84 PLANT STKUCTUEES 



temperatures and without any special display of energy. It 

 is important, because the food of all plants and animals de- 

 pends upon it, as it is the only known process by which inor- 

 ganic material can be organized. 



The process is called photosynthesis, or pliotot^iiiitux, 

 words indicating that the presence of light is necessary. 

 The mechanism on the part of the plant is the chloroplast, 

 which when exposed to light is able to do this work. The 

 process is often called " carbon assimilation," " chlorophyll 

 assimilation," " fixation of carbon," etc. It should be noted 

 that it is not the chlorophyll which does the work, but the 

 protoplasmic plastid stained green by the chlorophyll. The 

 chlorophyll manipulates the light in some way so that the 

 plastid may obtain from it the energy needed for the work. 

 Further details concerning it may be obtained by reading 

 § 112 of Plant Relations. 



It is evident that green plants must expose their chloro- 

 phyll to the light. For this reason the Algas can not live 

 in deep waters or in dark places. In the case of the large 

 marine kelps, although they may be anchored in considera- 

 ble depth of water, their working bodies are floated up 

 toward the light by air-bladders. In the case of higher 

 plants, specially organized chlorophyll-bearing organs, the 

 foliage leaves, are developed. 



56. Saprophytes. — Only cells containing chloroplasts can 

 live independently. In the higher plants, where bodies be- 

 come large, many living cells are shut away from the light, 

 and must depend upon the more superficial green cells for 

 their food supply. The habit of cells depending upon one 

 another for food, therefore, is a very common one. 



When none of the cells of the plant body contain chloro- 

 phyll, the whole plant becomes dependent, and must live as 

 a saprophyte or a parasite. In the cas(> of saprophytes dead 

 bodies or body products are attacked, and sooner or later all 

 organic matter is attacked and deconiposed by them. The 

 decomposition is a result of the nutritive processes of plants 



