eaks through the overlying cells a
differentiation of the tissues begins. In the axis of each of the
four divisions the cells divide lengthwise so as to form a
cylindrical mass of narrow cells, not unlike those in the stem of a
moss. Here, however, some of the cells undergo a further change; the
walls thicken in places, and the cells lose their contents, forming
a peculiar conducting tissue (tracheary tissue), found only in the
two highest sub-kingdoms. The whole central cylinder is called a
"fibro-vascular bundle," and in its perfect form, at least, is found
in no plants below the ferns, which are also the first to develop
true roots.

The young root and leaf now rapidly elongate, and burst through the
overlying cells, the former growing downward and becoming fastened in
the ground, the latter growing upward through the notch in the front
of the prothallium, and increasing rapidly in size (Fig. 67, _B_). The
leaf is more or less deeply cleft, and traversed by veins which are
continuations of the fibro-vascular bundle of the stalk, and
themselves fork once or twice. The surface of the leaf is covered with
a well-developed epidermis, and the cells occupying the space between
the veins contain numerous chloroplasts, so that the little plant is
now quite independent of the prothallium, which has hitherto supported
it. As soon as the fern is firmly established, the prothallium withers
away.

Comparing this now with the development of the sporogonium in the
bryophytes, it is evident that the young fern is the equivalent of the
sporogonium or spore fruit of the former, being, like it, the direct
product of the fertilized egg cell; and the prothallium represents the
moss or liverwort, upon which are borne the sexual organs. In the
fern, however, the sporogonium becomes entirely independent of the
sexual plant, and does not produce spores until it has reached a large
size, living many years. The sexual stage, on the other hand, is very
much reduced, as we have seen, being so small as to be ordinarily
completely overlooked; but its resemblance to the lower liverworts,
like _Riccia_, or the horned liverworts, is obvious. The terms
ooephyte (egg-bearing plant) and sporophyte (spore-bearing plant, or
sporogonium) are sometimes used to distinguish between the sexual
plant and the spore-bearing one produced from it.

The common maiden-hair fern (_Adiantum pedatum_) has been selected
here for studying the struc