xvi INTRODUCTION 



On the under side of the fronds of most species 

 numerous small dots may be found. These are 

 sometimes erroneously called the ** flowers " of the 

 fern, though structurally, they bear no resemblance 

 to flowers. These small bodies termed fruit dots 

 or sori (singular sorus) constitute the fruiting or- 

 gans of the fern. In their earlier stages they are, 

 in most species, covered with a membrane called 

 the indusium (plural indusia) which i3rotects a col- 

 lection of tiny globes called sporangia (singular 

 sporangium^. The indusium at the proper time 

 withers and disappears, exposing the sporangia. 

 In each sporangium is produced a large number of 

 tiny bodies called spores, which are liberated at 

 maturity by the snapping open of the sporangia 

 or spore-cases, and from these spores, under favor- 

 able circumstances, the new plants arise. In view 

 of this fact the natural inference might be that 

 they are seeds, but they bear no more resemblance 

 to seeds than the sori do to flowers. They only 

 correspond to seeds inasmuch as they serve to carry 

 the plants through a resting stage, and scatter the 

 species far and wide. A germinating spore does 

 not give rise immediately to a plant like the parent 

 as true seeds do. Instead, it develops into a small, 

 flat, green, heart-shaped body, scarcel}^ a quarter 

 of an inch across, called the prothallium (plural 

 prothallia) (Fig. 1). On the under side of the 

 prothallium are borne two minute organs, and by 

 the union of their contents a tiny fern develops. 



While spores are produced by all species of 

 ferns, some employ other methods of propagation 



