258 
PHYSIOLOGY. 
BOOK II. 
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 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 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 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. Thus, in the 
cultivated cherry, the plates of the medullary rays are very 
thin, the adhesions of them to the bark are very slight, and 
