I04 



MOSSES AND FERNS 



CHAP. 



The branching is monopodial and the branch represents 

 the ventral lobe of a leaf. After the first division by which 

 the two lobes of the leaf are separated, only the dorsal one 

 develops into the lamina of the leaf, which is thus in the seg- 

 ment from which a branch is to form, only one-lobed. In the 

 ventral cell three walls arise (Fig. 51), intersecting so as to 

 cut out a pyramidal cell of the same form as the apical cell of 

 the main axis, and the cell so formed af once begins to divide 



y. 



Fig. 51. — Diagram showing the ordinary method of branching in the acrogynous Jun- 

 germanniaceae (after Leitgeb). D,' Dorsal; V, ventral side of stem; X' X", apical 

 cells of the branches. The segments are numbered. 



in the same way, and forms a lateral axis of precisely the same 

 structure as the main one. 



The genus PJiysiotkun differs from all other known Acrog- 

 ynse in having a two-sided apical cell, instead of the typical 

 tetrahedral one — (Goebel (21), p. 287). 



The Sex-organs 



The plants in Porella are strictly dioecious and the two sexes 

 are at once recognisable. The males are smaller, and bear 

 special lateral branches which project nearly at right angles 

 from the main axis, and whose closely imbricated light green 



