74 INTERNAL STRUCTURE OF STEMS.—EXOGENOUS, 
scales or cataphyllary leaves, each of which has also the power 
of forming a leaf-bud in its axil. The presence of leaves with 
leaf-buds in their axils is therefore the essential characteristic of 
a stem, in contradistinction to a root, in which such structures 
are always absent. 
All Flowering plants, from the mode in which their axis is 
developed from the embryo in germination (page 13), must ne- 
cessarily have a stem, although such stem may be very short. 
Those which have this organ clearly evident are called caulescent, 
while those in which it is very short or inconspicuous are termed 
acaulescent or stemless. In Flowerless plants the stem is not 
necessarily present; thus it is absent in all Thallophytes, as 
already noticed (page 7). 
1. INTERNAL STRUCTURE OF THE STEM IN GENERAL.—A stem 
in its simplest condition consists merely of parenchymatous cells, 
with occasionally a central vertical cord of slightly elongated, 
somewhat thickened cells. Examples of such a stem may be 
commonly seen in Mosses (jigs. 9 and 10). Such a structure 
however would be unsuited to plants except those of low organ- 
isation, and we accordingly find, as a rule, that in all plants 
above the Mosses the stem is made up partly of parenchymatous 
cells, and partly of woody tissue and vessels of different kinds, 
by which the requisite strength and toughness are produced. 
In such stems therefore we distinguish two systems as already 
noticed (page 57), namely, a Parenchymatous or Cellular, and a 
Fibro-vascular ; and as the fibro-vascular tissue is arranged in 
longitudinal bundles in the midst of the parenchymatous, it has 
also been termed the Vertical or Longitudinal System, while the 
parenchymatous has been called the Horizontal System. 
In their internal structure the stems of plants are sub- 
ject to numerous modifications, all of which may be, however, 
in their essential particulars, reduced to three great divisions, 
two of which are found in the Phanerogamia, and one in the 
Cryptogamia. As illustrations of the two former, we may 
take an Oak and a Palm stem; of the latter, that of a Tree- 
fern. 
Upon making a transverse section of an Oak (fig. 176), we 
observe that the two systems of which the stem is composed are 
so arranged as to exhibit a distinct separation of parts. Thus 
we have a central one, m, called the pith ; an external one, ¢ e, or 
bark ; an intermediate wood, r, arranged in concentric layers 
or annual rings ; and little rays, b, connecting the pith and the 
bark, termed medullary rays. Such a stem grows essentially in 
diameter by annual additions of new wood on the outside of the 
previous wood, and hence it is called Exogenous (from two Greek 
words signifying outside growers). 
In a Palm stem no such distinction of parts can be noticed 
(fig. 177), but upon making a transverse section we observe a 
mass of parenchyma, m, distributed throughout it, and the 
