The additions to all exogenous plants are indicated in the stem or trunk, by concentric lines or circles. In the center there is a cellular substance called pith. When you take, therefore, a cross section of the trunk of this class, the structure and parts will be arranged thus—bark on the outside, pith in the center, and between these, concentric deposits of woody matter, and all connected into a solid mass by plates of comb-like tissue, radiating from the interior to the circumference, and termed medullary rays. A structure like this, so closely and firmly united, and filled up through all its parts, was surely intended for endurance; and yet out of this class of the vegetable tribes, nature has selected few of her carboniferous models. The plants of the period, as yet detected, are composed chiefly of cellular tissue, mixed up with the substance of the stem, and without pith, medullary rays, concentric woody deposits, or the binding ligament of bark. The hardy oak and tall slender cane may be taken as examples of the two modes of structure—the former allied to existing, the latter to extinct families. Another ground of distinction among plants consists in the leaves or flattened expansions, from which they derive all their grace and symmetry. This is farther connected with the seed and rudimentary organs, and gives rise to the division into _cotyledonous_ and _acotyledonous_ plants. The non-flowering or cryptogamous are all of the latter kind. The flowering or phonogamous not only belong to the former, but are again subdivided into monocotyledonous or dicotyledonous, according as their seed-vessels are possessed of one or of two lobes. Where there are two lobes the expansion of the germ upon bursting from the ground terminates in two imperfect leaves, by which the botanist can at once determine the class to which it belongs. The corns and grasses have single cotyledons, from one extremity of which descend the roots, and from the other the stem springs up, terminated with a single leaf.