526 BOTANICAL GAZETTE [DECEMBER 
ANTHERIDIA 
The antheridia originate in close proximity to the apical cell, 
arising in acropetal succession on the dorsal side of the thallus, 
usually singly, but occasionally two or three together. With 
further apical divisions they come to lie in 2 parallel rows on each 
side of the midrib, slightly sunken in the thallus by the develop- 
ment, from behind, of an involucral upgrowth. The mature 
antheridia are spherical and short-stalked and point diagonally 
outward and upward, each one being separated from the one pre- 
ceding it by sterile tissue (fig. 1): 
An antheridium initial appears as a papillate projection above 
the surface of the thallus, resembling closely one of the mucilage 
hairs with which it is associated. A transverse wall appears, 
dividing the initial into 2 nearly equal segments, the basal one 
remaining in the thallus and the outer one projecting above the 
surface of the thallus (fig. 2). The outer cell divides transversely 
into equal segments, forming a primary stalk cell and a primary 
antheridial cell (fig. 3). With further increase in size, the latter. 
divides by a median vertical wall, followed rapidly by a similar 
division in the stalk cell (fig. 4). One or two further transverse 
divisions complete the stalk, while a periclinal wall cuts off a pe- 
ripheral cell on one side of the antheridium, intersecting the first 
vertical wall near the top (fig. 5). A corresponding periclinal wall 
appears on the other side, followed by 2 more walls at right angles 
to the first two, intersecting both these and the first median division. 
As a result, 4 primary wall cells inclose 2 central cells, the entire 
structure being bisected by the original vertical wall. At this stage 
the involucre appears as an upgrowth of the thallus behind the 
young antheridium (fig. 9). It is built up by basal growth, and by 
the time the antheridium is mature, it consists of a scalelike sheath, 
6-10 cells in length. Whether these coverings are to be regarded 
as the beginnings of true foliar structures or merely as dorsal 
upgrowths of the thallus seems to be entirely a matter of opinion. 
If the complete involucre of Pellia be taken as representing the 
initial stage, a failure of the forward portion to develop would result 
in the precise condition found in Pallavicinia. Sphaerocarpus, pet- 
haps, represents an intermediate stage, as here the development of 
