302 PLANT STRUCTDRES 



164. Photosynthesis. — This is the process by which car- 

 bon dioxide and water are "broken up," their elements 

 recombined to form a carbohydrate, and some oxygen given 

 ojff as a waste product, the mechanism being the chloroplasts 

 and light. It has been sufficiently described in § 55, and 

 also in Plant Relations, pp. 28 and 150. 



165. Formation of proteids.— The carbohydrates formed 

 by photosynthesis, such as starch, sugar, etc., contain car- 

 bon, hydrogen, and oxygen. Out of them the living cells 

 must organize proteids, and in the reconstruction nitrogen 

 and sulphur, and sometimes phosphorus, are added. This 

 work goes on both in green cells and other living cells,- as 

 it does not seem to be entirely dependent upon chloroplasts 

 and light. 



166. Transfer of carbohydrates and proteids. — These two 

 forms of food having been manufactured, they must be 

 carried to the regions of growth or storage. In order to be 

 transported they must be in soluble form, and if not already 

 soluble they must be digested, insoluble starch being con- 

 verted into soluble sugar, etc. In these digested forms 

 they are transported to regions where work is going on, 

 and there they are assimilated — that is, transformed into 

 the enormously complex working substance protoplasm ; 

 or they are transported to regions of storage and there they 

 are reconverted into insoluble storage forms, as starch, etc. 



These foods pass through both the cortex and phloem 

 in every direction, but the long-distance transfer of pro- 

 teids, as from leaves to roots, seems to be mainly through 

 the sieve vessels. 



EESPIRATIOK 



167. Respiration. — This is an essential process in plants 

 as well as in animals, and is really the phenomenon of 

 "breathing." The external indication of the process is 

 the absorption of oxygen and the giving out of carbon di- 

 oxide ; and it goes on in all organs, day and night. When 



