96 



MOSSES AND FERNS 



CHAP. 



?. 



The HepaticcE Acrogynce 



Treubia and Haploniitrium, as we have seen, connect almost 

 insensibly the anacrogynous with the acrogynous Hepaticse. 

 The latter are much more numerous than the former, but much 

 more constant in form, and are doubtless a later speciaHsed 

 group derived from the former. While differing 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 off, the apical cell 

 becomes the mother cell 

 of an archegonium. The 

 study of any typical form 

 will illustrate the prin- 

 cipal characters of the 

 group. The species 

 selected, Porella {Mado- 

 theca) Bolanderi, is very 

 like the common and 

 ,^ ^ widely distributed P. 

 . -■ platyphylla, which corre- 



sponds with it in all 

 structural points. 



The plant grows 



Fig. 41. — Porella Bola7ideri {KvLSt.). A, Female plant, X 4 ; UDOn TOcks eSOeciallv 

 ? , archegonial branches ; B, an open sporogonium, 

 X4 ; C, a male plant, X4 ; (J, the antheridial branches. but alsO UpOn the trUnkS 



of trees, and forms dense 

 mats closely covering the substratum. It branches extensively, 

 but always monopodially, dichotomous branching never occurring 

 in the acrogynous Jungermanniaceae. The slender stem is com- 

 pletely hidden above by the two rows of closely-set, overlapping, 

 scale-like leaves. Upon the ventral side, which is fastened by 

 scattering rhizoids to the substratum, there is a row of much 

 smaller leaves (Amphigastria), more or less irregularly disposed. 

 The dorsal leaves are nearly oval in outline, but the two- 

 lobed form, that is very conspicuous in many species, is not so 

 noticeable here. The amphigastria are much smaller, and more 

 elongated than the dorsal leaves. The structure of the leaf is 



