180 
RUDIMENTS OF THE NATURAL SYSTEM OF BOTANY. 
year, we examined the growing wood of several trees, for the purpose of satisfy- 
ing ourselves on this subject. The results, however, were by no means so conclu- 
sive as we desired, probably because we had not provided magnifying-glasses of 
sufficient power, and other necessary auxiliaries. In the young shoots of the 
Horse Chestnut, (which is an excellent object for exhibiting the organs clearly,) 
we found a decided fibrous communication between the old wood and the leaves. 
The petiole of each leaf contains a quantity of fibres, visible to the naked eye.» 
which envelop the growing stem, and may be traced down it, at regular distances 
from each other, till they are covered by the next layer of fibres, which issue from 
the leaf immediately below the one examined ; nevertheless, this structure, so far 
from sanctioning the supposition that such fibres are originated in the leaves, appears 
to favour a directly contrary assumption. If the fibres emanated from the foliage, 
it is most reasonable to imagine that those proceeding from the uppermost leaf 
would, by being last formed, maintain their exterior position throughout their 
whole extent ; since it is extremely unlikely that they should be capable of pene- 
trating beneath all the other layers, and preserving their contiguity to the pith 
of the new shoots, and the wood of the previous year. And yet this last is 
undoubtedly the case ; so that the theory which attributes to the leaves the gene- 
ration of woody tissue, must be received with some degree of reserve. 
Besides being essentially characterised by wood which enlarges from the outside, 
the stems of Exogens are invariably encompassed by a cuticle or bark. This bark, like 
the pith, is composed of cellular tissue alone ; but an annual layer is deposited in the 
inside, which seems to have the same origin as, or one very similar to, the woody 
layer ; and, by its proximity to the atmosphere, the pressure of this latter, combined 
with the outward pressure, indeed, enlargement of the wood, causes it to assume a 
compactness of appearance resembling the woody tissue itself. When neither bark, 
wood, nor pith, are present to assist the student, he can refer to the leaves ; and 
from the description of them we have already given, the sub- class of any plant 
may be discovered. 
Such is a very brief delineation of the prominent distinguishing traits of Exogens. 
The reader who has by this means made himself acquainted with that first and 
greatest sub-class, will be prepared to follow us, at some subsequent period, into 
the further subdivisions of this arrangement ; from which we shall endeavour to 
conduct him, by a regular and pleasing route, to the lesser orders for a more minute 
inquiry into the peculiarities of individual plants. If we sometimes depart from 
the beaten path, to illuminate its obscurities by a borrowed light from other 
departments of botany, we hope, by accomplishing the design of those digressions, 
to conciliate the most captious, and inform those who have never had sufficient 
time, means, or inclination, to thread the mazes of botanical science. 
