Pei 
A. Pteridophyta,. Ferns and Fern Allies, 
Terrestrial or aquatic vlants with green foliage, having usually an 
underground stem which bears one or more leaves, these often tufted. Flowers 
and seeds none, reproduction being effected by spores which are formed in 
usually minute globose or watch-shaped sacs (the sporangia). These sporangia 
are in some cases assembled into small clusters (the sori) and may or may 
not be partially covered by a membrane of varied shape (the indusium),. The 
spores thus produced are usually of one size but may be of two sizes,in 
which case the larger are termed the megaspores and the smaller the micro- 
spores. On germination the spores give rise to a small,or even minute, plant 
known as the gametophyte. In general the spores when of one kind give rise to 
a monoecious thallus-like short-lived gametophyte (the prothallus), when of 
two kinds, the megasrores to an archegonial plant, the microspores to an 
antheridial plant. The gametophytes, whether monoecious or dioecious, alone 
bear the sex organs (the gametangia) and sex cells (or gametes). The product 
of fertilization grows within the female gamete-bearing organ (the archegonium’ 
end eventually into the familiar more or less long-lived svore-bearing plant 
(the sporophyte). Classification is based upon the habit of the sporophyte: 
and the nature of sporangial leaves (the sporovhylls) and of the spores. 
