362 PART III. THE CLASSIFICATION OF PLANTS. 



physis) is generally characterised by the presence of loose assimil- 

 atory tissue, rich in chloroplastids, the intercellular spaces of which 

 communicate with the outer air by means of the stomata. 



The hypobasal cell undergoes relatively few divisions. In the 

 Sphagnaceae, Andreaeaceae, and Archidium, it gives rise to a bul- 

 bous foot. In the Bryinese (e.g. Orthotrichum, Barbula, Atrichum 

 where the hypobasal cell undergoes a single division by a trans- 

 verse wall) the true foot is rudimentary, but it is functionally re- 

 placed (e.g. Phascum, Ephemerum, Polytrichum) by the dilated 

 lower end of the seta which constitutes a false foot. 



After fertilisation, the venter of the archegonium developes into 

 the calyptra which, for a time, keeps pace with the growth of the 

 contained embryo, but is eventually ruptured by the gradual 

 elongation of the seta. In Sphagnacese, in Archidium, and in 

 Phascum and Ephemerum among the lower Bryineae, the whole of 

 the ruptured calyptra remains as a sheath, the vaginula, round 

 the base of the short seta ; in the Andreaeaceae and in the higher 

 Bryineae the. lower portion remains as the vaginula, whilst the 

 upper portion is raised up like a cap (still called calyptra) on the 

 top of the elongating sporogonium. The floor of the receptacle 

 (i.e. the apex of the sexual shoot) is also stimulated to growth, form- 

 ing in most cases a conical projection on which are borne the para- 

 physes and the unfertilised archegonia, whilst in Sphagnaceae and 

 Andreaeaceas it elongates into the long pseudopodium (see p. 328) 

 The perichastial leaves also grow up round the lower part of the 

 seta or of the pseudopodium. 



The sporogonium, possessing, as it usually does, assimilatory 

 tissue and stomata, can assimilate the carbon dioxide of the air, and 

 can transpire actively. The supply of water necessary to meet the 

 loss by transpiration is obtained, together with salts in solution, 

 from the gametophyte, being absorbed by the true (hypobasal) or 

 the false (epibasal) foot, and it travels to the capsule through the 

 rudimentary xylem-tissue of the central strand present in the seta 

 of the higher forms. It is a point of considerable physiological 

 interest that the absorption of water in the first instance by the 

 gametophyte is apparently effected for the most part by the leaves 

 rather than by the rhizoids. 



The remarkable capacity for vegetative propagation manifested 

 by the gametophyte is shared by the sporophyte. It has been 

 ascertained that if portions of the capsule or of the seta, whilst 

 the cells are still living, be kept under favourable conditions, the 



