64 BOTANICAL GAZETTE [JANUARY 
formation of a definite oil cell from an ordinary vegetative cell 
is a common feature of the genus. No gemmae are. produced. 
The thallus grows by means of a single cuneate apical cell 
which cuts off segments from its four sides (fig. 1). The air cham- 
bers form a very extensive system of irregular air passages separated 
by thin partitions, but more or less connected (fig. 2). No chloro- 
phyllose filaments are formed. LrErrcEB (6) has observed that 
the secondary partitions arise from all sides of the primary air 
chambers and grow toward the center, and that the boundaries 
of the primary chambers are soon lost, resulting in the entire air 
chamber region becoming broken up into a uniform spongy tissue. 
The most complete study of the development of the air cham- 
bers among the Marchantiales is that of BARNES and Lanp (1). 
The situation in Reboulia is in agreement with the results of their 
investigations on other members of the group. The air chambers 
of Reboulia arise immediately behind the apical cell of the thallus 
by intercellular splittings which start at the surface of the thallus 
and progress inward (fig. 1), reaching the line of differentiation 
between the dorsal and ventral regions. Secondary splittings 
occur deep within the dorsal region, and do not reach the surface. 
No evidence was found to suggest centrifugal splitting in the region 
behind the apical cell, nor air chamber formation, as LEITGEB 
has described, by the upgrowth of adjacent cells of the thallus. 
The cells forming the air pore margins are attenuate, and are 
developed by segmentation from the adjacent epidermal cells 
(fig. 2). This type of air pore also occurs on the male receptacle, 
but those on the female receptacle are barrel-shaped (fig. 4). The 
air chambers in the female receptacle develop like those of the 
main body of the thallus. Cavers (2) has reported barrel-shaped 
air pores on the male receptacle of Reboulia hemisphaerica, but 
these were not present in the material used for this investigation. 
Sex organs 
The plants of Reboulia are monoecious, and the antheridia and 
archegonia are borne in separate median groups on the dorsal 
surface of the thallus. The antheridial group is only slightly 
raised above the general surface of the thallus, and the archegonial 
