I] 



THE GAMETOPHYTE OR PROTHALLUS 



15 



proportion of surface to bulk, and no serious resistance is offered to the 

 evaporation of water from it in dry air. Comparing the prothallus with the 

 Fern-Plant as regards the water-relation, the former is plainly less adapted 

 for life on land, and more immediately dependent on moisture. 



The prothallus thus constituted is capable in some cases of vegetative 

 propagation by "gemmae." But this gametophytic budding is less common 

 here than in the Bryophytes. 



Fig. 18. Successive stages in germination of the spores of Dryopteris 

 Filix-mas, to form the prothallus. (After Kny.) 



The dependence on moisture is still more obvious in the behaviour of 

 the sexual organs which the prothallus bears. These are male and female, 

 and they may be found on the same prothallus or on different prothalli 

 (Fig. 20, i). In the former case the antheridia, or male organs, commonly 

 appear first, and the archegonia, or female orga)is, later. There may thus be 

 a separation of the sexes either in time or in space. The flattened prothallus 

 of the ordinary cordate type usually bears both sex-organs (Fig. 19). When 

 grown under normal circumstances on a horizontal substratum it produces 

 them on its lower surface, the antheridia in the basal or lateral regions, the 



