MUSCI AND HEPATICAE 



363 



hitherto described, in being dependent 

 throughout its existence upon the gameto- 

 phyte ; in the absence of any appendages, 

 and normally o % f any branching whatever ; 

 also ir\ the fact that the spores all originate 

 from one continuous spore-sac contained in 

 the capsule, and not from separate spor- 

 angia. These features are common for 

 the Bryophyta, and mark a simpler grade 

 of evolution of the diploid generation. 

 Whether this is a primitive simplicity, or 

 a consequence of reduction, is a question 

 which can only be discussed on grounds 

 of broad comparison. But the probability 

 is that it is primitive. 



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 ex- 

 pansion will then account for the origin of the 

 capsule on the end of the seta. But this cap- 

 sule is a complicated body in the higher 

 Mosses. Its complexity arises from an ad- 

 vance in internal structure, which thus replaces 

 elaboration of external form. To under- 

 stand it the best approach is through develop- 

 ment. 



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. 307). A 

 spindle-shaped body is thus produced (Fig. 

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

 a central tract (endothecium), and a peripheral 

 tissue (amphithecium) (Fig. 306, C). The 

 former is the exclusive source of spore-forma- 

 tion ; the latter produces the external tissues 

 of the wall, parts of which are photo-synthetic 

 while young. Thus far the sporogonium 

 is enclosed in the growing venter of the 



3 



FIG. 306. 



Ceratodonpurpureus (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. 

 sps= outer spore-sac. 



