11 MUSCINE^—HEPATICyE— MARCH ANTIACEjE 15 



stoma-like slits upon the ventral surface lead to cavities where 

 great quantities of mucilaginous matter are secreted. 



The growth of the gametophore is usually due to the 

 division of a single apical cell. In some of the thallose forms, 

 e.g. Marchantiaceae, Anthoceroteae, a single initial cell is not 

 always to be recognised in the older thallus, but in these forms 

 a single initial always appears to be present in the earlier 

 stages. In the Jungermanniaceae, however, a single apical cell 

 is always distinguishable, but varies a good deal in form in 

 different genera, at least among the thalloid forms, or even in 

 the same genus. Among the foliose Jungermanniaceae it always 

 has the form of a three-sided pyramid. From the apical cell 

 segments are cut off in regular succession, and the first divisions 

 of the segments also show much regularity, and often bear a 

 definite relation to the tissues of the older parts. 



TJie Sexual Organs 



The archegonium is always traceable to a single cell, but 

 the position of the mother cell is very different in different 

 genera. In the simplest cases, eg. Riccia, SphcBrocarpus (Figs. 

 2, 29), the mother cell is formed from a superficial cell of one 

 of the youngest dorsal segments of the apical cell, close to the 

 growing point of an ordinary branch of the thallus, whose 

 growth is in no way affected by the formation of archegonia. 

 In such forms the archegonia stand alone, and about each is 

 developed a sort of involucre by the growth of a ring of cells 

 immediately surrounding the archegonium rudiment. In other 

 cases the archegonia are found in groups, e.g. Pallavicinia (Fig. 

 38), separated by spaces where no archegonia are found. 

 Here each group of archegonia has a common involucre. In 

 Aneura and most of the higher Marchantiaceae the archegonia 

 are found in the same way, but upon special modified branches. 

 In the foliose Jungermanniaceae the origin of the archegonia 

 is somewhat different. Here they are formed upon short 

 branches, where, after a small number of perichaetial leaves 

 have been formed, the subsequent segments of the apical cell 

 develop archegonia at once, and finally the apical cell itself 

 becomes the mother cell of the last-formed archegonium, and, 

 of course, with this the growth in length of the branch ceases. 

 With the exception of the Anthoceroteae, where the arche- 



