444 A TEXTBOOK OF THEORETICAL BOTANY 



of axillary branches, whose initial cells have been transformed into antheridia 

 at an early stage. The antherozoids closely resemble those of other Mosses. 

 The female branches are very short and clothed with leaves larger than 

 the normal, diminishing towards the apex where there is a group of three 

 archegonia of typical moss-form. The middle one grows from the apical 

 cell of the branch, and the two lateral ones from the two last segments cut 

 off from that apex before its transformation into an archegonium. 



The Sporogonium 



The ventral canal cell in the archegonium usually persists up till fertiliza- 

 tion. Cases have been recorded in which it apparently replaces the true 

 oosphere, and others in which it fuses with the oosphere, but nothing further 

 is known about these anomalies. The normal zygote cell divides first trans- 

 versely, and this is followed by other transverse divisions, so that the young 

 sporophyte forms a filament of six to twelve cells before any longitudinal 

 divisions occur. This is different from the development at this stage in 

 other Mosses and recalls that in Jungermanniales like Pellia. Growth of the 

 embryo is apical. The upper part of the filament becomes the spore capsule, 

 the lower part forms the very short seta and the enlarged foot, which acts 

 as an haustorium. The young sporophyte closely resembles that of Anthocei'os 

 in structure, but there is no basal meristem, and the capsule, instead of 

 elongating, becomes spherical (Fig. 435). As in Anthuceros the archesporial 

 layer develops from the amphithecium, the endothecium forming only the 

 columella. The spore layer is dome-shaped, lying over the top of the rounded 

 columella, and a disc-shaped operculum is differentiated, marked off by 

 a rim of thickened cells. The outer layer of the capsule consists of cubical, 

 strongly cuticularized cells, forming a very tough epidermis, but the inner 

 tissues and the columella are thin-walled and delicate. There are abortive 

 stomata present, which have no pores and whose guard cells contain no 

 chloroplasts, though the latter occur in all the other cells of the capsule. The 

 seta never elongates and the whole capsule remains, until mature, enveloped 

 in a calyptra formed by the archegonial wall, the lower part of which forms 

 a sac enclosing the foot, which is known as the vaginula. When the capsule 

 is ripe the top of the gametophyte axis rapidly elongates, emerging from 

 between the leaves of the archegonial branch and carrying up the whole 

 sporophyte, with the foot still embedded in the vaginula, on a leafiess stalk, 

 the pseudopodium. 



The thin-walled columella now shrivels away leaving an air space beneath 

 the spore layer, and the outer walls shrink till the capsule becomes cylindrical 

 (Fig. 436). This puts considerable pressure on the contained air, which 

 cannot escape because there are no stomatal apertures. The thickened cells 

 of the operculum, however, resist this shrinkage, with the result that a strain 

 is set up at the rim. The air pressure eventually overcomes this, and an 

 explosion occurs which blows off the lid to a distance of several feet and 

 scatters the spores broadcast. 



