VEGETABLE PHYSTOLOGY COMPOUND ORGANS. 
31 
medullary sheath seems to be very important, as, by its being 
cut through, the vitality of the plant above the incision is de¬ 
stroyed, although the bark, the pith, and the wood may be 
removed, and yet vitality may be maintained: it may be sup¬ 
posed that its office is to convey oxygen to parts inaccessible to 
the external air, and parting with it to the carbon of such parts, 
to produce carbonic acid; without the power of composing and 
decomposing which no part of a plant exposed to light could 
long exist. The wood is at once the support of all the de¬ 
ciduous organs of respiration, digestion, and fertilisation; the 
deposit of secretions peculiar to individual species; and the 
reservoir whence the newly-formed parts derive their suste¬ 
nance until they can establish communication with the soil. 
The hark acts as a protection to the young wood, and is also a 
medium in which the proper juices are finally elaborated in 
their descent, and by it they are horizontally communicated to 
the medullary rays, and by the latter deposited in the tissue of 
the wood: when young it is green like leaves; and the late 
Mr. Knight concluded that it exercised an influence on the 
fluids deposited in it, wholly analogous to that exercised by 
leaves : hence it has been named the <f universal leaf of veget¬ 
ables.” The medullary rays no doubt maintain a communi¬ 
cation between the bark, in which the secretions receive their 
final elaboration, and the centre of the stem, where they are 
at last deposited. 
The vascular system in exogens is confined to the space 
between the pith and the bark; it exists in immediate contact 
with the former, consisting, as already observed, of spiral 
vessels, forming a layer, called the medullary sheath. In the 
stems of endogenous plants the vascular and cellular systems 
are as distinct as in exogens; but, instead of being distin¬ 
guishable into several parts, the cellular system is a uniform 
mass, in which the vascular system is embedded in the form 
of thick fibres, having no tendency to collect in zones or 
wedges resembling wood. It is supposed to increase in di¬ 
ameter by the additions of fresh bundles of fibres, or vascular 
tissue, in the centre; whilst in exogens the increase takes place 
between the bark and the wood, and, consequently, at the cir¬ 
cumference. 
This part of the subject (which is an important one) might 
be extended to a much greater length, by entering into the 
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