LECT. VII.] A N ATOM Y OF STE M S . 313 



at i. 2. The density of the cellular substance 

 around the inosculating vessels constitutes the 

 diaphragm, which is interposed between the arti- 

 culations of this description of stem : and the new 

 joint does not appear hollow at first, the cellular 

 substance filling up its cavity as at k. for a short 

 space above the knot. There can be, therefore, 

 little doubt that the articulated culm is as much 

 one individual as the stipe ; and the truth of this 

 opinion is not shaken by our inability to explain 

 the object of its articulated structure. But if it 

 be allowable to hazard a conjecture, I would say, 

 that as the cords of vessels run in straight lines, 

 and are comparatively remote from the surface in 

 these stems, the enlargement at the knots is partly 

 intended to afford space for permitting branches 

 from these vessels to be given off to supply 

 the new joint which originates there: accord- 

 ingly we see branches added to the vascular cords 

 as they arrive at the knots; and these, after 

 passing through it, approach each other and dis- 

 appear in the vascular cords again pf the new 

 joint ; the necessity for the branches no longer ex- 

 isting. The original cords of the old stem ac- 

 tually terminate in the leaf, which is the limit of 

 the growth of the joint ; and in this respect gives 

 some colour of truth to Darwin's theory of the 

 individuality of the articulations. The cellular 

 substance itself, in the knot, is probably intended 

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