CHAP. III. 
STEM. 
257 
appears as veins : hence veins are always composed of bundles 
of woody tissue and spiral vessels. Thus situated, the veins 
are in the most favourable position that can be imagined for 
absorbing the fluid that, in the first instance, is conducted to 
the young pith, and that is subsequently impelled upwards 
through the woody fibre. 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 to 
convey that gas to parts inaccessible to the external air, and, 
parting with it to the carbon of such parts, to cause the pro- 
duction 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, by them to be deposited in the tissue of the wood. 
Hence, the character of timber is almost wholly dependent 
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 found so exactly drawn 
that the limits of the two are determined in the oldest spe- 
cimens as accurately as if they were fixed by rule and line : 
the woody tissue will be found uninterruptedly 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 they 
were at the time when the stock and scion were distinct indi- 
viduals. It is remarkable that the 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 never found elsewhere; as, for instance, gum 
s 
