44 PLANTS AND MAN 



dry season, cambium activity ceases entirely. When renewed 

 growth takes place the next spring, the wood presents a marked 

 contrast to that of the previous autumn which is contiguous 

 with it, having more and larger vessels and fewer fibers; it is this 

 line of demarcation between the summer wood of one season and 

 the spring wood of the next which results in an annual ring, of 

 which one is usually formed per year in the trunk of the tree. 



In a tree, then, we see the climax of organization of the plant 

 body. But with all its organs and tissues and elaborate specializa- 

 tion, the tree does not carry on any metabolic activities different 

 from those characteristic of a desmid or a liverwort; such struc- 

 tural complexity has become necessary because of the competi- 

 tive demands of land living. Soil is the part of the environment 

 containing water and the necessary minerals; therefore the tree 

 must have a part of its body in intimate contact with this source 

 of raw materials — the result being the root system. The utilization 

 of sunlight, on the other hand, necessitates photosynthetic tissues 

 exposed to the air, which is likewise the source of carbon dioxide 

 and oxygen in this part of the tree's environment. Hence the 

 aerial portions of the tree, making up the foliage. Connecting 

 these two important parts of the plant body, the stems, branches 

 and trunks become the arteries of conduction as well as the sinews 

 of support. The entire tree is a beautifully designed organism 

 with every part having a structure and an appearance correlated 

 with its special duty in the general maintenance of the tree's 

 existence. 



The Colorless Plant Body 



There are many plants, of varying degrees of body complexity, 

 which lack chlorophyll; therefore they can not carry on photo- 

 synthesis and lead the independent existence of autotrophic 

 green plants. Neither can they carry on the simple type of auto- 

 trophic metabolism without chlorophyll, characteristic of the 

 sulphur and iron bacteria. These heterotrophic bacteria and 

 fungi, like animals, are unable to live on inorganic materials; 

 unlike animals, they absorb their organic food from the environ- 

 ment rather than ingest it. This means a dependence upon 

 carbohydrates, fats and proteins produced by or existing in auto- 



