354 PART III. THE CLASSIFICATION OF PLANTS. 



where it is continuous with the tissue of the foot) by the archesporium. In 

 Notothylas. the archesporium is commonly co-extensive with the endothecium ; 

 but in some forms the archesporium is developed from only the external layer 

 of the endothecium, the rest of the endothecium constituting a columella (as in 

 the Bryineae among the Musci). 



In all three genera some of the cells derived from the archesporium are 

 sterile. In Dendroceros these cells develope into elaters with spiral thicken- 

 ings, each elater consisting of a row of several cells with an apparently con- 

 tinuous spiral band. In some species of Anthoceros (e.g. vicentianus, giganteus, 

 etc., constituting the subgenus Anthocerites) there are elaters quite similar to 

 those of Dendroceros : in other species (e.g. tuberculatus, glandulosus) the 

 elaters have the same form, but they have no spiral band ; in others (Icevis, 

 punctatus) the sterile cells do not form distinct elaters, but a network of sbort 

 cells, with spiral thickening, in the meshes of which lie the mother- cells of the 

 spores. In Notothylas the sterile cells do not develope into elaters ; they are 

 so arranged as to form chambers, in which lie the mother-cells of the spores. 



The sporogonium of Anthoceros and Dendroceros has no seta ; when the 

 apical growth has ceased, the capsule continues to elongate by basal growth, 

 and hence does not ever become fully mature throughout. In Notothylas the 

 sporogonium has a short seta, and elongates by intercalary growth, the grow- 

 ing-point being situated at the junction of the capsule with the seta ; but the 

 intercalary growth is of limited duration, so that the whole capsule becomes 

 mature throughout, and frequently becomes disconnected from the seta. The 

 pod-shaped capsule of Anthoceros and of Dendroceros splits from the apex 

 into two valves (Fig. 246). Stomata occur in the epidermis of the capsule in 

 most species of Anthoceros, but they appear to be wanting in the other genera. 



Since the archegonia are sunk in the tissue of the shoot, the calyptra, which 

 invests the developing embryo, is developed mainly from the surrounding 

 tissue, and only to a small extent from the wall of the arcliegonium. 



CLASS IV. MUSCI (Mosses). 



A. The GAMETOPHYTE. The protonema is more conspicuous in 

 the Musci than in the Hepaticae : it sometimes persists until the 

 sporogonia are developed and the spores are ripe (e.g. Ephemerum), 

 and in many cases the subterranean portion persists from year to 

 year. It is generally filamentous and much branched ; but in 

 some forms it is a flattened expansion (e.g. Sphagnum, Andreaea), 

 or cylindrical branched and shrubby (Andreaea), or it bears lateral 

 flattened expansions which are assimilatory organs (e.g. Andreaea, 

 Tetraphis, etc.). The filamentous protonema consists of a sub- 

 aerial and of a subterranean portion, which differ in that the cells 

 of the former contain chloroplastids, their walls are colourless, 

 and the septa are transverse ; whereas those of the latter do not 



