xliv OUTLINES OF BOTANY. 
gamous plants, during the first year of their existence consist anatomically 
of 
(1), a cellular system, or continuous mass of cellular tissue, which is 
developed both vertically as the stem or other parts increase in length, and 
horizontally or laterally as they increase in thickness or breadth, It sur- 
rounds or is intermixed with the fibro-vascular system, or it may exist 
alone in some parts of phznogamous plants, as well as in cryptogamous 
ones. 
(2), afibro-vascular system, or continuous mass of woody and vascular 
tissue, which is gradually introduced vertically into, and serves to bind to- 
gether, 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 
is never wholly wanting in any phenogamous plant. 
(3), an epidermis, or outer skin, formed of one or more layers of flat- 
tened (horizontal), 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 imme- 
diate action, but is wanting in those parts of aquatic plants which are con- 
stantly 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 re- 
gulate evaporation and respiration. They are chiefly found upon leaves, 
especially on the under surface. : 
195. When a phenogamous 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 
pheenogamous plants called Hxogens and Hndogens, 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 (Mono- 
cotyledons) the woody system is in separate small bundles or fibres running 
through the cellular system without apparent order, and there is usually no 
distinct 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 organisation, and the difference be- 
tween Exogens and Endogens, is often disguised or obliterated by irregulari- 
ties 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 relations to seasons of growth, and the 
epidermis has no stomates. 
197. In the Stem or branches, during the first year or season of hist 
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 
