MCSCINE5;. 193 



at its base by the growing sporogonium, upon which it remains, and 

 it is thus raised into the air, forming a " hood," the caljptra 

 (Figs. 192; 200 B). In the Sphagnacete the hood is not present; 

 in this order, as in the Liverworts, the archegonium remains at the 

 base of the sporogonium. The sporangium opens by circumsessile 

 dehiscence ; the upper portion (operculum) being separated along 

 a specially constructed ring of cells, and falls oft' like a " lid ' : 

 (Fig. 200). Only in a few forms (families 2 and 3) does any vari- 

 ation of this take place. Elaters are never found, but (with the 

 exception of Archidium) there is always present in the sporangium 

 a central mass of cells, the columella, which take no part in the 

 formation of the spores. The columella, in some, does not reach 

 quite to the operculum and in these cases the spore-sac is bell- 

 shaped and covers the columella (Andrecea, Fig. 190 ; Sphagnum, 

 Fig. 199 D) ; but in the majority of Mosses the columella extends 

 to the lid, so that the space containing the spores becomes a 

 hollow cylinder. 



The sporangium is generally raised on a long stalk ; in the great 

 majority this stalk is formed from the lower half of the oospore 

 and belongs to the asexual generation it is then known as the 

 seta. In Andreaea and Sphagmim the seta is very short, and the 

 sporangia are raised upon a long stalk (pseudopodium) developed 

 from the summit of the sexual generation (Figs. 190, 192). In 

 the latter figure an archegonium (a) is seen attached to the 

 pseudopodium, having been carried up with this during the course 

 of its development. The summit of the pseudopodium is enlarged 

 to embrace the foot of the sporogonium (Figs. 192, 199 D) 



A. The sporangium is supported on a pseudopodium ; the columella does 

 not extend to the operculum. 



Family 1. Sphagneae (Bog- Mosses). 



The protonema has been already described. The stem is 

 regularly branched owing to the fact that a branch, or collection of 

 branches, arises at every fourth leaf. These branches are closely 

 covered with leaves, some are erect, while others hang down and 

 surround the stem. No rhizoids are developed. These Mosses are 

 of a whitish-green colour, and when water is present are always 

 saturated with it like a sponge, the reason for this being found in 

 the construction of the stem and leaves. The stems are covered 

 by an external layer of large clear cells, without chlorophyll, but 

 with annular or spiral thickenings on the walls, which are also 



w. B. 



