CONSTRUCTION OF THE SHOOT SYSTEM 59 



FIG. 22. Venation of 

 leaf. 



of the upper surface become almost impermeable by 

 water. It is only after long soaking, therefore, that 

 any can find entrance. 



The danger from wind is perhaps greater than that 

 from rain. The leaf-blade, however, 

 though delicate and thin, is never- 

 theless very strong and not easily 

 torn. Running through it are the 

 ultimate endings of the vascular 

 strands we have already noted in the 

 root, the conducting tissue (Fig. 22). 

 These strands form the so-called veins 

 of the leaf and they constitute a net- 

 work of very tough fibrous bands 

 upon which the delicate tearable 

 tissue is supported. They generally 

 strengthen particularly the margins 

 and the apex of the leaf-blade and protect it from being 

 torn. The blade, therefore, when acted on by wind is 

 seldom either bent or curled, but is made to play as a 

 single rigid piece moving up and down without losing its 

 flatness for a moment. 



The danger of stripping from the twig is dealt 

 with differently. When the plant is of a sturdy, rigid 

 habit, the leaves are usually attached to the stem very 

 strongly, and are bent upwards so that the direction 

 of the wind must drive them towards the stem, and its 

 force cannot be felt between the latter and the leaf's 

 upper surface. More frequently, however, we find that 

 the blade of the leaf is attached to the twig by means of 

 a tough, flexible stalk, capable of movement in almost 

 every direction on its point of attachment. The elasti- 

 city is so great and so readily called into play that with 

 even the lightest breezes the leaves of most trees are 

 seen to swing to and fro with the greatest freedom. 



The form of the head of the tree is influenced by the 



