Exogens (Gr. outward, andto generate), a class of plants so called because their woody matter is increased by additions to the outside of that which first surrounds the central pith. As there are no specific limits to the age of exogenous trees, their diameter indefinitely increases by this annual process, a distinct external layer being added by each year's growth. The stem of an exogen consists of a central column of pith or medulla, woody zones, and bark. Processes from the central medulla called medullary rays cross the zones transversely. The bark of an exogen parts readily from the underlying wood at a particular season of the year, when a viscid secretion called cambium is produced between the wood and the inner surface of the bark. It is at this period that the leaves expand and the trunk lengthens. The woody fibres in the leaves are prolonged into the stem or trunk, passing down among the cambium, and adhering partly to the wood and partly to the bark of the previous year. By this means new living matter is continually deposited upon the outer portion of the woody stem and the inner portions of the bark.

It is in this part of the stem that the intensest vitality exists, the outer and older layers of the bark and the inner and older concentric rings of the wood becoming inert and falling off or decaying without injury to the vegetative parts. The office of the medullary processes is very important as means of communication between the centre of the stem and the outside layers or rings; and they are conduits, so to speak, by which the fluid matter passing down the bark can reach the wood next the medulla or pith. These processes, which resemble thin plates, are of a spongy nature similar to that of the pith from which they originated. They sometimes assume sinuosities and undergo partial obliteration; and sometimes the wood itself assumes an excessive irregularity. As these circumstances are to be found mostly in tropical exogenous trees, vines, and climbers, difficulty is sometimes experienced in perceiving from transverse sections their claims to be considered as exogens. This natural character of an outward growth in the exogens is associated with other peculiarities of development of other organs. Thus, the leaves have veins ramifying from the midrib outwardly to the circumference; or if there are several ribs, the veins are still of the same quality, so as to form an irregular network.

These veins never run parallel to each other without ramifications, and even some which appear to do so will be found to possess secondary veins. The leaves also fall away from the branches, being disarticulated from their places of insertion, leaving a clear scar behind. Certain foliolate organs, called stipules, are also frequently attached to the leaves, which is very unusual in endogens. The flowers are mostly quinary, that is, they have five sepals, five petals, and five stamens, or some multiple of that number. The tall and feathery outline of the palms is never seen in the exogens, as none of them depend on a single terminal bud for their developing growth. From the very germination of the seed the difference is apparent in the form of the embryo and in the dicotyledonous characteristics of the young plant.