52 PART I. MORPHOLOGY. [ 16 



the Fungi, the sporangium is multicellular. It is, however, unilo- 

 CM/ar, that is, it contains but one cavity in which spores are 

 developed, though this is sometimes chambered by incomplete walls 

 (trabeculce) as in Isoetes. 



In the Bryophyta, where the sporophyte apparently produces 

 only a single sporangium, termed the capsule or theca, 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 does not correspond to a single sporangium of a Fern or 

 a Phanerogam, but to at least a cluster (sorus} of such sporangia : 

 hence the exceptional complexity of its structure. 



In the Pteridophyta and the Phanerogams the sporophyte pro- 

 d uces 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 macrospor- 

 angia ; 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 (sporophylls) : 

 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 Phanerogams ; among 

 Gymnosperms, in the Taxese ; among Angiosperms, in the Poly- 

 gonace?e, Chenopodiacese, Amaranthacese, Primulacese, Composite, 

 Graminese, Naiadaceae, Piperacese, and others, the macrosporangia 

 being either terminal or lateral : the microsporangia are less com- 

 monly borne on the stem, but this is the case- in some Angiosperms, 

 such as Naias and Casuarina. 



The sporangia may be borne singly, or, as is more commonly the 

 case, in groups, each such group being termed a sorus. The spor- 

 angia of a sorus are generally quite distinct from each other ; but 

 in some cases (e.g. most Marattiaceae, Psilotum, Tmesipteris) the 

 sporangia are coherent, forming what appears to be a multilocular 

 sporangium but is really a synangium. 



In those heterosporous plants in which the sporangia are in sori, 

 the two kinds of sporangia are borne in distinct sori, the only ex- 

 ception to this rule is afforded by the Marsileacese, where each 

 sorus includes both microsporangia and macrosporangia. 



