156 MOSSES AND FERNS chap. 



sules are developed. This no doubt accounts for the extreme 

 rarity of the sporogonium in many Mosses, although in other 

 cases, e.g. Sphagnum, it would appear that the formation of the 

 sexual organs is a rare occurrence. These resemble in general 

 those of the Hepaticae, but differ in some of their details. 

 The leaves surrounding them are often somewhat modified, 

 and in the case of the male plants {Atridmiu, Polytrichmn) 

 different in form and colour from the other leaves, so that the 

 whole structure looks strikingly like a flower. As a rule, the 

 archegonial receptacles are not so conspicuous. The early 

 divisions of the archegonium correspond closely with those of 

 the Liverworts, but after the " cover cell " is formed, instead of 

 dividing by cross walls into four cells, it functions for some 

 time as an apical cell, and to its activity is due the entire 

 development of the neck. The venter is usually very much 

 more massive than in the Hepaticae, and the egg small. 



The antheridia, except in Sphagnum, are borne also at the 

 apex of the stem, whose apical cell does not always, at any rate, 

 become transformed into an antheridium, as we sometimes find, 

 especially in species of Atrichum and Polytrichum, that the axis 

 grows through the antheridial group and develops a leafy axis, 

 which later may form other antheridia at its apex. Where 

 the plants are dioecious the males are usually noticeably 

 smaller than the females. The antheridia, except in Sphagnum, 

 are very uniform in structure, and like the archegonium exhibit 

 a very definite apical growth (Fig. 90). The wall remains one- 

 layered, as in the Liverworts, and often the chloroplasts in its 

 cells become red at maturity, as in some Liverworts, e.g. 

 Anthoceros. The ripe antheridium is in most Mosses club- 

 shaped, and the sperm cells are discharged while still in connec- 

 tion, the complete isolation of the sperm cells only taking place 

 some time after the mass has lain in water. In Sphagnum the 

 antheridia are much like those of certain leafy Liverworts, and 

 stand singly in the axils of the leaves of the male branches. 



The sporogonium of the Mosses reaches a high degree of 

 development in the typical forms, and shows great uniformity, 

 both in its development and in the essential structure of the 

 full-grown sporophyte. With the exception of Sphagnum, which 

 will be referred to more specially later, the early growth of the 

 sporogonium is due to the segmentation of a two-sided apical 

 cell. The separation of the archesporium takes place, at a late 



