BOTANY. 15 
single terminal bud, as that just referred to, an example of which is to be 
found in the Palm, is called a phyllophor, or phyllogen. From this bud are 
developed the leaves with which the vascular bundles are connected, forming, 
as it were, their roots ; when the leaves of one bud decay another is produced 
in the centre, the bases of the leaves, as they die and fall off, leaving a scar 
on the stem. There is no way of determining the age of an endogen, a palm 
for instance, by examining a cross-section, since there are no rings of growth ; 
an approximation may, however, be found from the known length of the 
tree, elongation proceeding pretty uniformly, and at a determinate rate for 
different species. Occasionally there are several terminal buds, which 
may cause.a dichotomization or branching of the plant; in many, however, 
there is but a single one, whose decapitation is followed by the death of 
the tree. 
The third kind of stem, the acrogenous, or acotyledonous, 1s, in general 
appearance, not unlike that of most endogens, in beimg unbranched, of 
nearly uniform diameter, and bearing a tuft of leaves at the summit; the 
internal structure, however, always furnishes a ready means of distinction. 
Acrogens are rarely arborescent; a good illustration is, however, to be found 
in the Tree Fern, the stem of which is called a rachis or stipe. <A trans- 
verse section exhibits a circle of vascular tissue, composed of masses of 
various forms and size, near the circumference; the centre is either hollow 
or formed of cellular tissue. On the outside of the vascular circle there are 
cells covered by a cellular integument, representing an epidermis, often very 
compact, and formed originally of the bases of the leaves. The vascular 
bundles are all formed simultaneously, and their number depends upon that 
of the individual leaves or petioles. Although the acrogen is said to grow 
only from the top, yet, strictly speaking, there is a slight increase in diameter, 
as 1S shown im the separation of the rhomboidal leaf-scars, which originally 
were in contact. These scars, or cicatrices, are generally arranged in spiral 
series around the stem, which always carries their traces. 
In thallogens, which are simple expansions of cellular substance, some- 
times in definite directions, sometimes in all directions, there is no axis 
whatever, nothing but threads woven together or separate, or else cells, lobes, 
plates, or enlargements of various kinds. 
A few words as to the functions of the different parts of the stem must 
conclude this portion of our subject. The office of the pith, as already 
mentioned, is to convey nourishment to the young plant. By means of the 
medullary sheath a connexion is kept up between the central parts of the 
stem and the leaves, by means of spiral vessels, part of whose object may 
be the transmission of air. The medullary rays preserve a communication 
between the bark and the pith, and are directly connected with the formation 
of leaf buds and the matter of the cambium. ‘The bark protects the tender 
wood, conveys the elaborated sap downwards from the leaves, and is the 
medium in which many of the secretions are deposited. By means of the 
vascular bundles the crude sap is conveyed from the roots to the leaves. In 
woody fibre these bundles become ultimately choked up by the secondary 
deposits. 
15 
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