250 DUuPLER: AIR CHAMBERS OF REBOULIA 
cells around the pore (Fic. 17). Divisions tangential to the pore 
begin the formation of the roof of the chamber, forming a 
circle of small wedge-shaped cells (Fic. 18). As development 
proceeds the chambers become wider (Fics. 19-21), the growth 
of the roof keeping pace. At first the roof consists of but a 
single layer of cells, but very early in development periclin- 
al divisions occur, usually 
beginning at the margins of 
the chamber (Fic. 19) and 
proceeding until the roof be- 
comes, asa rule, two cells in 
thickness (Fic. 21). Very 
short outgrowths may project 
from the roof into the cham- 
ber. 
In sharp contrast to the 
simple pores of the thallus 
acle. Usually the young pore 
is open from the start, differ- 
ing in this respect from the 
young pores of Plagiochasma 
asshown by Miss Starr (16). 
The ‘‘barrel”’ is formed by 
periclinal divisions of the cir- 
cle of cells forming the margin 
of the young pore. The first 
Fic. 21. Vertical section of a young division results inan inner and 
chamber, showing the nearly complete an outer tier of cells (FIG. 20)- 
barrel-pore and a secondary plate project- The inner tier by a series of di- 
ing into the chamber, x iG, 22. — : 
Mature pore on female receptacle, x 433. visions forms the portion pro- 
jecting into the chamber; the 
outer tier by a series of similar but later divisions gives rise 
to the outer portion of the pore (Fic. 21). The inner projection 
of the pore consists of five or six tiers of cells, the outer of three 
or four, the circle next the pore becoming cutinized at maturity 
(FIG. 22). 
