. The structure and development of mosses and ferns (Archegoniatae). Plant morphology; Mosses; Ferns. 276 MOSSES AND FERNS CHAPi which comes to lie in a depression between the two lobes formed by the outer parts of the segments, and the prothallium 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 prothalliu
. The structure and development of mosses and ferns (Archegoniatae). Plant morphology; Mosses; Ferns. 276 MOSSES AND FERNS CHAPi which comes to lie in a depression between the two lobes formed by the outer parts of the segments, and the prothallium 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 yentral 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 prothalliurn. .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 fleshy 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 Dancsa (Brebner (i)) resembles more. Fig. 150.—Marattia Douglasii. A, Horizon- tal section of prothallium apex, with two initials, X160. B, Longitudinal section of a
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