The Plant. 87 



leaves the process of respiration or breathing, in which oxygen 

 of the inspired air combines with compounds of the plant, with 

 an accompanying elimination of carbon dioxide. This process 

 is most evident in darkness since it is not masked then by the 

 more extensive process of carbon dioxide assimilation. By com- 

 bining the carbohydrates as a basal material with nitrogen and 

 sulphur brought from the soil, the leaf cells produce a further 

 class of organic compounds known as proteins. Nitrogen and 

 sulphur usually enter the plant as highly oxidized compounds 

 and are built into the proteins after suffering reduction or loss of 

 oxygen. 



The leaf functions also as a temporary reservoir for migratory 

 compounds which, at the death of this organ, return into the 

 general circulation of food materials in the plant. This is true 

 particularly of trees and other perennial plants, whose dead 

 leaves are skeletons consisting chiefly of cellulose compounds and 

 unessential ash constituents like silica, the more important nu- 

 trient compounds and ash materials having returned to the stem 

 of the plant. 



Flowers, fruits, and seeds are pre-eminently seats of construc- 

 tive processes in which chemical reactions are especially active 

 and significant. Fragmentary protein structures, possibly the 

 amino-acids, are here withdrawn from solution in the sap cur- 

 rent and retained as finished proteins. Soluble carbohydrates 

 are converted to starch or to some of the fats, which are present 

 in the seeds. Ash constituents for the young plant of the next 

 generation are stored away as constituents of organic compounds. 

 Absorption of oxygen is especially marked in these organs and 

 may be accompanied by considerable heat production. In the 

 ease of the Italian arum lily it has been observed that the large 

 pistil absorbs in one hour nearly 30 times its volume of oxygen 

 with a resultant temperature of over 100 Fahr. 



The end of all this activity is the production of mature seed 

 containing a finished plant embryo, a store of food materials, and 

 enzymes to inaugurate the process of germination. At this stage 



