BARK. 1S9 



union of the stock and scion. This line will be sometimes 

 found so exactly drawn, that the limits of the two are well 

 defined even in old specimens : the woody tissue will be found 

 uninterruptedly continuous through the one into the other, 

 and the bark of the two indissolubly united ; but the medul- 

 lary rays emanating from the bark of each will be seen to 

 remain as different as it was at the time when the stock and 

 scion were distinct individuals. It is to be remarked, how- 

 ever, that bark has only a limited power of impelling secreted 

 matter into the medullary rays ; and that there are certain 

 substances which, although abundant in bark, are scarcely 

 found elsewhere; as, for instance, gum in a Cherry tree. 

 This substance exists in the wood in so slight a degree as 

 probably not to exceed in quantity what is to be found in 

 most plants, whether they are obviously gummiferous or not. 

 Are we from this to infer that the medullary rays have a 

 power of rejecting certain substances ? or, that their tissue is 

 impermeable to fluids of a particular degree of density? or, 

 that they only take up what settles down the bark through 

 its cellular system, and that gum, descending by the woody 

 system, is not in that kind of contact with the medullary 

 rays which is requisite in order to enable the latter to take 

 it up? 



As the bark, when young, is green like the leaves, and as 

 the latter are manifestly a mere dilatation of the former, it is 

 highly probable, as Knight believes, that the bark exercises 

 an influence upon the fluids deposited in it wholly analogous 

 to that exercised by the leaves, which will be hereafter ex- 

 plained. Hence it has been named, with much truth, the 

 universal leaf of a vegetable. In fact, in many succulent 

 plants, there is no other part capable of performing the 

 function of leaves. 



The business of the MEDULLARY RAYS is, to maintain 

 a communication between the bark, in which the secre- 

 tions receive their final elaboration, and the centre of 

 the trunk, in which they are at last deposited. This is 

 apparent from tangental sections of dicotyledonous wood 

 manifesting an evident exudation of liquid matter from the 



