340 PART IV. CLASSIFICATION. 



this way the gametophyte may be derived from the sporophyte 

 'by budding, without the intervention of spores. 



The principal orders of Mosses are the Sphagnacese and the 

 "Bryinese. 



Onler I. Sphagnaceae (Bog-Mosses). 



A. The GAMKTOPHYTE. The spore gives rise on germination to a fila- 

 mentous protonema ; when germination takes place in water, the proto- 

 nema remains filamentous and branches, but when it takes place on a 

 solid substratum the protonema 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 Morphology of the Adult Shoot. 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 

 (gametophores), the antheridia and archegonia being borne on distinct 

 branches, and in some species on distinct shoots. 



The 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 from the apex downwards. 



The 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 perichsetial 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 5 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 leaves vary in form according to their position ; thus stem-leaves, 

 branch-leaves, small scaly leaves, and involucral leaves may be dis- 

 tinguished. 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 cortex. The stem-leaves have, at the 

 base, a pair of lateral appendages, the auriculae. 



The leaf consists of a single layer of cells, and has no midrib. It is 

 made up of two kinds of cells : large empty cells of various forms with 

 perforated walls frequently with spiral or some similar form of thicken- 

 ing (Fig. 73, C}: small cells, arranged between the preceding, containing 

 protoplasm and chloroplastids. The relative arrangement of these two 

 kinds of cells affords a means of classification. Nostoc and other Algae 

 are frequently found in the large empty cells. 



