40 



MOSSES AND FERNS 



CHAP. 



the thallus Is composed of a series of large polyhedral air- 

 chambers arranged in several layers, and separated by walls 

 but one cell thick. The upper chambers communicate with 

 the outside by pores, quite like those of the Marchantiacese. 

 The ventral tissue and midrib are rudimentary, and the very 

 long pendent ventral lamellae are produced separately in trans- 

 verse rows, which, however, become displaced by the later 

 growth of the thallus, so that their original arrangement can 

 no longer be made out. Oil bodies like those found in the 

 Marchantiaceae occur. The terrestrial form, which growls on 

 the margins of ponds, etc., where the floating form is found, 

 is much more richly branched and more vigorous than the 

 floating form (Fig. lo). The ventral scales become shorter, 



and numerous wide but unthick- 

 ened rhizoids are formed, which 

 are almost completely lacking in 

 the floating form. The structure 

 of the reproductive organs and 

 sporogonium are essentially the 

 same as in Riccia. 



Garber ( i ) , w^ho has recently 

 studied the development of Riccio- 

 carpus, finds that it is not dioecious, 

 as has been frequently asserted, 

 but rather proterandrous — that is, 

 numerous antheridia are formed, 

 but some time before the first arch- 

 egonia develop. Occasionally no archegonia are formed. 



While the settling of the plant upon the mud is not a neces- 

 sary condition for the development of the reproductive organs, 

 as has been asserted by Leitgeb, still none are formed as a rule 

 upon plants growing in permanent ponds, while those growing 

 in temporary ponds regularly develop abundant reproductive 

 organs. In permanent bodies of water, vegetative multipli- 

 cation may be very rapid, and it has been found that after these 

 are frozen over, a certain number of the plants survive, some- 

 times sinking to the bottom, and resuming growth again in 

 the spring. 



The third genus, Tesselina (Oxymitra), represented by the 

 single species, T. pyramidata, is much less widely distributed, 

 belonging mainly to Southern Europe, but also found in Para- 





Fig. 10. — Ricciocarpus nutans. A, 

 Floating form; B, terrestrial 

 form, X2. 



