CHAP. V. 
ORIGIN OF WOOD. 
311 
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 imper- 
meable 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 ? or, that the latex, which flows exclusively through the 
cinenchyma, mingles but little with the medullary paren- 
chyma ? 
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 
a,n 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 succulent Cactaceae, 
Stapelias, and similar plants, there is no other part capable of 
performing the function of leaves. 
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 ap- 
parent from tangental sections of dicotyledonous wood mani- 
festing an evident exudation of liquid matter from the wounded 
medullary rays, although no such exudation is elsewhere 
visible. In endogenous plants, in which there appears less 
necessity for maintaining a communication between the centre 
and circumference, there are no special 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, undistin- 
guishable ; for we are scarcely aware of any appreciable dif- 
ference in the appearance of woody or vascular tissue; but 
the medullary rays (the silver grain of carpenters), differing 
X 4 
