22 VEGETABLE PHYSIOLOGY 



Yet a third reason may be given. The leaves are very 

 frequently so placed that they extend outwards from 

 the plant and lie nearly parallel to the surface of the ground. 

 In this way they present their edges to the wind and offer 

 as little obstacle as possible to its passage through the 

 tree, so minimising the risk of being torn off when the force 

 of the wind is strong. As the wind passes between them 

 they are made to rise and fall, but they offer much less 

 resistance to its force than they would if they were not 

 flattened. 



Besides the plants which we have been examining there 

 are other forms of terrestrial habits which possess only 

 weak axes, quite incapable of supporting any great 

 development of their shoot system. These obtain support 

 by clinging in various ways and holding by various mechan- 

 isms to other structures, such as the trunks of trees, walls, 

 &c. In some cases they develop accessory root systems 

 from some part of their shoots, such roots, usually of small 

 dimensions, penetrating their supports and so securing 

 anchorage. 



The first indication of structural differentiation in the 

 vegetative body of the plant is a change in the character 

 of the exterior, which has for its 

 object the protection of the plant 

 from external injurious influences. 

 This can be seen even among the 

 seaweeds, simple as is generally the 

 structure of members of this group. 

 Fucus and its allies, which form part 



of the class of the brown A1 g*> have 

 SHOWING CHARACTER OF their external cells much smaller, 



more closel y p ut to g ether ' and 



generally much denser than the rest 

 of their tissue (fig. 19). In the group 

 of the Mosses certain arrangements of this kind can be 

 seen. The common bog moss (Sphagnum) shows its stem 

 to have on the outside several layers of large empty cells 



