THE FERNS 137 



these cases the alternation of generations becomes 

 somewhat obscured, for on the male side the 

 prothallus is reduced to almost nothing, while the 

 female prothallus, though fairly large, remains to a 

 great extent enclosed within the megaspore. 

 In the Seed-plants, as already explained, this 

 reduction of the sexual individual goes still 

 further, the female prothallus, together with the 

 megaspore, remaining permanently enclosed in 

 the ovule and seed. At the same time, the 

 organs producing the two kinds of spores the 

 stamen with its pollen-sacs on the one hand, and 

 the carpel with its ovules on the other come to 

 be strongly differentiated, so that the terms male 

 and female are commonly applied to them. 



To return to the higher Spore-plants, in which 

 the characteristic alternation of generations is 

 more evident, we find that in the living vegetation 

 of the World they are represented by three great 

 groups, the Ferns, the Club-mosses or Lycopods, 

 and the Horsetails or Equisetums. 



The Ferns, familiar to every one, are char- 

 acterised by the great development of the leaf 

 in comparison with the stem; the frond is the 

 typical part of a Fern. The numerous spore- 

 sacs are borne either on the ordinary fronds 

 (usually on the back) or on special fronds or 

 leaflets; a cone is never formed. Most Ferns 

 have spores of only one kind; the small family 

 of the Water-ferns alone bears spores of two lands. 



