Marchantiacecz. 
165 
are purely dorsal outgrowths, formed one after another; the stalk 
has no furrow; the central dorsal tissue of the receptacle is poorly 
developed; hence the bulk of the receptacle is made up of the 
involucres, each with a single archegonium. In addition to this 
detailed agreement in the carpocephalum, Plagiochasma further 
resembles Cleveci in having usually a single ring of cells around each 
pore on the thallus, and in these cells radial thickening characteristic 
Fig. 22. Reboulia hemisplierica. A.—Vertical transverse section of a female 
receptacle and the adjacent portions of the thallus. Two developing sporogonia 
arc shown, each enclosed in its calyptra. The outer portion of the receptacle 
shows air-chambers with barrel-shaped pores, the inner portion consists of 
compact starch-bearing tissue, and the groove of the receptacle stalk shows in 
its upper portion numerous tuberculate rhizoids, in its lower portion a number 
of ventral scales. On the right, one of the band-like scales is shown arising 
from the tissue at the base of the stalk, x 20. 13 and C.—Two horizontal 
sections of a female receptacle, x 20. A is from the upper portion of the 
receptacle and shows the six cavities from which spring tuberculate rhizoids, 
the latter passing into the groove on the ventral surface of the stalk. B is 
taken at a lower level and shows six sporogonia in outline, each in its involucre. 
of the Astroporse. Four archegonia are usually formed, but 
frequently only two or three sporogonia, or even a single sporogonium 
may come to maturity. The upper portion of the capsule-wall 
