48 



MOSSES AND FERNS 



carpus, the original air-chambers become divided by the devel- 

 opment of partial diaphragms into secondary chambers, which 

 are not, hovirever, arranged in any regular order, .and communi- 

 cate more or less with one another. 



In Targionia (Figs. i8, 19), where the archegonia are 

 borne upon the ordinary shoots, the growth of the dorsal seg- 

 ments is so much greater than that of the ventral ones that the 

 upper part of the thallus projects far beyond the growing point, 

 j^ which is pushed under 



toward the ventral side. 

 A similar condition is 

 found in the archegonial 

 receptacles of other 

 forms, where this in- 

 cludes the growirig point 

 of the shoot (Fig. 21). 

 In Targionia the lacunae 

 are formed much as in 

 Fimbriaria, but they are 

 shallower and much wid- 

 er, and the pores corre- 

 spondingly few. The as- 

 similative tissue here re- 

 sembles that of Mar- 

 thantia and others of the 

 higher forms. It is 

 sharply separated from 

 the compact colourless 

 tissue lying below it, and 

 the cells form short con- 

 fervoid filaments more 

 or less branched and an- 

 astomosing, and except in the central part of the chamber united 

 with the epidermal cells. Under the pore, however, the ends 

 are free and enlarged with less chlorophyll than is found in 

 other cells. 



All of the Marchantiese except the aberrant genera Dumor- 

 tiera and Monoclea correspond closely to one or the other of the 

 above types in the structure of the thallus, but in the latter the 

 air-chambers are either rudimentary or completely absent, and 

 the ventral scales are also wanting. Leitgeb ( (7), vi., p. 124) 



Fig. j^.— Fimbriaria Californica. A, Vertical sec- 

 tion throueh the apex of a sterile shoot, show- 

 ing the formation of the air-chambers ; x, the 

 apical cell, X300; B, similar section through 

 an older part of the thallus, cutting through a 

 pore. X 100, 



