THE STRUCTURE OF STEMS. 221 



you liken the stem to a woven fabric, which part of it 

 would you say made up the warp, and which part 

 the woof? 



EXERCISE LXIX. 

 Second Year's Growth of Dicotyledonous Stems* 



In annual plants, like the melon, the cambium, of 

 course, perishes when the plant dies ; but in woody 

 plants this is the region of growth for all after-years. 

 You have seen that, in the primitive bundle (Fig. 

 386), there are two partial bundles, one of tissues 

 belonging to the woody system, and the other of 

 tissues belonging to the bark. The bark and wood 

 are connected by a delicate tissue of actively -mul- 

 tiplying cells, which may be easily seen with the mi- 

 croscope. As these cells are more gorged in spring, 

 the bark and wood are then more easily separable. 



Now, on the second year this cellular zone forms 

 in its interior, or next the wood, a new layer of wood, 

 precisely as in the former year, by the elongation of 

 some of its cells into fibres, and the conversion of 

 others into vessels, or ducts, while still others form 

 the parenchyma of the medullary rays. On the side 

 of the cambium next the bark there is similarly 

 formed from its cells a second layer of bark, precisely 

 like that of the first year. The new layer of bark 

 and the new layer of wood are, as before, transformed 

 cambium, but they are always separated by the true 

 cambium layer of vitally-active cells. 



