CHAP. II. 
EXOGENOUS STEMS. 
87 
cular system, until it is either much reduced in diameter or 
wholly disappears ; and in proof of this assertion, the Elder has 
been referred to, in which the pith is very large in the young 
shoots, and very small in the old trunks. Those, however, 
who entertain this opinion, seem not to consider that the 
diameter of the pith of all trees is different in different shoots, 
according to the age of those shoots ; — that in the first that 
arises after germination, the pith is a mere thread, or at least 
of very small dimensions — that in the shoots of the suc- 
ceeding year it becomes larger — and that its dimensions 
increase in proportion to the general rapidity of developement 
of the vegetable system : the pith, therefore, in the first- 
formed shoots, in which it is so small compared with that in 
the branches of subsequent years, is not small because of the 
pressure of surrounding parts ; it never was any larger. 
The pith is always, when first forming, a uniform compact 
mass, connected without interruption in any part ; but the 
vascular system sometimes developing more rapidly than itself, 
it occasionally happens that it is either torn or divided into 
irregular cavities, as in the Horse Chestnut, the Rice-paper 
plant, and many others ; or that it is so much lacerated as to 
lose all resemblance to its original state, and to remain in the 
shape of ragged fragments adhering to the inside of the vas- 
cular system : this is what happens in Umbelliferous and other 
fistular-stemmed plants. 
Sometimes the pith is much more compact at the nodes 
than in the internodes, as in the Ash ; whence an idea has 
arisen that it is actually interrupted at those places : this is, 
however, a mistake ; for in general there is no interruption 
of continuity, but a mere alteration in compactness. It does 
however sometimes happen, that the pith takes a large deve- 
lopement at the nodes, so as to cut off the vascular system of 
the internodes into almost distinct parts. This occurs in 
what are called articulated stems, as in Piper, Viscum, &c., 
and in the Vine when young. Dutrochet regards such cases 
as evidence that each internode is an independent creation in 
the beginning, and that it is only after having been growing 
for a period of time, varying in different cases, that the inter- 
nodes become connected by woody formations. 
G 4 
