258 PHYSIOLOGY. BOOK II. 



in a cherry tree. This substance exists in the wood in so 

 shght a degree as probably not to exceed in quantity what is 

 to be found in most plants, whether they are obviously gum- 

 miferous 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 exclusively, is not in that kind of contact 

 with the medullary rays which is required 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 pi'obable, as Knight believes, that the bark exercises 

 an influence upon the fluids deposited in it wholly analogous 

 to that exercised by the leaves, wliich will be hereafter explained. 

 Hence it has been named, with much truth, the universal leaf 

 of a vegetable. 



The business of the medullary rays is, no doubt, exclu- 

 sively to maintain a communication between the bark, in which 

 the secretions 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 wounded me- 

 dullary rays, although no such exudation is elsewhere visible. 

 In endogenous plants, in which there appears no necessity for 

 maintaining a communication between the centre and circum- 

 ference, there are no medullary rays. These rays also serve 

 to bind firmly together the whole of the internal and external 

 parts of a stem, and they give the peculiar character by which 

 the wood of neighbouring species may be distinguished. If 

 plants had no medullary rays, their wood would probably be, 

 in nearly allied species, undistinguishable ; for we are scarcely 

 aware of any appreciable difference in the appearance of 

 woody or vascular tissue ; but the medullary rays, differing in 

 abundance, in size, and in other respects, impress characters 

 upon the wood which are extremely marked. Tlius, in the 

 cultivated cherry, the plates of the medvdlary rays are very 

 thin, the adhesions of them to the bark are very slight, and 



