DIV. i PTERIDOPHYTA 501 



by the periplasm upon the exospore (in Horse-tails, Hydropterideae, 

 and some Ferns). 



The spores of the majority of the Pteridophytes are of one 

 kind, and give rise on germination to a prothallium, which produces 

 both antheridia and archegonia. In certain cases, however, the 

 prothallia are dioecious. This separation of the sexes extends in 

 some groups even to the spores, which, as MACROSPORES (megaspores), 

 developed in MACROSPORANGIA (megasporangia), give rise only to 

 female prothallia ; or as MICROSPORES, which are produced in 

 MICROSPORANGIA, develop similarly only male prothallia. In accord- 

 ance with this difference in the spores, a distinction may be made 

 between the HOMOSPOROUS and HETEROSPOROUS forms of the same 

 group ; but thte distinction has no systematic value in defining the 

 different groups themselves, as it has arisen independently in several 

 of them. 



The correspondence in the structure of their antheridia, archegonia, and 

 spore-mother-cells is in favour of a relationship between the Bryophyta and the 

 Pteridophyta. Though both groups may have had their origin from a common 

 group of Algae (p. 482), an independence in the further course of development 

 must be assumed in the two cases. In particular, it is impossible to derive the 

 sporophyte of the Pteridophyta from the sporophyte or sporogonium of the Moss. 

 While the latter without attaining any vegetative complexity comes to an end early 

 with spore- formation, the Fern sporophyte becomes differentiated into stem, leaf, 

 and root. The vascular bundles appear as quite new structures, the possession of 

 which enables the sporophyte to proceed to the development of a large complicated 

 and sometimes tree-like terrestrial plant ; this contrasts with the Bryophyta, where, 

 owing to the simple cellular structure and the absence of special water-conducting 

 channels, no great size can be reached. The plant only proceeds at a late stage to 

 the production of spores. The spore-mother-cells are formed endogeuously in special 

 parts of the leaf; these are indeed called "sporangia," but are not homologous 

 with the sporangia of Thallophyta. On this account it would seem advisable to 

 use a new term (sporothecae) for the so-called sporangia of Pteridophyta. The 

 spore-mother-cells, which may be most closely compared with the tetrasporangia 

 of Brown and Red Algae, correspond, rather than the sporothecae, to the sporangia of 

 Thallophyta. 



The gametophyte of the Vascular Cryptogams closes its development early by 

 the formation of sexual organs. The typical fern-prothallus hardly surpasses the 

 juvenile form of a thallus, while in the Bryophyta, on the other hand, the sexual 

 generation exhibits a progressive development ( 98 ). 



The Pteridophyta are divided into the following Classes.* 

 1. Filicinae. Ferns. Stem simple or branched, with well- 

 developed, alternate, often deeply-divided or compound leaves called 



[* To these must be added the recently established Class of the Psilophytales. This 

 includes the most simply organised Vascular Cryptogams. In some (Rhynia, Hornea) 

 the plant is rootless and leafless, consisting of a rhizome, branched cylindrical aerial 

 stems, and large terminal sporangia. In Asteroxylon and Psilophyton the stems bear 

 small simple leaves. A full account of these simple Vascular Cryptogams of Early 

 Devonian age will be found in Scott's Studies in Fossil Botany, 3rd ed., vol. i.] 



