THEORY OF VEGETATION. 



15 



Roots are composed of the neck or collar, of 

 the taproot from which the roots ramify, and of 

 fibrous rootlets, at the extremities of which are 

 the spongioles, which from the soil absorb the 

 nourishment of the tree. The spongioles, as seen 

 under a microscope, are composed of cellular 

 tissue, and are of the greatest importance to the 

 life of the tree. 



The trunk of the tree is composed of the pith, 

 the ligneous body, and the bark. The pith is 

 formed of cellular tissue, bound together by the 

 tubes of the vascular tissue. In this position these 

 tubes act as vessels of the medullary canal. From 

 the natural deviation of these vessels arise the 

 leaves and the buds, and after these the green or 

 growing shoot. The ligneous body lies between 

 the pith and the bark. If we cut through the 

 trunk transversely, the ligneous body is seen in 

 the form of concentric layers, each the growth of 

 one year. Permeating these layers are the medul- 

 lary rays which connect the medullary canal with 

 the circumference. If, however, the trunk be cut 

 through vertically, the ligneous fibres of the layers 

 are seen to be formed by the union of vessels 

 derived from the base of the leaves, and reaching 

 downwards to the spongioles. The ligneous fibres, 

 produced by the upper leaves, lie over those 

 produced by the lower leaves. Thus the ex- 



