ELEMENTARY BOTANY. 
XXV11 
laterally as they increase in thickness or breadth. It surrounds or is intermixed 
with the fibro- vascular system, or it may exist alone in some parts of phaenogamous 
plants, as well as in cryptogamous ones. 
2, a fibro-vascular system , or continuous mass of woody and vascular tissue, 
which is gradually introduced vertically into, and serves to bind together, the 
cellular system. It is continued from the stem into the petioles and veins of the 
leaves, and into the pedicels and parts of the flowers, and it is never wholly wanting 
in any phaenogamous plant. 
3, an epidermis , or outer skin, formed of one or more layers of flattened (hori- 
zontal), firmly coherent, and usually empty cells, with either thin and transparent 
or thick and opaque walls. It covers almost all parts of plants exposed to the 
outward air, protecting their tissues from its immediate action, but is wanting in 
those parts of aquatic plants which are constantly submerged. 
194. The epidermis is frequently pierced by minute spaces between the cells, 
called Stomates. They are oval or mouth-shaped, bordered by lips, formed of two 
or more elastic cells so disposed as to cause the stomate to open in a moist, and to 
close up in a dry state of the atmosphere. They communicate with intercellular 
cavities, and are obviously designed to regulate evaporation and respiration. They 
are chiefly found upon leaves, especially on the under surface. 
195. When a phsenogamous plant has outlived the first season of its growth, the 
anatomical structure of its stem or other perennial parts becomes more complicated 
and very different in the two great classes of phaenogamous plants called Exogens 
and Endogens, which correspond with very few exceptions to the two classes 
Dicotyledons and Monocotyledons (167), founded on the structure of the embryo. 
In Exogens (Dicotyledons) the woody system is placed in concentric layers between 
a central pith (198, 1), and an external separable bark (198, 5). In Endogens 
(Monocotyledons) the woody system is in separate small bundles or fibres running 
through the cellular system without apparent order, and there is usually no dis- 
tinct central pith, nor outer separable bark. 
196. The anatomical structure is also somewhat different in the different organs 
of plants. In the Root, although it is constructed generally on the same plan as 
the stem, yet the regular organization, and the difference between Exogens and 
Endogens, is often disguised or obliterated by irregularities of growth, or by the 
production of large quantities of cellular tissue filled with starch or other substances 
(192). There is seldom, if ever, any distinct pith, the concentric circles of fibro- 
vascular tissue in Exogens are often very indistinct or have no relation to seasons 
of growth, and the epidermis has no stomates. 
197. In the Stem or branches, during the first year or season of their growth, 
the difference between Exogens and Endogens is not always very conspicuous. In 
both there is a tendency to a circular arrangement of the fibro-vascular system, 
leaving the centre either vacant or filled with cellular tissue (pith) only, and a more 
or less distinct outer rind is observable even in several Endogens. More frequently, 
however, the distinction is already very apparent the first season, especially towards 
its close. The fibro-vascular bundles in Endogens usually anastomose but little, 
passing continuously into the branches and leaves. In Exogens the circle of fibro- 
vascular bundles forms a more continuous cylinder of network, emitting lateral 
offsets into the branches and leaves. 
198. The Exogenous stem, after the first year of its growth, consists of 
1, the pith , a cylinder of cellular tissue, occupying the centre or longitudinal 
axis of the stem. It is active only in young stems or branches, becomes dried up 
and compressed as the wood hardens, and often finally disappears, or is scarcely 
distinguishable in old trees. 
2, the 'medullary sheath , which surrounds and encases the pith. It abounds in 
spiral vessels (188, 3) and is in direct connexion, when young, with the leaf-buds 
and branches, with the petioles and veins of leaves, and other ramifications^of the 
system. Like the pith, it gradually disappears in old wood. 
3, the wood , which lies immediately outside the medullary sheath. It is 
formed of woody tissue (188, 2), through which, in most cases, vessels (188, 3) 
variously disposed are interspersed. It is arranged in annual concentric circles 
