442 BOTANY PART n 



of the Pteridophytes corresponds to the sporogonium, the prothallium 

 on the other hand to' the moss plant with its protonema ; although 

 both groups may have originated phylogenetically from a common 

 ancestor, they have followed altogether different directions in the 

 course of their further development. The correspondence in the 

 structure of their sexual organs, in particular, points to the existence 

 of a relationship between them ; on the other hand, their asexual 

 generations exhibit the greatest disagreement, so that it would not 

 seem admissible to regard the asexual generation of the Pteridophytes 

 as directly derived from the sporogonium of the Mosses. The 

 asexual and sexual generations of the Archegoniatae may, however, 

 be regarded as originally homologous. 



The existing Pteridophyta are classified as follows : 



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

 alternate, often deeply divided or compound leaves. Sporangia either 

 on the under side of the sporophylls, united in sori or free, or enclosed 

 in special segments of the leaves. 



Sub-Class 1. Filicinae eusporangiatae. Ripe sporangia with firm 

 wall composed of several layers of cells. Homosporous. 



Sub-Class 2. Filicinae leptosporangiatae. Ripe sporangia with 

 walls one layer thick. 



Order 1. Filices. Ferns, in the narrower sense. Homosporous. 

 Order 2. Hydropterideae. Water-Ferns. Heterosporous. \ 



2. Equisetinae. Horse-tails. Stem simple or verticillately branched, 

 with whorled, scale-like leaves forming a united sheath at each node. 

 Sporophylls peltate, bearing the sporangia on the under side, and 

 aggregated into a cone at the apex of each fertile shoot. 



Order 1. Equisetaceae. Horse-tails. Homosporous. 



3. Lycopodinae. Club Mosses. Stems elongated, dichotomously 

 branched, either forked or forming a sympodium, with leaves, in 

 many cases greatly reduced, or shortened and tuberous with awl- 

 shaped leaves. Sporangia arising singly in the form of firm-walled 

 capsules either from the stem in the leaf-axils, or from the leaf-base. 

 Tapetal cells persistent. 



Order 1. Lycopodiaceae. Club Mosses. Homosporous. 

 Order 2. Selaginellaceae. Heterosporous. 

 Order 3. Isoetaceae. Heterosporous. 



There are also various fossil groups, some of which are included 

 in the above divisions, while some form independent classes. 



