46 MOSSES AND FERNS chap. 



divides again by a transverse wall into a lower cell, which 

 develops no further, and a terminal cell from which the gemma 

 is formed. This terminal cell first divides into two equal cells 

 by a cross-wall (Fig. 12, B), and in each of these cells a similar 

 wall arises, so that the young gemma consists of four nearly 

 equal superimposed cells (Fig. 12, D). The wall III in Fig. 

 12, D, arises a little later than wall II, and is always more or 

 less decidedly concave upward. Each of the four primary cells 

 of the gemma is divided into two by a central vertical wall, 

 and this is followed by periclinal walls in each of the resulting 

 cells. At first the gemma is but one cell in thickness, but 

 later walls are formed in the central cells parallel to the 

 surface, so that it becomes lenticular. As it grows older there 

 is established on opposite sides (Fig. i 2, G, v, v') two growing 

 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 rhizoids. 

 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 characteristic 

 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 



