STEM AND BRANCHES. 



13 



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

 but must receive at least a passing glance. 



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



Fig. 10. — Cross-seci.1^ A the Ptem of an oak. 



Fig. 2'J.— Cellular tis.sue (pitli). Mngiiified. 



Fio. 21.— Ol)li(tuo 

 Bcctiou of line year- 

 old stem of nilanthus. 



shape but become polyhedral by mutual compression (Fig. 20). In other 

 word.s it is merelv cellular tissue, with feeble vitality and short-liv(>d. 

 Tlxough 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 abuiulant proi^oi'tion- 

 ately in herbs and suflfruticosc plants 

 than in woody perennials. In some 

 rapidly growing woody perennials, how- 

 ever, the young stems have a very largo 

 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. 

 During the stage of its active growth, the pith of 

 some plants abounds in mucilaginous principles, that 

 of the young shoots of sassafras being especially marked 

 in this respect, and being considerably used in medicine 

 on this account. 



Wood also possesses a cellular structure, but the 

 cells are of a differ nt shape from those of the pith, and 

 are differentl}' arranged. TLey 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 trausparei t walls, and thus permit the ready ingress and 



Fio. 32.— WofHl-cells. 

 Mttgmrted. 



