Fig. 52.—A, part of a plant of a leafy liverwort (Madotheca), × 2. B, part of the same, seen from below, × 4. C, a branch with two open sporogonia (sp.), × 4. D, a single sporogonium, × 8.
On other plants may be found numerous short side branches ([Fig. 53], B), with very closely set leaves. If these are carefully separated, the antheridia can just be seen as minute whitish globules, barely visible to the naked eye. Plants that, like this one, have the male and female reproductive organs on distinct plants, are said to be “diœcious.”
A microscopical examination of the stem and leaves shows their structure to be very simple. The former is cylindrical, and composed of nearly uniform elongated cells, with straight cross-walls. The leaves consist of a single layer of small, roundish cells, which, like those of the stem, contain numerous rounded chloroplasts, to which is due their dark green color.
The tissues are developed from a single apical cell, but it is difficult to obtain good sections through it.
The antheridia are borne singly at the bases of the leaves on the special branches already described ([Fig. 53], A, an.). By carefully dissecting with needles such a branch in a drop of water, some of the antheridia will usually be detached uninjured, and may be readily studied, the full-grown ones being just large enough to be seen with the naked eye. They are globular bodies, attached by a stalk composed of two rows of cells. The globular portion consists of a wall of chlorophyll-bearing cells, composed of two layers below, but single above ([Fig. 53], C). Within is a mass of excessively small cells, each of which contains a spermatozoid. In the young antheridium (A, an.) the wall is single throughout, and the central cells few in number. To study them in their natural position, thin longitudinal sections of the antheridial branch should be made.
Fig. 53.—A, end of a branch from a male plant of Madotheca. The small side branchlets bear the antheridia, × 2. B, two young antheridia (an.), the upper one seen in optical section, the lower one from without, × 150. C, a ripe antheridium, optical section, × 50. D, sperm cells with young spermatozoids. E, ripe spermatozoids, × 600.
When ripe, if brought into water, the antheridium bursts at the top into a number of irregular lobes that curl back and allow the mass of sperm cells to escape. The spermatozoids, which are derived principally from the nucleus of the sperm cells (53, D) are so small as to make a satisfactory examination possible only with very powerful lenses. The ripe spermatozoid is coiled in a flat spiral (53, E), and has two excessively delicate cilia, visible only under the most favorable circumstances.
The female organ in the bryophytes is called an “archegonium,” and differs considerably from anything we have yet studied, but recalls somewhat the structure of the oögonium of Chara. They are found in groups, contained in little bud-like branches (54, H). In order to study them, a plant should be chosen that has numbers of such buds, and the smallest that can be found should be used. Those containing the young archegonia are very small; but after one has been fertilized, the leaves enclosing it grow much larger, and the bud becomes quite conspicuous, being surrounded by two or three comparatively large leaves. By dissecting the young buds, archegonia in all stages of growth may be found.