ii M-USCINEJEHEPA TICsE MARCH ANT I ALES 15 



but these are not common. Mucilage cells of various kinds 

 are common. The secreting cells may be hairs on the ventral 

 surface, and especially developed near the apex, where the 

 mucilaginous secretion serves to protect against drying up ; or 

 they may be isolated (Marchantia) or rows of cells (Cono- 

 cephalus) within the tissue of the thallus. 



The growth of the gametophyte is usually due to the 

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

 e.g., Marchantiacere, Anthocerotes, 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 Jungermanniacese,.however, a single apical cell is always 

 distinguishable, but varies a good deal in form in different 

 genera, at least among the thallose forms, or even in the same 

 genus. Among the foliose Jungermanniacege it always has 

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

 ments 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. 



The 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, e.g., Riccia, Sphcerocarpus 

 (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 arche- 

 gonia. 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., Palla-, 

 vicinia (Fig. 38), separated by spaces where no archegonia are 

 found. Here each group of archegonia has a common invol- 

 ucre. In Aneura and most of the higher Marchantiaceae the 

 archegonia are found in the same way, but upon special modi- 

 fied branches. In the foliose Jungermanniacese the origin of 

 the archegonia is somewhat different. Here they are formed 

 upon short branches, where, after a small number of perichastial 

 leaves have been formed, the subsequent segments of the apical 



