Ill THE JUNGRRMANNIALES 89 



sections made parallel to the surface of the male branch. The 

 apex is occupied by an apical cell of the usual form, and the 

 cell divisions in the young segment are extremely regular. 

 The segment first divides into an inner and an outer cell, and 

 the former probably next into a dorsal and a ventral one. The 

 dorsal cell divides by a longitudinal wall into two nearly equal 

 cells, of which the inner one, dividing by a wall perpendicular 

 to the first, gives rise to the primary cell of the antheridium 

 (Fig. 40, A(^). This cell now projects above the surface of 

 the thallus, and divides into a single stalk cell, which under- 

 goes no further divisions, and the antheridium mother cell. 

 The divisions in the latter correspond to those in the other 

 Jungermanniales. First a vertical wall is formed, dividing 

 the young antheridium into two equal parts. Next, in each 

 of these, two walls arise intersecting each other as well as the 

 median wall, and dividing each half of the antheridium into 

 three cells, two peripheral ones and a central one. (A some- 

 what later stage than this is shown in Fig. 40, A.) The per- 

 ipheral cells do not reach to the top of the antheridium, and 

 next a periclinal wall is formed near the top of the central cells, 

 by which a third peripheral cell is formed in each half of the 

 antheridium, which now consists of two central cells and six 

 peripheral ones. The further divisions were not followed in 

 detail, but seem to correspond with those in the higher forms. 



Of the two first cells into which the dorsal cell divides, the 

 one which does not produce the antheridium together with the 

 inner of the two into which that cell first divides, form a par- 

 tition which rapidly increases in height with the growth of 

 the antheridia, and separates each from its neighbour by a 

 single layer of cells, so that the antheridia are sunk in cham- 

 bers, arranged in two rows, corresponding to the two series 

 of segments of the apical cell. 



In the other thallose anacrogynous forms, c. g., Polla- 

 vicinia (Fig. 41, A), the sexual organs are borne upon the 

 dorsal surface of the ordinary shoots, usually surrounded by 

 a sort of involucre. In most of these forms the apical cell is 

 of a different type from that of Anciira, but is variable even 

 in the same species. Thus in Pallavicinia cylindrica, while 

 the commoner form is nearly wedge-shaped, appearing four- 

 sided seen from the surface, and triangular in vertical section, 

 it may approach very nearly the two-sided type (Fig. 42, C). 



