200 PTERIDOPHYTA. 



According to the nature of the spores, the three classes of 

 the Vascular Cryptogams are each divided into isosporous and 

 heterosporous groups. 



I. The isosporous Vascular Cryptogams have only one kind 

 of spore. The prothallium developed from this is in some cases 

 monoecious, bearing both antheridia and archegonia ; but in others 

 there is a distinct tendency for each prothallium to bear only 

 antheridia or archegonia (dioecious) true Ferns and Lycopodium. 



In Equisetum there is only one kind of spore, but two kinds of 

 p.rothallia are developed, one of which bears only antheridia. 

 (male), the other only archegonia (female) ; but the one that 

 bears antheridia may be transformed into the one that bears 

 archegonia and vice versa. 



II. In the higher group, heterosporous Vascular Cryptogams 

 (Selaginella and Isoetes, etc.), there are two distinct kinds of 

 spores, the small, microspores, and the large, macrospores. The 

 microspores are male, and produce prothallia which bear only 

 antheridia. The macrospores are female, and produce prothallia 

 which bear only archegonia. 



Corresponding to this difference in the spores, there is also 

 found a difference in the development of the prothallium. In the 

 Isosporeae the prothallium is large, and either green, leaf-like, and 

 provided with rhizoids (most of the Ferns, Horsetails, etc.), or 

 subterranean, pale-coloured, and globular (Ophioglossum, Lycopo- 

 dium). It lives vegetatively for a fairly long time, and generally 

 produces a large and varying number of archegonia and an- 

 theridia. The prothallium in the Heterosporeas is gradually more 

 and more reduced, its independent and vegetative life becomes of 

 less and less importance, it becomes more dependent on the mother- 

 plant, and projects from the spore very slightly, or not at all. 

 The antheridia and archegonia become reduced in number to one, 

 and also degenerate in point of development. 



It may here be remarked that the gradual development of the- 

 asexual generation, the development of the two kinds of spores, 

 and the progressive reduction of the prothallium and sexual 

 organs which is found in this Division, is continued to the Gymno- 

 sperms and Angiosperms. The microspores are in these called 

 pollen-grains, and the male prothallium is very rudimentary. 

 The macrospores arc termed embryo-sacs, and the female prothal- 

 lium, the endosperm. 



The asexual generation, sporophyte. When the oosphere r 



