THE GENERAL CHEMISTRY AND PHYSICS OF PLANT-LIFE. 269 



(b) The Effect of Temperature upon Plant-life. 



As every chemical process takes place in a certain tempera- 

 ture, so likewise are the various life-processes of plants depend- 

 ent upon certain temperatures ; among different plants the same 

 life-process may be dependent upon different temperatures. 

 However, the temperature suitable to a given process may vary 

 considerably, so that we usually speak of a minimum, optimum, 

 and maximum temperature. According to SACHS, these three 

 "cardinal points " for the germination of our cereals are approx- 

 imately at 0, 28 (optimum), and 40 C. 



In reference to experimental physiology it may be mentioned 

 that there are special apparatus for the determination of the 

 rate of growth within definite periods of time. Such are the 

 auxanometers of SACHS, WIESNEE, and BARANETZKY. 



Below the optimum the curve of growth rises and falls with 

 the temperature-curve. The curves at least tend in the same 

 general direction, though they may not be parallel. It, however, 

 requires great care to determine the influence that each individ- 

 ual factor has upon growth. Some observations in regard to 

 these difficulties will be in order. When observing the influence 

 of a constant temperature upon the growth of a plant which is 

 at the same time exposed to a variable illumination, for example 

 growth during day and night, we encounter a complication 

 (SACHS '). The shoot-axis of Dahlia, for instance, shows a 

 maximum growth in the early morning ; in the afternoon a reduc- 

 tion ; before sunset another increase. It is evident that in this 

 case the growth-curve would not extend parallel with the temper- 

 ature-curve. According to the observations made by SACHS and 

 BARANETZKY, there is a periodicity of growth independent of 

 temperature and light, which is manifest in a rise and fall extend- 

 ing over a variable period of time. - The most important and 

 most general phenomenon in this periodicity due to internal 

 causes is the grand period of growth (Sachs) : each transverse 

 zone of a root, of a stem, etc., begins to grow slowly, then grows- 

 more rapidly, and after having reached the maximum gradually 

 decreases until it ceases to grow entirely. Such periodicity is. 

 not due to external causes. 



1 Vorlesungen, p. 680. 



