12 PLANT LIFE. 



the liquid condition. The following, amongst other reasons, 

 explains this statement : the various substances constituting 

 plant food are brought into contact with the protoplasm 

 through the agency of water in which they are held in 

 solution ; this substance also conveys assimilated food from 

 its point of origin to those portions of the plant that require 

 food, but cannot prepare it directly for themselves. In the 

 passive condition of plant life, the amount of water present 

 is reduced to the minimum, but is probably never entirely 

 absent. 



(c) Temperature. — ^The vital functions of plants appear 

 as a rule to be confined between the temperatures o° C. 

 ( = 32° F.) and 50° C. (= r22'' F.) ; but the same functions 

 have very different limits between these two extremes in 

 different plants, and even in the same individual different func- 

 tions have different limits. Gourd seeds will not germinate at 

 a temperature below 13° C. ( = 55"4° F.), nor at one above 

 42° C. (=io7'6° F.), whereas in barley the limits to ger- 

 mination are 5°C. ( = 41° F.)and37°C. ( = 98'6°F.). Between 

 the two extremes of temperature at which any function can 

 be exercised there is one point, the optimum temperature, 

 at which that function is performed with the greatest activity, 

 and any departure from this point in the direction of the 

 maximum or minimum becomes less and less favourable for 

 the performance of that function. 



id) Light is an indispensable factor in connection with 

 green plants, inasmuch as the development of chlorophyll 

 is dependent on light ; and further, the work done by 

 chlorophyll can only be exercised under the influence of 

 light. Various other forces less general in their influence 

 on plant life do not call for special notice at present. 



It must be clearly recognized that favourable conditions in 



