176 THE STORY OF THE PLANTS. 



the air by the plant in its leaves, and afterwards 

 worked up by it into sugars, starches, protoplasm, 

 and chlorophyll. 



Stems are usually covered outside for purposes 

 of protection by a more or less thick integument, 

 which in trees and shrubs assumes the corky form 

 we know as bark. Bark consists of dead and 

 empty cells, thickened with a lighter thickening 

 matter than wood, and presenting as a .rule a 

 rather spongy appearance. But beneath the bark 

 comes a distinct layer of living material, inter- 

 posed between the corky dead cells of the integu- 

 ment and the woody dead cells of the interior. 

 This living layer extends over stem, twigs, and 

 branches : it forms the binding and connecting 

 portion of the entire plant community ; it links 

 together in one united whole the living material 

 of the leaves and shoots with the living material 

 of the roots and rootlets. It is thus the stem, 

 above all, that gives to the complex plant colony 

 of foliage and flowers whatever organic unity and 

 individuality it ever possesses. 



All situations, however, are not alike. Just as 

 here this sort of leaf succeeds, and there that, so 

 in stems and branches, here this form does best, 

 and there again the other. The shape of the stem 

 and branches, in fact, is the shape of the entire 

 plant colony ; and it is arranged to suit, on the 

 average of instances, the convenience of all its 

 component members. Much depends on the shape 

 of the leaves ; much on the conditions of wind or 

 calm, shade or sunshine. 



Some plants are annuals. These require no 

 large and permanent stem ; they spring from the 



