FUNCTION.] DIFFICULTIES MET. 199 



The case of a ring of red bark always forming red wood 

 beneath it, is precisely of the same nature. After the new 

 bark has adhered to the mouths of the medullary rays of the 

 stock, and so identified itself with the horizontal system, it is 

 gradually pushed outwards by the descent of woody matter 

 from above through it ; but, in giving way, it is constantly 

 generating red matter from its horizontal system, through 

 which the wood descends, and thus acquires a colour not 

 properly belonging to it. With regard to the instances of 

 grafts overgrowing their stocks, or vice versa, it seems that 

 these are susceptible of explanation on the same principle. 

 If the horizontal system of both stock and scion has an equal 

 power of lateral extension, the diameter of each will remain 

 the same ; but, if one grows more rapidly than the other, 

 the diameters will necessarily be different : where the scion 

 has a horizontal system that developes more rapidly than that 

 of the stock, the latter will be the smaller, and vice versa. 

 It is, however, to be observed, that in these cases plants are 

 in a morbid state, and will not live for any considerable 

 time. 



Another case was, that if a large ring of bark be taken 

 from the trunk of a vigorous elm or other tree, without being 

 replaced with anything, new beds of wood will be found in the 

 lower as well as upper part of the trunk ; while no ligneous 

 production will appear on the ring of wood left exposed 

 by the removal of the bark. Now this is so directly at vari- 

 ance with the observations of others, that it is impossible to 

 receive it as an objection until its truth shall have been 

 demonstrated. It is well known, that, if the least continuous 

 portion of liber be left upon the surface of a wound of this 

 kind, that portion is alone sufficient to establish the commu- 

 nication between the upper and lower lips of the wound ; but, 

 without some such slight channel of union, it is contrary to 

 experience that the part of a trunk below an annular incision 

 should increase by the addition of new layers of wood until 

 the lips of the wound are united, unless buds exist upon the 

 trunk below the ring. The horizontal parenchymatous 

 system may, however, go on growing, and so form new 

 layers. 



