266 PHYSIOLOGY. BOOK II. 



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

 oradually pushed outwards by the descent of woody matter 

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

 generatino- red matter from its horizontal system, through 

 which the wood descends, which thus acquires a colour that 

 does not properly belong to it. With regard to the instances 

 of o-rafts overgrowing their stocks, or vice versa, it is obvious, 

 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 diiferent : 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 

 altogether in a morbid state, and will not live for any consi- 

 derable 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 any thing, 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 ex- 

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

 variance 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 communi- 

 cation 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. 



Those who object to the theory of wood being generated by 

 the action of leaves, either suppose, -^ 1st, that liber is deve- 

 loped by alburnum, and wood by liber ; or, •2dly, that " the 

 woody and cortical layers originate laterally from the cambium 

 furnished by pre-existing layers, and nourished by the descend- 



