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In the ferns the vascular tissue branches off and extends into the branches of the frond. As we shall see when we discuss evolutionary trends in vascular tissue, this branching is correlated with the appearance of side branches or leaves along the stem. In the simpler vascular plants these leaf traces simply separate off from the central cylinder or stele of vascular tissues and extend toward the leaf bases. In the ferns and more complex vascular plants, they extend right into the frond branches and into the twigs and the leaves. This branching of a main stem is the functional basis for frond formation. It involves three related processes: overtopping, planation, and webbing. Overtopping refers to the occurrence of lateral extensions, but with further growth of the higher part of the plant. The result is an elongation of the plant because one branch is the topmost. Planation describes the planar distribution of branches; that is, they tend to extend in one plane. The presumed advantage of this is to present a broad surface for the absorption of sunlight. Webbing is the appearance of leaf tissue between the branching stem tissue. This, too, aids in presenting a broad surface for absorbing light. Fern fronds formed from overtopping, planation, and webbing are called megaphylls; ferns are megaphyllous plants.

It is worth looking more closely at the tracheid cells of the xylem. These are the transport cells and, within the vascular plants as a whole, we find four kinds. In ferns themost common kind of tracheid is the scalariform one, but ones with bordered pits are found in certain fern species.Fern root structures are best considered prostrate stems, lying along the ground, from which fine filaments of cells extend into the earth or other substratum, such as rocks or even the bark of trees. Ferns are world wide in distribution, but most typically are restricted to moist areas. One important reason for this restriction is that water--from rain, dew, or other sources--is necessary for fertilization to occur. The male gametes of ferns are flagellated and behave like actively swimming sperm cells. There must be water on the gametophyte surface to allow the sperm to swim from the structures where they are formed to the structures in which the nonmotile eggs are situated.

Most species have sporophytic fronds under 1 m tall, but there are still larger ferns growing in special tropical or semitropical habitats. In the late Paleozoic, there were more tree ferns, but evergreens and especially modern flowering shrubs and trees appear to have competed successfully with the tree ferns and replaced them as our dominant modern large plants.