LETTER IV. 



37 



derive the first materials for its development in spring 

 from the succulent pith of that shoot. This pith is 

 simply a store of nourishment laid up in the cells, 

 which fill the medullary cavity of the shoot ; and it is 

 placed in closest proximity to the bud for the supply 

 of its earliest and more immediate wants. Thus the 

 shoot or stem yielding nourishment to the bud (besides 

 being to it, as we have seen, a stand or support on 

 which and around which to grow), may fairly be said 

 to serve to the young plant, which is to emanate from 

 the bud, the purpose of a temporary soil. This pur- 

 pose answered, and the supply of nourishment there 

 exhausted, the growing plant issuing from the bud will 

 send down a root ; and this root, passing alongside the 

 stem and the root of the plant of the bygone year, will 

 ultimately reach the ground. In its course down- 

 wards, encased in its own layer of bark, it will pass 

 between and part asunder the bark and the woody 

 tissue of the stem and root of that plant, traversing 

 in its passage the cellular tissue which connected these, 

 and which, stored with nutritious juices as the pith 

 was, will supply all its intermediate wants. On reach- 

 ing the ground — its permanent soil — it will thence 

 derive what further supplies it needs. 



11. I have repeatedly spoken of the roots as creep- 

 ing down, and as passing from the lower end of the 

 stems of the plants above downwards, into the soil. I 

 use this expression meanwhile analogically and pro- 



