34 ALTERNATING GENERATIONS 
only appendages other than rhizoids and occasional hairs are small flat 
scales borne on the lower surface. The gametophyte is thus of very simple 
form (Fig. 17). The sexual organs are borne 
in acropetal succession on its upper surface, 
and are sunk in depressions. The sporophyte, 
which results from fertilisation of the ovum by 
A spermatozoids motile through water as in Ferns, 
is a small spherical body, with no distinction 
of apex and base (Fig. 18). It consists of a 
single layer of cells forming a peripheral wall, 
a which is, however, disorganised before the 
ANY aS ripeness of the spores. The latter are pro- 
MAMAS BP duced by a tetrad division of the spore-mother- 
Fic. 17. cells, which occupy the whole internal space of 
Riccia minima. A, thallus of the sporogonium (Fig. 18a); on germination 
natural size. £4, the same in vertical 
section, showing two sporogonia sank the spores yield fresh gametophytes. Thus the 
in the tissue of the thallus. Magnified. : ane 
(After Bischoff.) two generations are here as distinct from one 
D 
another, structurally and in origin, as in the 
Fern, though both are of small size and simple forni. It is to be 
noted, however, that the sporophyte is throughout its life dependent 
Ricciocarpus natans. Theupper figure 
shows the spherical spore-mother-cells 
surrounded by nutritive material. The 
Ricciocarpus natans. Young sporogoniain longitudinal section, lower shews the tetrads formed from 
surrounded by the archegonial wall. The younger (666) shows them: the sporogonial wall (shaded) is 
the amphithecium (shaded) surrounding the sporogenous cells: in — still seen surrounding them, and covered 
the older (560) these are separated, as the free, and rounded — externally by the archegonial wall of two 
spore-mother-cells. (After Garber.) cell-layers. 666. (After Gather.) 
upon the gametophyte, both mechanically and physiologically, and that it 
is an almost entirely undifferentiated, spore-producing body. 
