16.] CHAPTER II. SPECIAL MORPHOLOGY OF THE MEMBEKS. 71 



necessarily puts a term to the life of the organism. It is in these 

 plants that distinct sporangia are to be found. 



In the Algae and Fungi, the sporangium, when present, con- 

 sists of a single cell (or a ccenocyte). It may be comparatively 

 undifferentiated, as in Bangiaceae and other Florideae, where it is 

 simply a cell of the thallus the contents of which form one or 

 more spores ; or it may be a distinctly differentiated organ as 

 in the Cutleriaceoe, Sphacelarieae, Ectocarpese, Laminariese, and 

 some Florideae, among the Algae ; and in the Phycomycetes and 

 Ascomycetes among the Fungi. 



In all plants higher than the Algae and the Fungi, the sporan 

 gium is multicellular. It is, however, unilocular, that is, it contains 

 but one cavity in which spores are developed, though this is some- 

 times chambered by incomplete walls (trabeculce) as in Isoetes. 



In the Bryophyta, where the sporophyte apparently produces 

 only a single sporangium, termed the capsule or tJieca, this organ 

 constitutes the whole (Riccia) or a considerable portion of the 

 sporophyte. Its structure is simple in Riccia and other Hepaticse, 

 but it becomes highly elaborate in the true Mosses (e.g. Polytri- 

 chum). It must, however, be borne in mind that the theca of the 

 Bryophyta is not completely homologous with a single sporangium 

 of a Fern or a Phanerogam, but with at least a cluster (sorus) of 

 such sporangia : hence the exceptional complexity of its structure. 



In the Pteridophyta and the Phanerogams the sporophyte pro- 

 duces a number of sporangia. In the heterosporous forms there 

 are two kinds of sporangia which respectively produce the two 

 kinds of spores : those which produce macrospores are termed 

 macrosporangia ; those which produce microspores, microsporangia. 

 In the Phanerogams the macrosporangium is commonly termed 

 ovule, and the microsporangium pollen-sac. 



When the shoot of the sporophyte is differentiated into stem 

 and leaf, the sporangia are generally borne on the leaves (sporo- 

 phylls) : but in some plants they are borne on the stem. This 

 is the case in most Selaginellas, among the Pteridophyta : the 

 macrosporangia (ovules) are borne on the stem in various Phane- 

 rogams ; among Gymosperms, in the Taxea3, and Gnetaceae ; among 

 Angiosperms, in the Polygonaceae, Chenopodiacese, Amaranthaceae, 

 Primulaceae, Composite, Gramineae, Naiadaceae, Piperaceae, and 

 others, the macrosporangia being either terminal or lateral : the 

 microsporangia are less commonly borne on the stem, but this is 

 the case in some Angiosperms, such as Naias and Casuarina. 



