II MUSCINE^—HEPATICJE—MARCHANTIALES 49 



investigated D. irrigua, whose thallus is characterised by a 

 pecuhar areolation composed of projecting cell plates, and 

 came to the conclusion that these were the remains of the walls 

 of the air-chambers, whose upper parts, with the epidermis, 

 were thrown off while still very young. He had only herba- 

 rium material to work with, but in this he detected traces of the 

 epidermis and pores in the younger parts. I examined with 

 some care fresh material of D. trichocephala, from the Hawa- 

 iian Islands, and find that in this species, which has a perfectly 

 smooth thallus without areolations, that no trace of air-cham- 

 bers can be detected at any time. Vertical sections through 

 the apex show the initial cells to be like those of other Marchan- 

 tiaceas, and the succession of segments the same, but no indi- 

 cations of lacuna can be seen either near the apex or farther 

 back, the whole thallus being composed of a perfectly contin- 

 uous tissue without any intercellular spaces, and no distinct 

 limit between the chlorophyll-bearing and the colourless tissue. 

 As Dumortiera corresponds in its fructification with the higher 

 Marchantieoe, the peculiarities of the thallus are probably to 

 be regarded as secondary characters, perhaps produced from 

 the environment of the plant, and species like D. irrigua would 

 form transitional stages between the typical Marchantiaceous 

 thallus and the other extreme found in D. trichocephala. 



Sexual Organs 



The structure and development of the sexual organs are 

 very uniform among the Marchantiaceae. In Fimbriaria Cali- 

 fornica, which is dioecious, the antheridial receptacle forms a 

 thickened oval disc just back of the apex. Not infrequently 

 (Fig. I, A), when the formation of antheridia begins not long 

 before the forking of the thallus, both of the new growing 

 points continue to develop antheridia for a time, and the recep- 

 tacle has two branches in front corresponding to these. The 

 receptacle is covered with conspicuous papillae which mark the 

 cavities in which the antheridia are situated. Vertical longi- 

 tudinal sections through the young receptacle show antheridia 

 in all stages of development, as their formation, like those of 

 Riccia, is strictly acropetal. The first stages are exactly like 

 those of Riccia, and the primary cell divides into two cells, a 



pedicel and the antheridium proper. The divisions in the lower 

 4 



