VEGBTATIVE 0K6ANS OF PLAKTS. 



299 



is commonly known as the silver-grain. The botanist 

 terms them pith-rays, or medullary 

 rays. 

 Fig. 51 exhibits a section of 

 A spruce wood, magnified 300 di- 

 ^ ameters. The section is made 

 allleugthwise of the wood-cells, four 

 j// of which are in pai't represented, 

 and outs 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 diicts. 

 Camiium of Exogens. — The 

 Fig. 51. growing part of the exogenous 



stem is between tlie 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 formative zone, from which, on the one 

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

 develop. In the cambium, likewise, the pitli-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 bai'k 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 ofE smoothly is im- 

 possible. 



Minute Structure of Exogenous Stems.— The ac- 

 companying figure (52) will serve to convey an idea of 



