46 MOSSES AND FERNS CHAP. 



is established on opposite sides (Fig. 13, G, v, v') the grow- 

 ing points, which soon begin to develop in the manner found in 

 the older thallus, and come to lie in a depression, so that the 

 older gemmae are fiddle-shaped. The gemma stands vertically, 

 and there is no distinction of dorsal and ventral surfaces. The 

 cells contain chlorophyll, except here and there the cells with 

 oil bodies, and an occasional large colourless superficial cell. 

 Among them are small club-shaped hairs, which secrete a 

 mucilage that swells up when wet, and finally tears away the 

 gemmae from their single-celled pedicels. 



The further development of the gemmae depends upon their 

 position as to the light. Whichever side happens to fall down- 

 ward becomes the ventral surface of the young plant, and the 

 colourless cells upon this surface grow out into the first rhi- 

 zoids. The two growing points persist, and the young plant 

 has two branches from the first, growing in exactly opposite 

 directions. As soon as it becomes fastened to the ground the 

 dorsiventrality is established, and upon the dorsal surface the 

 special green lacunar tissue and the epidermis with its charac- 

 teristic pores are soon developed, while the ventral tissue loses 

 its chlorophyll, and soon assumes all the characters found in 

 the mature thallus. 



The branching of the thallus is in most cases dichotomous, 

 as in Riccia, but occasionally, as in Targionia (Fig. i, E), the 

 growth is largely due to the formation of lateral adventitious 

 branches produced from the ventral surface. 



In structure and development the sexual organs correspond 

 closely to those of the Ricciaceae, but they are always formed 

 in more or less distinct groups or "inflorescences." As might 

 be expected, this is least marked in the lower forms, especially 

 the Corsinieae (Leitgeb (7), vol. iv.), where the main distinc- 

 tion between them and the lower Ricciaceae is that in Corsinia 

 the formation of sexual organs is confined to a special region, 

 and that the archegonia do not have an individual envelope as 

 in Riccia, but the whole group of archegonia is sunk in a com- 

 mon cavity, which is of exactly the same nature as that in 

 which each archegonium is placed in the latter. In most of 

 the Marchantieae, however, both antheridia and archegonia 

 are borne in special receptacles, which in the case of the latter 

 are for the most part specially modified branches or systems of 

 branches, raised at maturity upon long stalks (Fig. 21). The 



