Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/125

Rh The Hepaticse, commonly known as Liverworts, are small plants of varied forms found on damp ground, moist parts of trees, or floating on the water. The lowest representatives consist simply of a single layer of cells forming a green membrane or patch, as in Anthroceros (Fig. 113), or there is a double layer of cells, as seen in Sphaerocarpus. In Marchantia (Fig. 114) the layers of cells are more numerous and thicker than in the lower forms just mentioned, and the upper and lower surfaces are clothed with a skin or epidermis, on the upper surface of which stomata are seen; stomata are holes in the epidermis through which air can pass from the outside of the plant to the inside. In the middle of the frond or body of the Riccia a distinct line or midrib is seen. This line in the Jungermannia (Fig. 116) becomes a well-defined stem, with leafy appendages on each side. The Jungermanniae are therefore the first plants in which we meet with the structures so characteristic of higher plants, viz., the stem and leaves; the apparent stems of the Fucoidae and Floridae being composed only of a greater number of denser cells, not essentially different from the adjacent parts either in structure or function. While the higher forms of Liverworts, in having stems and leaves, exhibit a marked progress, the lower forms in their cellular structure differ in no way from the Green Algae. The structure of the reproductive apparatus, however, in all Liverworts is much more complex than that of the Algae. In the concave receptacles, supported by stalks, as seen in Marchantia (Fig. 114), are found oval cellular bodies, the so-called Antheridia (Fig. 120, C), which contain spiral filaments (Fig. 120, D) capable of moving after having escaped from the Antheridia. The convex-lobed bodies terminating the stalks of the same plant (Fig. 115) contain flask-shaped bodies, the Archegonia