I. Annales du Jardin Botanique de Buitenzorg, vol. XII., 

 1895. 



