276 



MOSSES AND FERNS 



CHAP. 



which comes to He in a depression between the two lobes formed 

 by the outer parts of the segments, and the prothalhum assumes 

 the heart-shape found in most homosporous Ferns. The sec- 

 ondary initial cells vary in number with the width of the inden- 

 tation in which they lie. Seen from the surface they are oblong 

 in shape, but in vertical section are nearly semicircular (Fig. 

 150, B). Basal segments are cut off by a wall that extends 

 the whole depth of the prothallium, and the segment is then 

 divided by a horizontal wall into a dorsal and ventral cell of 

 nearly equal size. The divisions are more numerous in the 



ventral than in the dorsal 

 cells of the segment, this 

 difference first being mani- 

 fest some distance back of 

 the apex. Owing to this, a 

 strongly projecting, nearly 

 hemispherical cushion - like 

 mass of tissue is formed 

 upon the ventral surface. 

 The superficial cells of both 

 sides of the prothallium have 

 a well-marked cuticle. Nu- 

 merous brown rhizoids, 

 which, like those of the sim- 

 pler Liverworts, are uni- 

 cellular and thin - walled, 

 grow out from the cells of 

 the lower surface, especially 

 from the broad midrib. The 

 full-grown prothallium in 

 M. Douglasii is sometimes a 

 centimetre or more in length 

 (Fig. 151), and tapers from the broad heart-shaped forward 

 end to a narrow base. In Angiopteris (Farmer (3) ) it is more 

 nearly orbicular. In both genera it is dark-green in colour, 

 looking very much like the thallus of Anthoceros Icevis, and like 

 this too is thick and fieshv in texture. A broad midrib extends 

 for nearly the whole length of the thallus and merges gradually 

 into the wings, which are also several-layered, nearly or quite 

 to the margin. 



The prothallium of Dancua (Brebner (i)) resembles more 



Fig. 150. — Marattia Douglasii. A, Horizon- 

 tal section of prothallium apex, with two 

 initials, Xi6o. B, Longitudinal section 

 of a similar growing point; d, dorsal; v, 

 ventral segment. 



