78 BOTANY. 



sugar, and a subtraction of a molecule of water (H^O) 

 yields the formula of starch (0,Hj„OJ. These changes 

 may be expressed as follows : 



00, + H,0 = II^OO, = 0H,0 + (0„ set free), 

 and 6(CH,0) = C,H„0, = C,H„0, + H.G. 



Now while starch is probably not formed in such a direct 

 way, it is worthy of note that in the chemical changes which 

 take place between the absorption of carbon dioxide and 

 the appearance of starch in the chloroplasts there is a set- 

 ting free of oxygen precisely as required by the expression 

 above. Moreover, in some cases the carbohydrate formed in 

 photosyntax is not starch, but glucose, or even oil or other 

 physiologically equivalent compounds. These carbohy- 

 drates are taken into the protoplasm as constituents of its 

 substance, from which it may build a cellulose wall (O^Hj^Oj), 

 or form glucose (OJij^Oj), sucrose (C,jH,jO„), inulin, 

 gums, oils, acids, etc. About one half of the dry substance 

 of plants is carbon, all of which has been obtained from 

 the carbon dioxide of the air by the process outlined above. 

 137. Nitrogen-assimilation. — -Another important assim- 

 ilative process is that by which nitrogen is obtained. This 

 substance, although not present in such large quantity as 

 carbon, is of high importance on account of its entering 

 largely into the composition of protoplasm. Inasmuch 

 as about 80 per cent of the air is free nitrogen, it might 

 be supposed that plants derive it from this source, but 

 careful experiments show this not to be the case. On: the 

 contrary, the nitrogen is derived from compounds in the 

 air, soil, and water, chiefly in the form of nitrates of 

 various bases (e.g., soda, potash, lime, ammonia, .etc.), 

 or some ammonia salt (e.g., the nitrate, chloride, sulphate, 



