VEGETATIVE ORGANS 01' PLAKTS. 



299 



is commonly known as the silver-grain. The botanist 

 terms them pith-rays, or medullary 

 rays. 



Fig. 51 exhibits a section of 

 vspruce wood, magnified 300 di- 

 ameters. The section is made 

 lengthwise of the wood-cells, four 

 of which are in part represented, 

 aud cuts across the pith-rays, 

 whose cell-structure and position 

 in the wood are seen at m, n. 



Branches have the same struct- 

 ure as the stems from which they 

 spring. Their tissues traverse 

 those of the stem to its center, 

 where they connect'with the pith 

 and its sheath of spiral ducts. 



Cambium of Exogens. — The 

 growing part of the exogenous 

 stem is between the fully formed wood and the ma- 

 ture bark. There is, in fact, no definite limit where 

 wood ceases and bark begins, for they are connected by 

 the cambial or formabive zone, from which, on the one 

 hand, wood-fibers, and on the other, bast-fibers, rapidly 

 develop. In the cambium, likewise, the pith-rays which 

 connect the inner and outer parts of the stem continue 

 their outward growth. 



In spring-time the new cells that form in the cambial 

 region are very delicate and easily broken. For this 

 reason the rind or bark may be stripped from the wood 

 without difBculty. In autumn these cells become thick- 

 ened and indurated — become, in fact, full-grown bast and 

 wood-cells — so that to peel the bark off smoothly is im- 

 possible. 



Minute Structure of Exogenous Stems. — The ac- 

 companying figure (53) will serve to convey an idea of 



Jig. 51. 



