224 



THE SECOND BOOK OF BOTANY. 



In our picture of the woody bundle (Fig. 386) of 

 the first year, the portion next the pith is shown, 

 consisting of spiral ducts; and you saw (Fig. 385) 

 that the spiral ducts of all the woody bundles form a 

 sheath around the pith. But this sheath is not re- 

 produced in after-years. There are no spiral ducts 

 in the wood of the second year. They are never 

 found in woody stems, except around the pith. 



The stem, then, is made up of two distinct parts, 

 the wood and the bark. Fig. 396 represents a por- 



FIG. 396. 



K L. P. L. k LMC.C. F. v. F. v. F.V.T. 



M 



tion of a woody stem three years old. At M, you 

 see the pith ; T is the medullary sheath, and Y, F, 

 the woody layer of the first year, followed by Y, F, 

 Y, F, the layers of the second and third years. After 

 the cambium (C) is the green layer of the bark (M C), 

 and the three successive yearly rings of bark, marked 

 L, P, L, P, L, P. 



The green layer does not increase at all after the 

 first year, as the corky layer shuts out the light on 

 which its growth depends, and finally it perishes en- 



