LETTER V. 



41 



shoot, grows and extends itself horizontally. In the 

 course of each season provision is made for that which 

 is to be developed the next, just as provision is made 

 for the evolution of every part of the new plants. 

 That is to say, equally with a set of buds, a layer of 

 embryo cellular tissue is each year produced for the 

 purposes of the next. This layer has received the 

 name of Cambium^ and is found on the side of the 

 medullary ring next the bark. On the return of 

 spring it begins to grow, and rapidly extends itself by 

 the development of new cells. These in due time are 

 traversed by a new set of 

 woody bundles {a a a) as 

 represented in figure 3. 

 These bundles are the roots 

 of the new plants emanating 

 from the buds, and they 

 pass down in such manner 

 as that the new medullary 

 rays are on a line with those 

 of last year's. The whole is 

 bounded by a new layer of bark {h b 6), lying inside 

 that of the previous year's plant. And it may be 

 remarked in passing, that, while the cellular tissue 

 serves permanently to bind those woody bundles or 

 roots together, it serves to them also, in their passage 

 to the soil, the like office which the pith in the medul- 

 lary cavity does to the bud — that of supplying their 

 present needs in respect of nourishment. 



