. The structure and development of mosses and ferns (Archegoniatae). s dif-ferentiated earlier than in each of the four primary cells ofthe endothecium, as seen in trans-verse section, a periclinal wallarises which at once separates thearchesporium from the columella(Fig. 115, D). The outer spore-sac has but two lavers of cells, andthe capsule wall three, and betweenthem the large lacuna is formed asin Funaria; but in Phascum as inEphemerum, the separation of thecells is complete. In the seta aslightly-developed central cylinder of conducting tissue is de-veloped, derived, as in Fun
. The structure and development of mosses and ferns (Archegoniatae). s dif-ferentiated earlier than in each of the four primary cells ofthe endothecium, as seen in trans-verse section, a periclinal wallarises which at once separates thearchesporium from the columella(Fig. 115, D). The outer spore-sac has but two lavers of cells, andthe capsule wall three, and betweenthem the large lacuna is formed asin Funaria; but in Phascum as inEphemerum, the separation of thecells is complete. In the seta aslightly-developed central cylinder of conducting tissue is de-veloped, derived, as in Funaria, from the endothecium, but inPhascum it is much less conspicuous. Pleuridium (, A) in its later stages corresponds exactly to Phascum, ex-cept that the capsule is more slender. In both of these generathe seta remains short, but is perfectly evident. Whether theabsence of a distinct operculum in the cleistocarpous Mosses isa primitive condition, or whether they are reduced forms, it isimpossible to determine positively from a study of their Fig. 116.—Pleuridium suhulatum,X20. VI. THE BRYALES 217 Stegocarpco Very much the larger number of Mosses belong to thisgroup, which is primarily distinguished from the foregoing bythe presence of an operculum. Of course among the 7000 ormore species belonging here there are many differences in struc-ture; but these are mainly of minor importance morphologically,and only the more important differences can be considered here. As we have already seen, there is great uniformity in thegrowth of the stem, which, with the single exception of Fis-sidens, has always a three-sided pyramidal apical cell. InFissidens this is replaced by a two-sided one, but even here ithas been found (Goebel (8), p. 371) that the underground
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