REPOllT ON THE PHILOSOPHY OF BOTANY. 37 



descending in parallel tubes from the origin of the leaves, and 

 from no other place : 4thly, That in all cases where obstacles 

 are presented to the descent of such tubes, they turn aside, and 

 afterwards resume their parallelism when the obstacle has been 

 passed by : 5thly, That in endogenous plants, such as Palms, and 

 in some exogenous trees, such as Lignum Vita, they cross and 

 interlace each other in a manner which can only be accounted 

 for by their passing downwards, the one over the other, as the 

 leaves are developed : and, finally, That the perfect organization 

 of the wood is incompatible with a mere deposit of secreted 

 matter. To all which the following evidence has been added 

 by M. Achille Richard. He states * that he saw in the pos- 

 session of Du Petit Thouars a branch of Robinia Pseudacacia, 

 on which Robinia hispida had been grafted. The stock had 

 died, but the scion had continued to grow, and had emitted 

 from its base a sort of plaster, formed of very distinct fibres, 

 which surrounded the extremity of the branch to some distance, 

 and formed a sort of sheath ; thus demonstrating incontestibly 

 that fibres do descend from the base of the scion, to overlay 

 the stock. 



To this several objections have been taken, the most im- 

 portant of which are the following. If wood were really or- 

 ganized matter, emanating from the leaves, it must necessarily 

 happen that in grafted plants the stock ought in time to acquire 

 the nature of the scion, because its wood would be formed en- 

 tirely by the addition of new matter, said to be furnished by 

 the leaves of the scion ; so far, however, is this from being the 

 fact, that it is well known that in the oldest grafted trees there 

 is no action whatever exercised by the scion upon the stock, 

 but that, on the contrary, a distinct line of organic demarcation 

 separates the wood of the one from the other, and the shoots 

 emitted from the stock by wood said to have been generated 

 by the leaves of the scion, are in all respects of the nature of 

 the stock. Again, — if a ring of bark from a red-wooded tree 

 is made to grow in the room of a similar ring of bark of a white- 

 wooded tree^ as it easily may be made, the trunk will increase 

 in diameter, but all the wood beneath the ring of red bark will 

 be red, although it must have originated in the leaves of the 

 tree which produces white wood. It is further urged, that in 

 grafted plants the scion often overgrows the stock, increasing 

 much the more rapidly in diameter, or that the reverse takes 

 place, as when the Pavia littea is grafted upon the common 

 Horse-chestnut, — and that these circumstances are inconsistent 



» yuufcaiu- E/emcns de la BoiaiiiqKC, 5mc edit. p. 105. 



