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When a teed is sown, and the embryo-plant within it grows, tissue is de¬ 
veloped in three directions, upwards, downwards, and horizontally. The part 
which developes upwards, seeking the light, is the ascending axis or stem 
around which the leaves are arranged; that which descends downwards, avoid¬ 
ing the light, is the descending axis or root; that horizontally is the medullary 
system; the part from which the two axes start is the crown or collar, and was 
formerly erroneously considered by some to be the seat of vegetable vitality. 
Plants are divided into three classes, according to the internal structure of their 
stems: exogens, ondogens, and acrogens. Exogens, or outside growers, to 
which class all the forest trees of the northern United States belong, increase 
in an outward direction, a layer of wood being formed each year on the out¬ 
side of that of the year preceding; hence a cross section of the wood of an 
exogen exhibits a number of zones or circles, each indicating a year’s growth. 
Endogens, or inside growers, as Palms, Indian Corn, and the grasses, increase 
by additions made internally. There is no distinction in the stems of these 
plants of pith, medullary rays, wood, and bark, as in exogens. The structure 
of the seeds as well as of the stems of these two classes of plants is different. 
The seed of an oxogen has two cotyledons or seed-lobes, hence they are called 
dicotyledons. Endogens have only one cotyledon, and are called monoco¬ 
tyledons. In a dicotyledonous plant the pith, composed of cellular tissue, 
occupies a great portion of the young stem. It is full of fluid, which is 
employed in the nourishment of the young shoot. After serving this tempo¬ 
rary purpose, it dries up. Immediately surrounding the pith is the medullary 
sheath, a fibro-vascular layer of vessels, which extend into the leaves; beyond 
are bundles of porous and ligneous vessels, inserted like wedges between the 
medullary rays. In succeeding layers or circles of wood the medullary sheath 
is not repeated. Surrounding the w r ood is the bark, which consists of several 
layers. The liber, or inner bark, or bast layer, as it is sometimes called, from 
the use made of it, consists of elongated vessels chiefly; it increases by very 
thin layers on its inside, which may be separated from each other like the 
leaves of a book, hence the name liber. Next to the liber is the cellular 
envelope, consisting of cellular tissue. It is sometimes called the green bark, 
as the cells generally contain green coloring matter, called chlorophyle. It is 
not so extensively developed as the outer or 6uberous layer of bark, which 
forms the substance known as cork; outside of these layers is the epidermis, 
which serves a temporary purpose, and in time dries up and peels off. 
The heart-wood, or duramen, of an exogenous tree is well known to be 
