LIFE HISTORY OF A FERN 



159 



spores may be formed; a sorus will consist of fuUy lOO sporangia, usually 

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

 be fully so fertile pinnas 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 wiU thus, on a 

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



148. TjT)es 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. 



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 aU grades of trans- 

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



