GROUP III. PTERIDOPHYTA. 375 



(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 ; 

 PsilotaceaD) forming a synangium (see p. 72) : 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 (eusporangiate) , 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 (rnicrospores) and large spores (macrospores) , 

 and are said te be heterosporous. 



The sporangia of the heterosporous forms are distinguished as 

 microsporangia and macrosporanyia according to the kind of spores 

 which they develope : and when the sporophylls bear either only 

 microsporangia or only macrosporaiigia they are distinguished as 

 microsporophylls and tnacrosporophylls. The number of macrospores 

 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 GAMETOPHYTE. The spore, on germination, gives rise to 

 a prothallium which is the gametophyte. It is very small and in- 

 conspicuous, as compared with the sporophyte ; its body is, gener- 

 ally speaking, thalloid ; there is no vascular tissue in its structure, 

 and in many cases it does not become free from the spore. It 

 usually lives through but one short period of growth. 



In any one of the homosporous forms, the prothallia developed 

 from the spores are all essentially alike ; generally speaking, any 



