GROUP III. PTERIDOPHYTA. 349 



portion of the shoot, so as to constitute a flower : but in some 

 cases (e.g. Osmunda, Ophioglossacese, Marsileaceae) they differ in 

 form and structure from the foliage-leaves. .In the Equisetinse 

 the sporophylls are highly specialised, and are grouped into cones 

 (flowers) at the ends of the fertile branches : similar cone-like 

 flowers, with less specialised sporophylls, occur in various 

 Lycopodinse. 



The sporangia are unilocular, though in Isoetes they are incom- 

 pletely chambered by trabeculse : they are developed singly or in 

 groups (sori) ; in the latter case they are usually distinct, but in 

 some cases they are coherent (Marattiacese, except Angiopteris ; 

 Psilotacese) forming a synangium (see p. 52) : the synangium 

 should not, however, be regarded as the result of the cohesion of 

 originally distinct sporangia, but as a group of sporangia which 

 have not separated. The sporangium is developed either from a 

 single superficial cell (leptosporangiate) ; or from a group of super- 

 ficial cells (cusporangiate), and sometimes from deeper cells as 

 well : the mother-cells of the spores are derived from an arche- 

 sporium which is either a single hypodermal cell or a group of 

 hypodermal cells. 



The spores produced in the sporangia, are single cells, with 

 generally two coats, endospore and exospore. Many of the Pterido- 

 phyta produce spores which are all quite alike, whence they are 

 said to be homosporous ; whereas others produce spores of two 

 kinds, small spores (micros pores) and large spores (macrospores or 

 mcgaspores), and are said to be heterosporous. 



The sporangia of the heterosporous forms are distinguished as 

 microsporangia and macrosporangia, according to the kind of 

 spores which they develope : and when the sporophylls bear either 

 only microsporangia or only macrosporangia they are distinguished 

 as microsporophylls and macrosporophylls. The number of macro- 

 spores produced in the macrosporangium is generally small, though 

 they are numerous in Isoetes : thus there are four in Selaginella, 

 only one in the Hydropterideae. 



The spores are generally set free by the dehiscence of the 

 sporangia : but in Salvinia the whole sporangium falls off and the 

 spores germinate within it. 



B. THE G-AMETOPHYTE. The spore, on germination, gives rise 

 to a prothallium which is the gametophyte. It is very small 

 and inconspicuous, as compared with the sporophyte ; its body is, 

 generally speaking, thalloid ; there is no vascular tissue in its 



