310 
PHYSIOLOGY. 
BOOK II 
are in the most favourable position that can be imagined for 
absorbing the fluid which, in the first instance, is conducted 
to the young pith, and which is subsequently impelled up- 
wards through the woody tissue. So essential is the medullary 
sheath to vegetation in the early age of a branch, that, as is 
well known, although the pith and the bark, and even the 
young wood, may be destroyed, without the life of a young 
shoot being much affected ; yet, if the medullary sheath be 
cut through, the pith, bark, or wood, being left, the part above 
the wound will perish. It may be supposed, considering the 
large proportion of oxygen it contains, that its office is in part 
to convey that gas to parts inaccessible to the external air, 
where it may combine with the carbon of such parts, and 
cause the production of carbonic acid ; without a power of 
composing and decomposing which, no part exposed to light 
can long exist. 
The BARK acts as a protection to the young and tender 
wood, guarding it from cold and external accidents. It is 
also the medium in which the proper juices of the plant, in 
their descent from the leaves, are finally elaborated, and 
brought to the state which is peculiar to the species. It is 
from the bark that they are horizontally communicated to 
the medullary rays, which deposit them in the tissue of the 
wood. Hence, the character of timber is almost wholly de- 
pendent upon the influence of the bark, as is apparent from a 
vertical section of a grafted tree, through the line of 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 uninter- 
ruptedly continuous through the one into the other, and the 
bark of the two indissolubly united ; but the medullary 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 distipct individuals. It is to be remarked, however, 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 else- 
where; as, for instance, gum in a Cherry tree. This sub- 
stance exists in the wood in so slight a degree as probably 
