The plants are of two kinds, male and female. The two different organs, then, are developed on different plants. In appearance, however, before the beginning of the structures which bear the sexual organs they are practically the same. The thallus is flattened like nearly all of the thalloid forms, and branches in a forked manner. The color is dark green, and through the middle line of the thallus the texture is different from that of the margins, so that it possesses what we term a midrib, as shown in figs. 257, 261. The growing point of the thallus is situated in the little depression at the free end. If we examine the upper surface with a hand lens we see diamond-shaped areas, and at the center of each of these areas are the openings known as the stomates. [Illustration: Fig. 258. Section of antheridial receptacle from male plant of Marchantia polymorpha, showing cavities where the antheridia are borne.] =484. Antheridial plants.=—One of the male plants is figured at 257. It bears curious structures, each held aloft by a short stalk. These are the antheridial receptacles (or male gametophores). Each one is circular, thick, and shaped somewhat like a biconvex lens. The upper surface is marked by radiating furrows, and the margin is crenate. Then we note, on careful examination of the upper surface, that there are numerous minute openings. If we make a thin section of this structure perpendicular to its surface we shall be able to unravel the mystery of its interior. Here we see, as shown in fig. 258, that each one of these little openings on the surface is an entrance to quite a large cavity. Within each cavity there is an oval or elliptical body, supported from the base of the cavity on a short stalk. This is an antheridium, and one of them is shown still more enlarged in fig. 259. This shows the structure of the antheridium, and that there are within several angular areas, which are divided by numerous straight cross-lines into countless tiny cuboidal cells, the _sperm mother cells_. Each of these, as stated in the former chapter, changes into a swiftly moving body resembling a serpent with two long lashes attached to its tail. [Illustration: Fig. 259. Section of antheridium of marchantia, showing the groups of sperm mother cells.] [Illustration: Fig. 260. Spermatozoids of marchantia, uncoiling and one extended, showing the two cilia.] =485.= The way in which one of these sperm mother cells changes into this spermatozoid is very curious. We first note that a coiled spiral body is appearing within the thin wall of the cell, one end of the coil larger than the other. The other end terminates in a slender hair-like outgrowth with a delicate vesicle attached to its free end. This vesicle becomes more and more extended until it finally breaks and forms two long lashes which are clubbed at their free ends as shown in fig. 260. [Illustration: Fig. 261. Marchantia polymorpha, female plants bearing archegoniophores.]