STEM AND BRANCHES. 



13 



Their structure deserves more attention than we can give in this place, 

 but must receive at least a jDassing glance. 



Pith is but an aggregation of thin- walled cells, originally spherical in 



Fig. 19. — Cross-section of the stem of an oak. 



20.— Cellular tissue fpitli). Magnified. 



Fig. 21. — Oblique 

 section of oneyear- 

 olJ sten^i of ailanthus. 



Duri 



the 



shape but become pol^-hedral by mutual compression (Fig. 20). In other 

 words it is merely cellular tissue, with feeble vitality and short-lived. 

 Though active in the young and growing shoot, it soon becomes inert, and 

 not unfrequently decays long before the 

 plant reaches its term of existence. It 

 is commonly more abundant proportion- 

 ately in herbs and suffruticose plants 

 than in woody perennials. In some 

 rapidly growing woody perennials, how- 

 ever, the young stems have a very large 

 proportion of pith, as seen in the ailan- 

 thus (Fig. 21). As it exists in most 

 exogenous stems, it might very properly 

 be viewed solely as a relic of their in- 

 fancy. 



stage of its active growth, the pith of 

 some j)lants abounds in mucilaginous principles, that 

 of the young shoots of sassafras being especially marked 

 in this respect, and being considei-ably used in medicine 

 on this account. 



Wood also possesses a cellular structure, but the 

 cells are of a different shape from those of the pith, and 

 are differenth' arranged. They are commonly elongate- 

 cylindrical, tapering at each end, placed side by side, 

 and overlapping at the ends (Fig. 22) in such manner 

 as to form more or less tough, strong fibres. In early 

 youth they have transparent waUs, and thus permit the ready ingress and 



Fig. 22.— Wood-cells. 

 Magnified. 



