EXOGENOUS PLANTS, or ExoaxNs (Gr. exo, outwards; gennao, to produce), are those in which the woody substance of stem increases by bundles of vascular tissue added externally. The exogenous stem contains a central pith (q.v.), from which medullary rays proceed to the bark (q.v.), and the bark is very distinct from the fibro vascular or woody part which it surrounds. The exogenous is thus very different in structure and manner of growth from the endogenous or the acrogenous stem. Amidst the cellluar substance of the young stem, when it has developed itself from the seed, woody cords are seen connecting the cotyledons, and afterwards the leaves, when these appear, with the root, in the central axis of which they join. A section of the stem exhibits the cellular substance traversed by vascular bundles (woody fiber), which in the section are more or less wedge-shaped, radiating from the center, but yet not pro longed into the center itself, which, even to the greatest age of the stem, remains occu pied by the cellular pith. Additional bundles are interposed, as growth proceeds, diminishing the proportion of cellular substance in the stem, yet without these bundles ever becoming so compacted together as to cut off the communication between the cel lular center of the stem and its bark, which is maintained by means of the medullary rays, often, indeed, imperceptible to the naked eye, but always present even in the hardest and most close-grained wood. The woody layers which are formed in succes sive years, as new leaves and branches are developed, are formed amidst the cambium (q.v.), into which the woody fibers of the new leaves descend, between the bark and the former wood. Thus the concentric circles are formed, usually one for each year's growth, distinguishable even in the most matured timber, and by which the age of trees is very commonly computed. The beginning of each new layer is generally marked by a greater abundance of porous vessels, the openings of which are conspicuous in the transverse section. In pines, the line of separation between the layers is marked by greater density of texture, and often by deeper color. The age of trees cannot, how ever, be calculated with perfect certainty from the concentric circles of the stem, as any circumstance which temporarily arrests the growth in any summer, may produce an effect similar to that ordinarily produced by the change of seasons; whilst in the trees of tropical countries, at least where the wet and dry seasons are not very marked, con centric circles are often not to be discovered.
The structure of the branch of an exogenous tree perfectly corresponds with that of the stem. The vascular bundles of the stem or branch form a loop where a leaf begins, and those of the leaf and its axillary bud spring from the loop. The roots of exogen ous plants have nit a central pith like the stem, but in a few trees, as the horse-chest nut, the pith is prolonged to some extent into the root.
Anomalies are not unfrequently to be met with in the structure of exogenous stems, and particularly among the twining woody plants of tropical countries. There are also very many herbaceous plants, in which, although the structure agrees with that of an exo genous tree in its first year, no further development is ever attained; whilst in many, even this is very imperfectly reached; but yet these are on other accounts unhesitatingly classed with exogenous plants. The exogenous stem and dicotyledonous seed are so constantly found together, that the designation exogenous plants is often applied to that great division of the vegetable kingdom, which is also called dicotyledonous. See BOTANY. Exogenous plants are also characterized by a particular mode of germination, with reference to which they are called exorhizal (Gr. exo, outwards; rhira, a root), the radicle simply lengthening, and not having to break through the coat of the embryo. The leaves of exogenous plants generally exhibit a net-work of veins, instead of the parallel veins characteristic of endogens, and a greater proportional breadth of leaf usually accompanies this reticulated venation.
Exogenous plants are far more numerous than endogens. All the trees and shrubs of Britain, and those of temperate and cold climates generally, are exogenous, as well as very many herbaceous plants of these parts of the world, and many trees, shrubs, and herbaceous plants of the tropics. Almost all trees, except palms and a few liliacem, pandanaeece, and tree-ferns, are exogenous.