If the leaf of a Monocotyledonous plant be held up to the light, the principal veins will be found to form parallel lines of nearly equal thickness, the central one being very little, if any, larger than the others (see _a_ in _fig._ 103). The trees belonging to this division are all natives of the tropics, and their softest and newest wood is in the centre, where fresh deposits are made every year inside the old wood; and hence, these trees are called endogens, which signifies, to increase from within. The wood of these trees has neither medullary rays nor concentric rings; and a section of it appears pierced with numerous holes (_b_), as may be seen by cutting off a slice of bamboo. The germination of a Monocotyledonous plant, with the cotyledon remaining in the ground, is shown at _c_. [Illustration: FIG. 103.—MONOCOTYLEDONOUS PLANT.] The Dicotyledones and the Monocotyledones have all visible flowers, and are hence called Phanerogamæ; but the Acotyledones have no visible flowers, and they are hence called Cryptogamæ, which signifies that their flowers are hidden. The most remarkable of the Cryptogamous plants are the ferns, some of which become lofty trees; the wood of which is in curious wavy lines, as it appears to be formed by the footstalks of the decayed leaves growing together and becoming woody. The veins in the leaves or fronds of the ferns are forked. Besides the great divisions already mentioned, the Dicotyledonous plants have been divided into the Dichlamydeæ, or those having both calyx and a corolla; and the Monochlamydeæ, or those having only a calyx; but there are so many exceptions, as to render this division of little value. The Monochlamydeæ are not subdivided, but the Dichlamydeæ are again divided into the Thalamifloræ, in which the petals and the stamens grow separately out of the thalamus or flat part of the receptacle, and generally from below the pistil; the Calycifloræ, in which the stamens and petals are either attached to the calyx, or to a lining of it formed by the dilated receptacle; and the Corollifloræ, in which the petals grow together, so as to form a cup for the pistil, and which have the stamens attached to the corolla, but quite distinct from the calyx. The Monocotyledones have also been re-divided into the Petaloid, or those with regular flowers, like the bulbous plants, and Orchidaceæ; and the Glumaceous plants, or those with scales or glumes instead of petals, as in the sedges and the grasses. The Acotyledones are divided into those with leaves, as the ferns; and those without leaves, as the mosses, lichens, and fungi.