LIFE HISTORY OF A FERN 



159 



spores may be formed; a sorus wiU consist of fully 100 sporangia, usually 

 more; 20 is a moderate estimate of the sori on an average pinna; there may 

 be fully so fertUe pinnae on one well-developed leaf, and a strong plant 

 would bear 10 fertile leaves. 48 X 100 X 20 X 50 X 10 = 48,000,000. 

 The output of spores on a strong plant in the single season will thus, on a 

 moderate estimate, approach the enormous number of fifty millions." 



148. Types of Sporophylls.— In many ferns the leaves 

 serve both vegetative and reproductive functions in about 



Fig. 119— The cinnamon fern (Osmunda cinnamomea), showing foliage 

 leaves and sporophylls. (Photo by Elsie M. Kittredge.) 



equal degree, as in the case of Polypodium mentioned 

 above. In some species, however, there are two kinds of 

 leaves— one devoted entirely to vegetative functions, and 

 another to the reproductive, or spore-producing function 

 (Fig. 119) ; between these two extremes all grades of trans- 

 ition are found (Fig. 120) . But however widely the sporo- 



