LETTER XII. 



129 



(and which extends from the summit of the last year's 

 shoot to the extreme point of last year's root), there 

 is ultimately evolved three distinct tissues — woody 

 tissue, cellular tissue, and bark tissue, — the bark lying 

 outside and enclosing the other two, the cellular con- 

 necting the wood and the bark. Grant that all these 

 are formed as they lie {in situ.) It is not, however, 

 the whole account of the matter. For, from the lower 

 end of the Cambium-layer, — at the point where it is in 

 contact with the tip of last year's root, a new root 

 comes off which does in fact creep doicniuards^ — which 

 descends into the soil, just as from the bud at the other 

 end of the Cambium-layer the shoot ascends into the 

 air. 



12. We shall presently consider in what way the 

 transformation into woody tissue of the intermediate 

 (and far greater) portion of the Cambium-layer is 

 effected. Meanwhile, let us compare what has just 

 been described as occurring in the tree with what 

 occurs in the seedhng plant. We shall see that the 

 analogy between them is as complete as possible. 

 From the embryo contained in the seed, there passes 

 upwards the shoot, and downwards the root — the 

 shoot and the root being united at a point called the 

 neck. In like manner, from the summit of a tree 

 fifty years old there rises upwards a new shoot, and 

 from its opposite extremity in the soil there passes 

 downwards a fresh root — separated, however, the one 



I 



