GROUP II. BRYOPHYTA : MUSCI. 363 



superficial cells will grow out into protonemal filaments. In this 

 way the gametophyte may be derived from the sporophyte by 

 budding, without the intervention of spores ; when this occurs it is 

 a case of apospory (see p. 87). 



The Mosses are classified as follows : 



Order I. Sphagnaceae. Order III. Archidiacea?. 



,, II. Andreasaceae. ,, IV. Bryineee. 



Order I. Sphagnaceae (Bog-Mosses). 



A. The GAMETOPHYTE. The spore gives rise on germination to a filamentous 

 protonema ; when germination takes place in water, the protonema remains 

 filamentous and branches, but when it takes place on a solid substratum the 

 prutonema assumes the form of a branched cellular expansion attached to the 

 substratum by root-like protonemal filaments. In either case adult shoots are 

 developed as branches upon the protonema. 



The Morplwloyy of the Adult Snoot. The shoot is radially symmetrical, and 

 is differentiated into stem and leaves. It consists of a main axis bearing 

 numerous lateral branches. 



Growth is effected, in both the main axis and the lateral branches, by means 

 of a growing -point in which there is a three-sided apical cell. 



The Sexual Organs are borne on specially modified lateral branches (gameto- 

 phores), the antheridia and archegonia being borne on distinct branches, and in 

 some species on distinct shoots. 



A branch which bears antheridia (antheridiophore) is elongated and is covered 

 with small, closely packed, imbricate leaves, by the side of each of which an 

 antheridium is developed. The antheridium, which is raised upon a long stalk, 

 is spherical ; it opens by the splitting of the wall into valves irorn the apex 

 downwards. 



A branch which bears archegonia (archegoniophore) is short ; it bears at its 

 apex a group of (1-5) archegonia, surrounded by rather large involucral leaves 

 with rudimentary perich as tial leaves. 



The Structure of the Adult Shoot. The main axis has no central strand ; it 

 consists of a mass of elongated thin-walled parenchymatous cells, which gradually 

 passes over into an external zone of prosenchymatous cells, the walls of which 

 are thick and brown ; externally to this is a cortex, consisting of 1-5 layers of 

 cells which are usually empty, and have large holes in their walls (except the 

 Sphagna cuspidata) ; in some cases (Sphagna cymbifolia) the cortical cells have 

 spiral thickenings. 



The cortex of the lateral branches generally consists of one or two layers of 

 cells. In all, except the Sphagna cymbifolia, the external walls of the cortical 

 cells are not perforated, though the lateral and transverse walls are perforated ; 

 except in the case of certain large retort-shaped cells, situated near the inser- 

 tion of the leaves, which have an external opening at the upper end. 



The leaves vary in form according to their position ; thus stem-leaves, branch- 

 leaves, small scaly leaves, and involucral leaves may be distinguished. They 

 are sessile, and have a broad insertion ; in most cases the leaf is connected with 

 the prosenchymatous tissue of the stem, the leaf-tissue extending through the 



