m THE JUNGERMANNIALES loi 



stitute this family, differ from all other Hepaticse in having 

 the leaves radially arranged, and not showing the dorsiventral 

 form that characterises all the others. The plants are com- 

 pletely destitute of rhizoids but possess a rhizome-like basal 

 part, from which the leafy axes arise. The latter have well- 

 developed leaves arranged more or less distinctly in three rows. 

 The stem grows from a tetrahedral apical cell, as in the acrog- 

 ynous forms, but in Haplomitrium at least the apical cell does 

 not develop into an archegonium. The archegonia are in this 

 genus borne at the end of ordinary shoots, but in Caiobryum 

 the end of the female branch becomes much broadened and 

 the numerous archegonia stand crowded together. In this 

 case it is possible that the apical cell of the stem may finally 

 produce an archegonium. Much the same difference is ob- 

 servable in the arrangement of the antheridia. 



The Acrogyn^ 



Treubia and Haplomitrium, as we^have seen, connect al- 

 most insensibly , the anacrogynous with the acrogynous Jun- 

 germanniales. The latter are much more numerous than the 

 former, but much more constant in form, and are doubtless a 

 later specialized group^derived from the former. While dif- 

 fering in the form and' arrangement of the leaves and other 

 minor details, they are remarkably constant in their method of 

 growth and in the position of the sexual organs, especially 

 the archegonia. These are always- formed upon- special 

 branches, where, after a varying number of segments are cut 

 ofif, the apical cell becomes the mother cell of an archegonium. 

 The study of any typical form will illustrate the principal 

 characters of the group. The species selected, Porella (Ma- 

 dotheca) Bolanderi, is very like the common and widely dis- 

 tributed P. platyphylla, which corresponds with it in all struct- 

 ural points. 



The plant grows upon rocks, especially, but also upon the 

 trunks of trees, and forms dense mats closely covering the 

 substratum. It branches extensively, but always monopodi- 

 ally, dichotomous branching never occurring in the acrogynous 

 Jungermanniales. The slender stem is completely hidden 

 above by the two rows of closely-set, overlapping, dorsal 

 leaves. Upon the ventral side, which is fastened by scattering; 



