164 
F. Cavers. 
when young. The receptacle is rounded or oval, with a single 
anterior growing-point which produces the antheridia in acropetal 
succession ; sometimes the apex branches during the development 
of the antheridia, so that the receptacle is forked in front (Fig. 21, c). 
Fig. 21. Rcboulia hemispherical A.—Longitudinal section of male receptacle 
the antheridial cavities are shaded, and the scales and rhizoids on the ventral 
surface are shown in part, x 20. B.—Transverse section of male receptacle, 
x 20. C.—Horizontal section of a receptacle with two growing-points, the 
latter showing the youngest antheridia, whilst the oldest occupy the posterior 
part of the receptacle, x 20. D.—Part of A, x 150, showing on the right the 
upper portion of an antheridium and its cavity, on the left an air-chamber 
opening by a small barrel shaped pore ; the cells around the air-chamber 
contain chloroplasts. 
The growing-point may proceed to form ordinary tissue again, 
leaving the receptacle behind, and several receptacles may he 
produced one after another; or a female receptacle may be formed 
immediately in front of the male receptacle ; or the apical growth 
may cease after the development of a male receptacle so that the 
latter stands at the margin of the thallus. 
Plagiochasma (Fig. 19,1.), which stands at the base of this group, 
strikingly resembles Clevea (and differs from all the remaining 
Operculates) in the development of the carpocephala. The latter 
