MCALLISTER: MORPHOLOGY OF THALLOCARPUS 123 
The gelatinous material persists for some time, disappearing 
only as the thick common wall is formed around the spore tetrad 
(Fic. 14). The sporogenous cells all round up to form spore 
mother cells and all of the spore mother cells undergo division 
and all of the resulting cells form spores. None become sterile 
nutritive cells nor elater-like cells. 
After the division of the spore mother cell the resulting cells 
become separated by a thin wall but remain closely attached and 
finally become surrounded by a thick, papillate common wall 
(Fics. 14, 15). These spore walls remain thin and undeveloped 
until the thick retaining wall of the spore ball is completely formed 
(Fic. 14). The outer face of the tetrahedral spore is slightly 
roughened with projections which in cross section appear crenu- 
late (Fic. 16). While thickened to a marked degree the wall of 
this outer face is not more than half as thick as the retaining wall 
of the spores of the tetrad. The inner faces of the spore are 
thickened only slightly. 
It will be seen from the above that the structure and the 
development of the gametophyte and the sporophyte of Thallo- 
carpus is practically identical with that of Riccia. The adhesion 
of the spores of the tetrad in a spore ball and the lack of peg-like 
thickenings in the rhizoids of the former are the only characters 
upon which a separation of the two genera could be based. a 
The morphology of the plant is nevertheless that of a Riccia 
rather than of a Sphaerocarpos. The adhesion of the spores is 
certainly very scanty evidence upon which to base relationship 
to the latter genus, for not all of the Sphaerocarpos species have 
spores remaining attached at the time of their shedding. The 
spores of S. cristatus M. A. Howe separate long before maturity and 
those of S. Donellii Aust. separate as they come to maturity (8). 
The occurrence of peg-like rhizoid thickenings throughout the 
Marchantiales and their Jack in the Jungermanniales _ mee 
character of greater phylogenetic significance. While it is prob- 
able that this character should receive more consideration than 
the spore characters, nevertheless, in view of the essential identity 
of the structure of the gametophyte of Thallocarpus with that of 
Riccia, it would seem extreme to remove the genus Thallocarpus 
from the Marchantiales on the grounds of this character alone. 
