80 PLANT STUDIIW 



bubbles in a test tube, as in an ordinary chemical experi- 

 ment for collecting gas over water, and testing it in the 

 nsual way. 



Some very important things are learned by tliis experi- 

 ment. It is evident that some process is going on witliin 

 the leaf which needs light and wdiich results in giving (iff 

 oxygen. It is further evident that as oxygen is elinanated, 

 the process indicated is dealing with suljstances which 

 contain more oxygen tlian is needed. The amount of 

 oxygen given off may be taken as the measure of the work. 

 The more oxygen, the more work ; and, as we have observed, 

 the more light, the more oxygen; and no light, no oxygen. 

 Therefore, light must be essential to the work of which the 

 elimination of oxygen is an external indication. Tliat this 

 process, wdiatever it may be, is so essentially related to 

 light, suggests the idea that it is the special process which 

 demands that the loaf shall be a light-related organ. If so, 

 it is a dominating kind of work, as it (^hielly determines 

 the life-relations of foliage leaves. 



The process thus indicated is known as p/id/osi/u/Jicsis, 

 and the name suggests that it has to do with the arrange- 

 ment of material with the help of liglit. It is really a ])ro- 

 cess of food manufacture, Ijy wlii(di raw nniterials arc made 

 into plant food. This process is an exceedingly important 

 one, for upon it depend the lives of all plants and aninnds. 

 The foliage leaves mav be considered, thci'efor(\ as sj)ei'i(il 

 orga?is of pkotosyiilJiesiK. They are sjiecial organs, not ex- 

 clusive organs, forany green tissue, whether on stem or fruit 

 or any part of tlie plant bodv. mav do the same woi'k. It 

 is at once appai'ent, also, that, dui-ing the night the ])rocess 

 of jihotosyiithcsis is not going on, and therefore during the 

 night oxygen is not being given olf. 



Another part oF this pi-occss is not so easily observed, but 

 is so closely related to the elimination of oxygen that it 

 must be mentioned. Carbon dioxide occurs in the air to 

 which the foliage leaves are exposed. It is given olf from 



