GROUP II. BRYOPHYTA. 317 



cerotaceae and in nearly all Musci the archesporium is a layer of 

 cells : it is generally the external layer of the endothecium, but 

 in most of the Anthocerotaceae and in the Sphagnacese it is the 

 innermost layer of the amphithecium. In those forms where the 

 archesporium is a layer of cells, the internal sterile tissue of the 

 endothecium constitutes what is termed the columella. The arche- 

 sporial cells are either themselves the mother-cells of the spores, 

 or they undergo division to form these cells. Each mother-cell 

 gives rise to four spores ; the nucleus divides into two, and each 

 of these divides again ; the protoplasm aggregates round the four 

 nuclei, constituting four cells which surround themselves with a 

 proper wall and which are the spores. They do not usually all 

 lie in one plane, but are placed tetrahedrally. The mature spore 

 is a cell, consisting of a mass of protoplasm, with a nucleus, and 

 containing chloroplastids, starch-grains and oil-drops ; the wall 

 consists of two layers of the usual structure (see p. 50). During 

 the formation of the spores the mother-cells become isolated from 

 each other, floating freely in a mucilaginous liquid in the interior 

 of the capsule. 



The escape of the spores from the capsule is effected in various 

 ways. In some cases the wall of the capsule simply decays (e.g. 

 Riccieae, Phascum) ; or it splits into valves (e.g. Jungermanniaceae) ; 

 or the upper part is thrown off as a lid or operculum (e.g. some 

 Marchantieae, Sphagnaceae, most Bryineae). 



On being set free, the spores germinate, when the conditions 

 are favourable, giving rise to the protonema. The brittle exo- 

 spore being ruptured, the contents, covered by the endospore, 

 then generally grow out in the form of a filament, which is the 

 beginning of the protonema. In some rare cases (e.g. Pellia) cell- 

 divisions take place within the spore before the exospore is 

 ruptured, so that the protonema is from the first a mass or a layer 

 of cells. 



The Bryophyta (Muscineae) are divided into two classes, the 

 distinctive characters of which are as follows : 



Class III. HEPATICJE (Liverworts). 



Gametophytic Characters. Protonema, generally short-lived, 

 inconspicuous, a flattened expansion. 



Adult shoot, generally dorsiventral ; thalloid in many forms ; 

 has unicellular root-hairs ; no trace of vascular tissue : leaves 

 (when present) destitute of a midrib. 



Sporophytic Characters. The sporogonium remains within the 



