BOTANY. 14 
The stem does not always stand in the air (aerial); it is sometimes below 
ground (subterranean). ‘The latter are sometimes called roots; true roots, 
however, differ from these, in never possessing scales (rudimentary leaves) 
or nodes, from which may be developed eyes (rudimentary buds). The 
crown of the root is a shortened stem, often partly subterranean, and remain- 
ing in some plants after the upper portions have withered. This it is which 
constitutes the persistent portion of the ascending axes of the perennial 
plant. Here the internodes are very short, and the nodes crowd so closely 
together that there appears to be no stem. A rhizome or root-siock is a 
stem running along the surface of the ground, partially covered with soil, 
and sending out leaf buds from the upper side, and roots from the lower. 
This is seen in ferns, iris, &c. <A pserdo-bulb is an enlarged bulbose aerial 
stem, and is succulent, sometimes with numerous spiral cells and vessels, and 
a thick epidermis. A soboles is a creeping subterranean stem, sending 
roots from one part, and leaf buds from another. <A tuber is produced by a 
swelling of the internodes, caused generally by a deposit of starchy matter, 
as in the potatoe. The eyes of the potatoe are the leaf buds on the 
abbreviated and highly expanded stem. <A corm is a solid underground 
stem, which, of a roundish form, neither creeps nor roots, and is invested 
by series of imbricated scales, as in the tulip. It developes a second corm to 
one side, which feeds on the first and destroys it, itself to be devoured in turn 
by a successor. 
Stems, with respect to their structure, are either exogenous, endogenous, 
or acrogenous. Erogenous stems (erogens) are those which increase inde- 
finitely by layers applied to the outside. Stems are endogenous (endogens) 
when the bundles of vascular tissue are produced in definite fasciculi, and 
converged towards the interior, all additions being made in the interior. In 
the acrogenous stem (acrogens) the vascular bundles are all developed simul- 
taneously, and not in succession, the elongation of the stem depending on the 
union of the basis of the leaves,or the petioles, and the extension of the 
growing point or summit. In addition to the above, we have thallogenous 
plants (thallogens) where there is simple elongation or dilatation, without leaf 
buds or leaves, and dictyogens, where the stem has the structure of endogens, 
and the roots nearly that of exogens, as in Smilax. We shall now refer to these 
more particularly; premising, however, that there are modifications of the 
embryo which run parallel to those of the stem, the exogen having a germ with 
two seed lobes or cotyledons (hence dicotyledonous) ; the endogen, one with 
but a single cotyledon (monocotyledonous); and no lobe whatever in the acro- 
gen (acotyledonous). 
In the exogenous or dicotyledonous stem, we have the type of most trees 
of temperate climates, embracing both a cellular and vascular system. The 
cellular system includes the outer bark, the medullary rays, and the pith; the 
vascular the inner bark, the woody layers, and the medullary sheath. In 
the earlier stages of growth the young exogen is almost entirely cellular ; 
after a time, however, we perceive wedge-shaped bundles, edges of which 
point towards a common centre, arranged around a central cellular mass 
called pith, which is connected with the outer bark by means of cellular 
Il 
