i 9 2o] HAUPT—FOSSOMBRONIA 324 



apical cell of the pyramidal type. This series, of course, is not a 

 truly phylogenetic one, but represents a sequence of hypothetical 

 stages through which the Jungermanniaceae acrogynae have 

 probably passed in the course of their evolution. 



Sex organs 



The plants of Fossombronia cristula are monoecious; the sex 

 organs are dorsal and scattered over the stem in the leaf axes. 

 The antheridia and archegonia are more or less separately grouped, 

 but both kinds may occur in the same leaf axis (figs. 7, 8). There 

 is no time relation in the appearance of the sex organs; antheridia 

 may precede or follow the archegonia, and this sequence may be 

 repeated several times in any order. 



The question of the differentiation of sex in F. cristula is an 

 interesting one. Inasmuch as the thallus is bisexual and there is 

 no definite sequence of antheridia and archegonia, sex must be 

 determined at some other point in the life history than at the 

 reduction division, or at one of the divisions of the apical cell. 

 Up to the formation of the first horizontal wall in the initial, no 

 differentiation of sex has occurred. Moreover, as the first vertical 

 wall determines the kind of sex organ to be produced, sex probably 

 is determined at the division concerned with the formation of the 

 first gametogenous cell. It would be an interesting experiment to 

 attempt to control sex in this plant by external conditions, as the 

 sex organ initials probably contain the possibilities of both sexes. 



Antheridium. — The antheridia develop in small groups, either 

 separately or with archegonia, in acropetal succession from the 

 immediate dorsal segments of the apical cell. Each group comes 

 to lie in the axis of a leaf which acts as an involucral organ, protect- 

 ing the group from behind. There is no special involucre developed, 

 as in many of the strictly thallose Jungermanniales, for, as the 

 writer has pointed out in his study of Pallavicinia (4) , the anther- 

 idial involucre of the thallose forms is strictly homologous with the 

 involucral leaf of the foliose forms. 



In the development of the antheridium of F. cristula, the 

 initial becomes papillate (fig. 9), and by a transverse division a 

 basal cell is cut off from an outer cell. A second transverse wall 



