Fig. 279.
Anthoceros gracilis. A, several gametophytes, on which sporangia have developed; B, an enlarged sporogonium, showing its elongated character and dehiscence by two valves, leaving exposed the slender columella on the surface of which are the spores, C, D, E, F, elaters of various forms, G, spores. (After Schiffner.)
503. Antheridia.—The antheridium arises from an internal cell of the thallus, a cell just below the upper surface. This cell develops usually a group of antheridia which lie in a cavity formed around this cell as the thallus continues to grow. They are situated along the middle line of the thallus, and can be seen by making a section in this direction. The antheridia are oval or rounded, have a wall of one layer of cells which contains the sperm cells, and each antheridium has a slender stalk. The sperms are like those of the true liverworts.
504. Archegonia.—The archegonia are also borne along the middle line of the thallus. Each one arises at an early stage in the development of the tissue of the thallus from a superficial cell, but the archegonium does not project above the surface. The venter therefore which contains the egg is deep down in the thallus, the wall of the neck is formed from cells indistinguishable from the adjoining cells of the thallus and opens at the surface.
Sporophyte of Anthoceros.
505. The Sporogonium.—The sporogonium is developed from the fertilized egg, fertilization resulting of course from the fusion of one of the sperms with the nucleus of the egg. From the lower part of the embryo certain cells elongate and push out like rhizoids into the thallus (gametophyte), but never reach the outside so that the sporogonium derives its nutriment from the gametophyte in a parasitic manner like the true liverworts. It is surrounded at the base by a sheath, an outgrowth of the gametophyte.
506. Growing point of the sporogonium.—A remarkable thing about the sporogonium of anthoceros, and its relatives, is that the growing point instead of being situated at the free end is located near the base, just above the nourishing foot. Thus the upper part of the sporogonium is older. In the old sporogonia there may be ripe spores near the free end, young ones near the middle, and undifferentiated growing tissue near the base. A longitudinal section of a sporogonium just as the spores are ripening will show this.
507. Structure of the sporogonium.—A longitudinal section of the sporogonium shows that the spore-bearing tissue occupies a comparatively small portion of the sporogonium. In the section there is a narrow layer (two cells thick) on either side and joined at the top. In the entire sporogonium this fertile tissue is in the shape of an inverted test tube situated inside of the sporogonium. The wall of the sporogonium is about four cells thick. The sterile tissue inside of the spore-bearing tube is the columella. The cells of the wall contain chlorophyll, and there are true stomata with guard cells in the epidermal layer.
508. Spores and elaters.—In the spore-bearing tissue there are two layers of cells (the archesporium). Each cell is a potential mother cell. The cells, however, of alternate tiers do not form spores. They elongate some what and are somewhat irregular and sometimes divide or branch. They are supposed to represent rudimentary elaters. The cells in the other tiers are actual mother cells, and each one forms four spores.
509. The sporophyte of anthoceros represents the highest type found in the liverworts. The spongy green parenchyma forming the wall, with the stomata in the epidermal layer, fits this tissue for the process of photosynthesis, so that this part of the sporophyte functions as the green leaf of the seed plants. It has been suggested by some that if the rhizoids on the nourishing foot could only extend outside and anchor in the soil, the sporophyte of anthoceros could live an independent existence. But we see that it stops short of that.