Endogens (Gr. within, and to produce), a class of plants so called because their stems increase in diameter by the deposition of new woody matter in the centre, in contradistinction to exogens, whose stems increase by the formation of a new layer of wood outside of that previously formed, and immediately beneath the bark. In endogens the stem has no medullary rays, concentric rings, or apparent distinction of pith, wood, and bark, but consists of fibres of woody or vascular tissue, distributed with little apparent regularity through the cellular system of the stem. They may be traced from the base of the leaves downward, some passing into the roots, and others curving outward until they lose themselves in the rind or cortical integument, which differs from the bark of exogens in that it does not increase by layers, and cannot be separated from the wood. As the plant grows, new threads or fibres spring from the freshly formed leaves, and passing first down the centre of the stem crowd the old ones out, and are finally directed toward the rind. In some plants the rind, being soft, is capable of unlimited distention ; in others it soon indurates, and the stem consequently ceases to grow in diameter.
The best example of this class of plants is the palm, whose branchless trunk, rising from 30 to 150 feet from the ground, and terminated by a simple cluster of foliage, has a striking and majestic appearance. The growth of this tree is from the terminal bud, and if the bud is destroyed the tree perishes. In some instances, as in the doum palm of Upper Egypt, and the pandanus or screw pine, two terminal buds appear and branches then shoot forth. The asparagus is an example of endogenous growth. Endogens are monocotyledonous; the veins of their leaves are almost uniformly in parallel lines connected by simple transverse bars; their flowers are trimerous, or have their sepals, petals, stamens, and styles in threes. They luxuriate in hot and humid climates, and comprise the greater number of plants contributing to the food of man, and but a small proportion of poisonous plants. They are generally shorter lived than exogens, though the dragon tree and others, whose growth is not limited by the hardening of the cortical integument of the stem, may attain a great age.
The average age of the palms is perhaps 200 or 300 years.