54 STUDY OF COMMON PLANTS. 



Indian corn. In this the fibro -vascular bundles are scat- 

 tered through the fundamental tissue so that there is no 

 Monocotyle- manifest distinction of pith, wood, and bark, and 

 dons, both here and in other members of the same 



class certain mechanical arrangements of much interest 

 present themselves. In the stem of Indian corn a strong 

 cylindrical band of sclerenchyma is placed just beneath 

 the epidermis, a disposition of the mechanical elements 

 adapted to secure the greatest strength with the least 

 amount of material ; and the same principle is carried out 

 in the bundles themselves, the sheaths of which are much 

 thickened radially, thus aiding materially in preventing 

 bending of the stem, and also protecting the vessels and 

 other conducting elements. 



The stem of dicotyledons presents a rather more com- 

 plicated structure. As seen in the apple shoot, which 

 may be taken as a representative, the pith, 

 wood, and bark are arranged concentrically. 

 In the bark, as a rule, three layers may be distinguished, 

 viz., outer bark or cork, middle bark or green layer, con- 

 sisting chiefly of large cells containing chlorophyll and 

 other materials, and inner bark or bast, characterized by 

 the presence of sieve-tubes, usually with bast libers and 

 some parenchyma. Between the inner bark and wood is 

 the cambium zone, which during the growing season is a 

 layer of delicate cells, by the multiplication of which new 

 wood and bark are produced. The Avood consists of the 

 large vessels, the openings of which are conspicuous on 

 transverse section, wood fibers which constitute the 

 greater part of its substance and give the wood its 

 rigidity, and the medullary rays, to which in many species 

 are added the wood-parenchyma cells. The pith consists 

 of large cells which commonly present no distinctive 



