PH AOPHYCE.X 109 
are of two kinds, plurilocular (containing pre- 
sumptive gametes) and unilocular. 
The Thallus of Ralfsia forms leathery crusts on 
rocks, &c., and though at first almost circular, be- 
comes ultimately of irregular outline. It attains 
considerable thickness in the more central portion, 
and decreases towards the margin. The cells, each 
with one chromatophore, are in vertical series and form 
a parenchymatous tissue; those of the margin effect- 
ing by division the extension of the thallus, while 
the superficial cells similarly add to its thickness. 
It is attached to the substratum either directly or 
by means of root-hairs. On the upper surface there 
occur single hairs, or in other cases tufts, sometimes 
springing from pits which, however, do not appear to 
exhibit a special development like the cryptostomata 
of other orders. By a process of overlapping of new 
crusts on older ones, the thickness of the thallus is 
often considerably increased. In Zithcderma there is 
no notable difference from Ral/sia in the develop- 
ment and structure of the thallus. 
The Reproductive Organs.—Until recently only 
unilocular sporangia were known in the case of 
Ralfsia. They are obovate and arise as lateral 
processes from septate hairs, which are in turn 
direct prolongations of the superficial cells of the 
thallus. Though of lateral origin, the sporangia 
assume a terminal position by pushing aside the 
true terminal shoot. The plurilocular sporangia, 
recently discovered by Mr. Batters, are of similar 
origin, but with them are no paraphyses. The 
hairs or paraphyses grow mainly, if not wholly, 
