PROTHALLUS 301 



at the same time horizontal divisions in the middle region of 

 the plate form the cushion, which is several cells thick. The 

 green thalloid plant thus established is known as a prothallus, 

 and when mature is usually heart-shaped, sometimes attaining 

 a diameter of over a centimetre (Fig. 167). All the cells are 

 alike and contain chloroplasts. From the underside, especially 



FIG. 167. Fern-prothallus (magnified about thirty-five times), from the 

 under-surface. A considerable number of antheridia (an.} are seen, 

 on the older part, and the projecting necks of twelve archegonia (ar.), 

 in the region of the cushion, from which also numerous rhizoids (r.) 

 arise, g.p., growing point. 



in the region of the cushion, scattered superficial cells grow out 

 as colourless rhizoids (r.), which serve for anchorage and the 

 absorption of moisture. 



When the prothalli grow densely crowded, they often remain 

 more or less filamentous, a condition which is normal for most 

 of the Filmy Ferns. 



The prothallus is the Fern-plant (gametophyte) that bears 

 the sexual organs. These are of the same general type as in 



