418 Lang. — On the Morphology of Cyathodium. 
appear In acropetal succession until the full number, about five, has been 
formed, so that the youngest archegonium is situated furthest from the 
anterior margin of the branch. This will be clear from Fig. io, which 
represents a longitudinal section through the specimen in Fig. 9. The 
apical cell is visible some distance from the margin, and the youngest 
archegonium is close to it, while further forward is an older archegonium. 
It is also evident from this section that the basal tissue underlying the 
archegonia is of greater thickness than is usual even in the midrib of this 
Species. After the full number of archegonia have been laid down the 
involucre commences to form around the group. The sides of the V 
(Fig. 13) first grow up as plates of cells a single layer thick. Later the 
growth involves the apex itself (Fig. 14), and this leads to the involucre 
being composed of two lobes united at the hinder end. The development 
of the involucre is independent of fertilization. It is throughout one layer 
of cells thick, and bears neither rhizoids nor ventral scales on its outer 
surface. Fig. 16 represents two fully developed involucres, the one on the 
left surrounding a group of unfertilized archegonia. while that on the right 
encloses two sporogonia. The involucre is roofed in above by a prolongation 
of the margin of the thallus bearing air-chambers (Figs. 14, 48). 
In C. cavernarum , on the other hand, the apex does not become 
displaced, so that the archegonia stand on what is actually as well as 
morphologically the upper surface of the thallus (Figs. 34-36). The arche- 
gonia are in this case roofed in by a single layer of cells, which may 
be regarded as morphologically equivalent to the epidermal layer of the 
last formed and incomplete air-chamber (cf. Fig. 34). The basal tissue 
bearing the archegonia is two layers of cells thick ; the further growth 
of the apical region produces only one layer of cells. Thus the involucre 
in this species is equivalent to the basal tissue of the thallus itself, and 
is several layers thick behind, but further forward is a single layer. The 
lower surface of the involucre bears ventral scales and sometimes rhizoids 
(Fig- 37 )- 
The structure and development of the archegonium in the two species 
is like that in other Marchantiaceae. A series of developmental stages 
of the archegonium of C. foetidissimum is shown in Fig. 15, a-d , and 
a young archegonium of C. cavernarum in Fig. 34. One point of difference 
between the two species is of interest as occurring within a genus. In 
C. foetidissimum there is no stalk-cell, while in C. cavernarum there is 
a stalk-cell which from the first projects above the surface. The mature 
archegonium is borne on a short stalk, the wall of the venter is a single 
layer of cells thick, and there are six rows of neck-cells. The central series 
consists of ovum, ventral canal-cell, and eight or nine neck canal-cells which 
may not all be separated by cell-walls. 
