34 PLANTS AND MAN 



phere. Mosses can be considered as representing a body plan in 

 which the erect habit and some speciaHzation of tissues have 

 appeared. But many improvements were necessary before plants 

 could successfully colonize the variety of habitats of mountains, 

 plains and deserts which they occupy today. Adequate tissues to 

 protect the plants against water loss, special conductive channels 

 to form a vascular system, and skeletal tissues had to appear. 

 The first to develop these were the ferns and their relatives the 

 horse-tails and club-mosses; in the geologic past these plants were 

 the dominant forms of land vegetation, forming the first forests. 

 But the culmination of adaptation for land living is to be seen in 

 the seed plants, which have superseded the fern group in the 

 struggle to cover the earth's surface with vegetation. Since 

 fundamentally the ferns are like the seed plants in their main- 

 tenance structures, only the latter need to be considered. Of these 

 the maple tree can be considered as representative of the entire 

 group. 



A Woody Land Plant 



In the MAPLE TREE the constructive metabolic work is local- 

 ized in the foliage, of which each leaf is in reality a factory special- 

 izing in the manufacture of sugar. The entire body plan of the 

 tree is designed to place the foliage in the most advantageous 

 position for carrying on its work and to provide the factories 

 with the necessary raw materials. Sunlight being essential for 

 photosynthesis, one of the main functions of the trunk is to raise 

 the foliage to the light so that each of the thousands of individual 

 leaves can get its share of solar energy; thus there is a network of 

 branches and stems which entails skeletal tissue in order to pro- 

 vide a framework which can rise far into the air and support tons 

 of weight. To best expose the leaves to the light the plant body 

 must often raise the foliage a considerable distance from the earth. 

 Since the earth is the sole source of the water and mineral salts, 

 the trunk and branches assume a second function in addition to 

 support; through their conductive channels the materials taken 

 in by the absorptive system pass freely to the leaves; more freely 

 than would be true it the water and dissolved salts had to diffuse 

 slowly from cell to cell in their course from roots to leaves. The 



