OUTLINES OF BOTANY. 
XXV11 
§ 2. Arrangement of the Elementary Tissues, or Structure of the Organs of Plants. 
193. Leaves, young stems, and branches, and most parts of phsenogamous 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 surrounds or is intermixed with the fibro- 
vascular system, or it may exist alone in some parts of phsenogamous 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 pe- 
dicels and parts of the flowers, and is never wholly wanting in any phsenogamous plant. 
3, an epidermis, or outer skin, formed of one or more layers of flattened (horizon- 
tal), 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-sliaped, 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 phsenogamous 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 (1985). In Endogens (Monocotyle- 
dons) 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 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 sel- 
dom, 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 then - 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, how- 
ever, 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 com- 
pressed 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 connection, when young, with the leaf-buds arid 
