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 

 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, 

 which is, however, disorganised before the 

 ripeness of the spores. The latter are pro- 

 duced by a tetrad division of the spore-mother- 

 cells, which occupy the whole internal space of 

 the sporogonium (Fig. i8A); on germination 

 the spores yield fresh gametophytes. Thus the 

 two generations are here as distinct from one 

 another, structurally and in origin, as in the 



Fern, though both are of small size and simple form. It is to be 

 noted, however, that the sporophyte is throughout its life dependent 



PIG. 17. 



Riccia minima. A, thallus of 

 natural size. B, the same in vertical 

 section, showing two sporogonia sunk 

 in the tissue of the thallus. Magnified. 

 (After Bischoff.) 



FIG. 18. 



Ricciocarpus natans. 

 surrounded by 



is natans. Young sporogonia in longitudinal section, 

 surrounded by_ the archegonial wall. The younger ( X 666) shows 

 the amphithecium (shaded) surrounding the sporogenous cells : in 

 the older (xs6o) these are separated, as the free, and rounded 

 spore-mother-cells. (After Garber.) 



SA. 



Ricciocarfots natans. The upper figure 

 shows the spherical spore-mother-cells 

 surrounded by nutritive material. The 

 lower shows the tetrads formed from 

 them : the sporogpnial wall (shaded) is 

 still seen surrounding them, and covered 

 externally by the archegonial wall of two 

 cell-layers. X666. (After Gather.) 



upon the gametophyte, both mechanically and physiologically, . and that it 

 is an almost entirely undifferentiated, spore-producing body. 



