CHAPTEB XI. 

 THE PLANT. 



FOR a description of the structure and general life-history of plants the 

 reader will naturally turn, not to a work on agricultural chemistry, but 

 to a treatise on botany. Nevertheless, a brief account of the functions 

 of the various parts and their special adaptation for carrying on the 

 processes of life may usefully be given here. 



Germination. A seed is essentially a germ or embryo, together 

 with a store of reserve material from which the future plant is to be 

 formed (endosperm or cotyledons). The embyro is the only portion 

 of the seed which is really alive, the endosperm, e.g., of barley or wheat, 

 is merely a store of food l ; hence it is possible to transplant the embyro 

 from one seed to another without destroying its power of growth. The 

 nature of the food stored in a seed varies ; there is always a consider- 

 able amount of albuminoid matter and either starch or fat. 



Seeds suffer little change by keeping, provided they be protected 

 from moisture ; if not already dry they lose water, and in some cases 

 carbon dioxide, but these changes soon cease and no further loss occurs. 

 Vitality may be retained for several years. When placed under suitable 

 conditions, seeds germinate. The most important circumstances affect- 

 ing germination are 



(1) Moisture. 



(2) Temperature. 



(3) Access of oxygen. 



(4) Eemoval of carbon dioxide. 



Moisture is essential and acts by producing considerable swelling, 

 accompanied by a rise of temperature. 



A suitable temperature is also requisite. As a rule, no germination 

 occurs below 3 or above 49. The limits as well as the optimum 

 temperature vary with different seeds. 



For every plant there can be found three important temperatures, 

 viz., the minimum, optimum and maximum temperatures, at which 

 growth occurs or at which germination proceeds. 



Thus, the following table gives the three points for the germination 

 of several plants, in degrees centigrade : 



1 Steward, however, has shown that the purely endospermie tissues of barley and 

 maize respire, absorb oxygen and evolve carbon dioxide, thus affording evidence of 

 vitality, even in these cells (Ann. of Botany, 1908, 415). 



(219) 



