48 



MOSSES AND FERNS 



CHAP. 



Riccia, and the succession of dorsal and ventral segments is 

 the same ; but here the development of the ventral segments 

 is much greater, and there is not the formation of the median 

 ventral lamellae as in Riccia, but the two rows of ventral scales 

 arise independently on either side of the midrib, very near the 

 growing point, and closely overlap and completely protect 

 the apex. The formation of the lacunae in the dorsal part 

 of the thallus begins earlier than in Riccia, and corresponds 



very closely to what ob- 

 A • tains in Ricciocarpus. The 



pits are at first very narrow, 

 but widen rapidly as they 

 recede from the apex. In 

 the epidermal cells sur- 

 rounding the opening of 

 the cavity, there are rapid 

 divisions, so that the open- 

 ing remains small and forms 

 the simple pore found in 

 this species. As in Riccio- 

 carpus, the original air- 

 chambers become divided 

 by the development of 

 partial diaphragms into sec- 

 ondary chambers, which are 

 not, however, arranged in 

 any regular order, and com- 

 municate more or less with 



Fig. 13. — P'iiiibriaria Californica (Hampe). A, Ver- c^wp pnn't'hpr 

 tical section through the apex of a sterile shoot, show- 

 ing the formation of the air-chambers ; jtr, the apical In Ta7'gionia (Figs, 16, 

 cell, X300: B, similar section throueh an older part \ i ,i i 



of the thallus, cutting through a pore, X 100. I 7)> whcre the archcgonia 



are borne upon the ordi- 

 nary shoots, the growth of the dorsal segments is so 

 much greater than that of the ventral ones that the upper 

 part of the thallus projects far beyond the growing point, 

 which is pushed under toward the ventral side. A similar 

 condition is found in the archegonial receptacles of other forms, 

 where this includes the growing point of the shoot (Fig. 19). 

 In Targionia the lacuna; are formed much as in Fimbriaria, 

 but they are shallower and much wider, and the pores corre- 

 spondingly few. The assimilative tissue here resembles that 



