HORNED LIVERWORTS'. 241 



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 anthe- 

 ridia 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 fer- 

 tilized 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 (gameto- 

 phyte), but never reach the outside so that the sporogonium derives its 

 nutriment from the gametophyte in a parasitic manner like the true liver- 

 worts. 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 spo- 

 rogonium 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 spo- 

 rogonium 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 colu- 

 mella. 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 



