THE PTERIDOPHYTA : FILICALES, THE FERNS 479 



The reproductive organs are generally borne either on the leaves or in 

 the axils between the leaves and the stem. They consist of little capsules 

 called sporangia, in which spores are developed. These spores may be all 

 of the same size, when the plant is said to be homosporous, or thev may 

 be of two different sizes and the plant is then spoken of as heterosporous. 

 In the heterosporous types the smaller spores are termed microspores and 

 are developed in microsporangia, while the larger spores, which are 

 generally produced in smaller numbers, are termed megaspores, and are 

 formed in megasporangia. 



From the fact that the members of the Pteridophyta produce spores we 

 realise that they represent sporophytes, being comparable with the sporo- 

 gonium of the Bryophyta. The spores on germination give rise to the 

 gametophyte plants, which are usually small, insignificant structures, some- 

 times living an independent life and assimilating food material for them- 

 selves, but often living enclosed within the spores or else saprophytically 

 underground. We see, therefore, that one of the most characteristic features 

 of the Pteridophyta is that the sporophyte has become the dominant part of 

 the life-cycle while the gametophyte has been much reduced. For this 

 reason we must be careful not to compare, for example, the Fern plant with 

 the Moss plant, since they belong to opposite parts of the cycle of alternation 

 of sporophyte and gametophyte. 



CLASSIFICATION 



The classification of the Pteridophyta is somewhat complicated 

 and elaborate, for in considering the various groups which comprise this 

 assemblage of plants we must take into consideration the many and varied 

 fossil forms which are no longer represented in the present-day flora. 

 These we shall consider later, in Volume III. 



It is, however, impossible to give a clear classification of the group without 

 reference to these fossil types, and the following scheme is an outline of an 

 inclusive arrangement : — 



A. PSILOPSIDA. 



1. Fsiloiales, e.g., Psilotum. 



2. Psilophytales, e.g., Rhynia, Asteroxylon (fossils). 



B. LYCOPSIDA. 



1. Lycopodiales, e.g., Lycopodium, Selaginella, Lepidodendion 



(fossil). 



2. Isoetales, e.g., Isoetes. 



C. SPHENOPSIDA. 



1. Equisetales, e.g., Eqiiisetum, Calamites (fossil). 



2. Sphenophyllales, e.g., Sphenophyllum (fossil). 



D. PTEROPSIDA. 



Fib'cales, e.g., Dryopteris, Pteridium, Osmiinda, Hvnienophyl/uin, 

 Marattia, Botrychium, Azolla. 



