CHAPTER XII. 



THE RELATIONS OF PLANTS TO EXTERNAIi 



AGENTS. 



§ I. Temperature. 



241.— General Eolations. The functions of plants are 

 possible only between certain limits of temperature of the 

 air, water, or soil, yarying considerably for each species. In 

 eyery plant there is a certain minimum temperatuj-e, below 

 which all functional activity ceases ; thus in most instances 

 plants become inactive when the temperature approaches 

 0° Cent. (3^° Eahr.). On the other hand, there is a maxi- 

 mum beyond which activity ceases ; this ranges in different 

 plants from about 35° to 50° Cent. (95° to 133° Fahr.). Be- 

 tween these two extremes is the temperature at which the 

 greatest activity takes place ; this has been termed the opti- 

 mum. 



In any particular plant, the maxima, optima, and minima 

 are not exactly alike for all functions, some being performed 

 at temperatures considerably above or below those at which 

 others cease. It is furthermore to be observed that, in gen- 

 eral, there is a simple suspension of activity at temperatures 

 a few degrees below the minimum, whereas above the max- 

 imum the death of the organ ensues ; in the former a resto- 

 ration of the normal temperature is soon followed by a re- 

 sumption of activity ; in the latter the activity cannot be 

 restored, even under the most favorable conditions. 



242.— Absorption of Water as Affected by Temperature. 

 The absorption of water and watery solutions is greatly 

 affected by changes in the temperature of the absorbing 

 organs, as the roots of the higher plants. Thus Sachs 

 found "that the roots of the tobacco-plant and gourd no 



