Ill MARCHANTIE^ 49 



of Marchantia and others of the higher forms. It is sharply- 

 separated from the compact colourless tissue lying below it, 

 and the cells form short confervoid filaments more or less 

 branched and anastomosing, 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 the other cells. 



All of the Marchantiese except the aberrant genus Duinor- 

 tiera 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 ^ investigated D. 

 irrigua, whose thallus is characterised by a peculiar 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 herbarium 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. tricJiocepJiala, from the Hawaiian Islands, and 

 find that in this species, which has a perfectly smooth thallus 

 without areolations, that no trace of air-chambers can be 

 detected at any time. Vertical sections through the apex 

 show the initial cells to be like those of other Marchantiaceae, 

 and the succession of segments the same, but no indications 

 of lacuncE can be seen either near the apex or farther back, 

 the whole thallus being composed of a perfectly continuous 

 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 

 Marchantieai, 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. tridwcepJiala. 



The structure and development of the sexual organs are 



very uniform among the Marchantieae. 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 



^ Leitgeb (7), vol.'vi. p. 124. 

 E 



