. Botany of the living plant. Botany; Plants. Ml'SCI AND HEPATICAE 471 of the wall, parts of which are photosynthetic while young. Thus far the sporogonium is enclosed in the growing venter of the archegonium (Fig. y>6). As it develops further the lower part remains thin, forming the seta, which may be traversed by a conducting strand (Fig. 364). But the distal part enlarges to form the capsule. A layer of cells is there cut off from the periphery of the endothecium, and acquires dense contents. This is the arche- sporium, which is shaped like a barrel without ends {arch. Fig. 364). Within
. Botany of the living plant. Botany; Plants. Ml'SCI AND HEPATICAE 471 of the wall, parts of which are photosynthetic while young. Thus far the sporogonium is enclosed in the growing venter of the archegonium (Fig. y>6). As it develops further the lower part remains thin, forming the seta, which may be traversed by a conducting strand (Fig. 364). But the distal part enlarges to form the capsule. A layer of cells is there cut off from the periphery of the endothecium, and acquires dense contents. This is the arche- sporium, which is shaped like a barrel without ends {arch. Fig. 364). Within it is the large-celled water-storage tissue of the columella. The amphi- thecium, limited now by a superficial epidermis with stomata, forms a lacunar photo-synthetic tissue, with a large and continuous air-space outside the archesporium. This tissue is specially active in that region called the apophysis, where the stomata are most frequent (st., Fig. 364). In some Mosses it is enlarged as a very effective organ of nutrition. As the development proceeds the cells of the archesporium divide repeatedly, forming a thick cylinder of sporogenous cells surrounding the columella, and limited externally by a double layer of cells of the amphithecium. This con- stitutes the spore-sac. Its cells then separate, and rounding off in a liquid that fills the sac, each undergoes tetrad-division, and finally produces four spores. Reduction takes place as usual, common numbers of chromosomes for Mosses being 12-6. The numbers are low for Bryophytes generally. The mature spore is very minute, and almost spherical, and it contains globules of oil. Meanwhile above the fertile region certain inner cells of the amphithecium undergo changes of induration of the cell-walls to form the peristome, which is closely related to the liberation of the spores {p., Fig. 364). As its structure differs in detail in various Mosses, it provides facts valuable in their classification. The case of Fontinalis serves
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