4/0 



BOTANY OF THE LIVING PLANT 



may be found, providing for ventilation 

 (Fig. 364, St.). But such tissues dry up at 

 maturity, so that the capsule is then full 

 of the yellowish dusty spores. 



The sporophyte thus constructed is 

 dependent throughout its existence upon 

 the gametophyte : it bears no appendages, 

 and normally it never branches. The 

 spores all originate from one continuous 

 spore-sac contained in the capsule. These 

 features are common for the Bryophyta, 

 and mark a simpler grade of evolution of 

 the diploid generation than is seen in the 

 Pteridophyta and Seed-Plants. Whether 

 this is a primitive simplicity, or a con- 

 sequence of reduction, is a question which 

 can only be discussed on grounds of broad 

 comparison. 



The Moss Sporogonium is without any of 

 those lateral appendages which are so con- 

 spicuous a feature in Vascular Plants. Its 

 external form contrasts with theirs in being 

 simple. It is essentially a spindle, with polarity 

 defined as apex and base. Transverse expan- 

 sion accounts for the origin of the oval capsule 

 on the end of the seta. But this capsule is a 

 complicated body in the higher Mosses. Its 

 complexity arises from an advance in inter- 

 nal structure, which thus contrasts with its 

 simplicity of external form. To understand it 

 the best approach is through development. 



The zygote first divides by a basal wall, 

 which is transverse or slightly oblique to the 

 axis of the archegonium. This at once defines 

 the polarity. It is succeeded by a brief apical 

 growth in the epibasal half, with a two-sided 

 apical cell. The hypobasal half also segments, 

 but less regularly, boring its way downwards 

 into the tissue of the parent (Fig. 366). A 

 spindle-shaped body is thus produced (Fig. 

 365, A, B). Subdivision of the segments gives 

 a central tract (endothecium), and a peripheral 

 tissue (amphithecium) (Fig. 365, C). The 

 former is the exclusive source of spore-forma- 

 tion ; the latter produces the external tissues 





B 



9 •■ 9 



Fig. 365. 



Ceratodon purpureus (after Kienitz- 

 Gerloff) . A , B, young embryo seen 

 from points of view at right angles to 

 one another. C = an older embryo. 

 gg = outer limit of the endothecium. 

 sp = outer spore-sac. 



