396 CONCLUSION 



But the power which green plants have of construct-, 

 ing carbohydrates from carbon dioxide and water led, 

 broadly speaking, to the formation of more carbo- 

 hydrate material (sugars) within the cell than could 

 be used for the formation of new protoplasm, since the 

 available nitrogen in the form of nitrates, though 

 widely distributed, is -nothing like so abundant as the 

 carbon dioxide and water. This excess carbohydrate 

 material is condensed in the form of the polysaccharides 

 starch and cellulose, the latter covering the protoplast 

 in the form of the characteristic cell wall, which is such 

 an important feature of the structure of plants and 

 very largely determines the configuration and structure 

 of their bodies and the direction of their evolution. 



Under most conditions of life green plants make new 

 protoplasm slowly, and their bodies contain a far smaller 

 proportion of protoplasm and proteins, and a far larger 

 proportion of carbohydrates, than those of animals. 

 They grow slowly, continuously, and more or less 

 indefinitely, and correspondingly their bodies are much 

 less highly integrated than the bodies of the higher 

 animals. An " individual plant " is much less of an 

 " individual " than an individual higher animal, such 

 as an insect or a vertebrate. Large parts of it can be 

 cut off with little or no injury to the rest of the body, 

 and any sufficient portion (in some cases very small 

 fragments indeed) can grow into a new plant under 

 suitable conditions. 



These features are shared by the colourless plants — 

 the fungi — which have not the power of making carbo- 

 hydrates from carbon dioxide and water. But fungi 

 must live where they can obtain at least their carbo- 

 hydrate food ready made, and thus they are not 

 pioneers, like the green plants, but camp followers, 



