DIV. I PTERIDOPHYTA 503 



Order 6. Lepidodendraceae. Extinct. Heterosporous ; re- 

 peatedly dichotomously branched trees. 



5. Pteridospermeae. Extinct plants with the habit of large ferns. 

 Heterosporous with microsporangia and seed-like macrosporangia. 

 Derived from Eusporangiate Ferns. 



CLASS I 

 Filieinae (Ferns) ( l > 92 ' 112> 115 ) 



The great ^majority of existing Pteridophytes belong to the 

 Ferns, taking the group in a wide sense. Two sub-classes are 

 distinguished according to the structure of the sporangia. The 

 Eusporangiate Ferns are characterised by sporangia, the thick wall 

 of which consists of a number of layers of cells. They open by a 

 longitudinal split. The Leptosporangiate Ferns, on the other hand, 

 have sporangia which, when mature, have their wall formed of one 

 layer of cells, and dehisce transversely or longitudinally. Stipules, 

 which are found at the base of the frond in the former group, are 

 wanting in the Leptosporangiatae. Differences also, exist in the 

 prothallus and in the structure of the sexual organs. Only in some 

 groups of Leptosporangiatae is there a perispore deposited on the 

 outside of the exospore. 



While in earlier geological periods the Eusporangiatae were abundantly 

 represented, they now include only two families, each with a few genera. They 

 appear to represent the more ancient type of Ferns and to stand nearest to the 

 forms from which the Filieinae have been derived. Along with them, even in 

 palaeozoic times we have the Leptosporangiatae, from which in later cretaceous 

 and tertiary times the Hydropterideae have branched off as a small group of 

 aquatic or marsh-growing Ferns. In the Hydropterideae only among Ferns the 

 spores are differentiated into microspores and macrospores. 



Sub-Class I. Eusporangiatae 

 Order l. Marattiaceae ( 116 ) 



This order, perhaps the most primitive of existing Ferns, includes about 20 

 stately tropical ferns with thickened tuberous stems and usually very large leaves 

 provided with two stipules at the base. The sporangia are situated in groups 

 (sori) on the under surface of the leaves, and are either free (Angiopteris) or united 

 to form an oval capsule-like body, the chambers of which are the sporangia. The 

 prothallium in contrast to that of the Ophioglossaceae is a green, heart-shaped 

 thallus, resembling that of a Liverwort and growing on the surface of the soil. 

 It is sometimes dichotomously branched. The sexual organs resemble those of 

 the following order but are developed on the lower surface of the prothallus. An 

 endophytic fungus occurs in the cells of the prothallus. 



