12 ? BOTANY. 
processes called medullary rays. At first the pith is large, and occupies a 
large proportion of the plant; the medullary rays, also, are of considerable 
thickness ; subsequently, by the increase of the old wedges and the 
development of new ones between them, the medullary rays become more 
restricted. Such is the structure of a young shoot during the first year. 
At the end of the second year, the shoot is found to have increased in 
diameter by the formation of a zone of vessels consisting of porous and 
woody tissue, and a zone of fibrous bark, the medullary rays bemg continued 
from within outwards, the number of such zones increasing year after 
year. 
Taking up the components of stem in proper succession, we begin with 
a more particular examination of the pith. This, the central portion, 
consists of cellular tissue, the cells, often hexagonal, diminishing towards 
the circumference. Pith is at first of a greenish color, and full of fluid; | 
subsequently this disappears. leaving a light colored, spongy, dry mass. 
Sometimes, in drying, it separates into regular cavities, as in the Walnut 
and Jessamine; in this case it is said to be discoid or disciform. More 
frequently the cavities, when they exist at all, are of irregular shape. 
Occasionally there are vessels in the pith; sometimes, also, regular deposits. 
The elder exhibits an abundant pith; rice paper consists of sections of pith, 
the exact origin of which is, however, still undecided; some ascribe it to a 
species of Adschymomene. The object of the pith is to furnish nourishment 
to the young buds, for which purpose it is often filled with dextrine or 
starch, convertible into sugar by the process of vegetation. When the woody 
circle of the first year is complete, the pith remains stationary as to size ever 
afterwards. 
The medullary sheath consists of fibro-vascular or spiral vessels imme- 
diately including the pith, projections of which pass through this sheath into 
the medullary rays. A few woody fibres are usually intermingled with the 
spiral vessels. ‘This sheath is in direct gommunication with the leaf buds 
and the veins of the leaves, and carries up oxygen liberated by the decompo- 
sition of carbonic acid or of water, conducting it into the leaves. 
Woody Layers. During the first year, the vascular cylinder consists of 
an internal layer of spiral vessels forming the medullary sheath, and 
external bundles of porous and ligneous vessels. Subsequently, the layer of 
spiral vessels is not repeated, but concentric zones of porous vessels and of 
pleurenchyma are formed, constituting, in the tree, the woody circles. 
Exogenous plants are sometimes termed cyclogens, from their exhibiting 
these concentric circles. A transverse section of a branch or trunk of a 
tree usually shows these concentric circles very clearly, each one of which 
is generally supposed to represent the growth of a year. The circle of large 
pores usually seen to separate contiguous layers, is composed of the mouths 
of porous vessels. The distinctness, as well as the size of these circles, varies 
in different plants, and even in different parts of the same section. Neither 
is the number of rings in a cross-section to be taken as an indication of the 
true age of a tree, since there is good reason for supposing that two and 
even more rings may be formed in a single year, while one ring may occupy 
12 
