FERNS 



339 



through the decaying mass of leaves obtaining also some food. 



Another kind of dimorphism is present in several of our common 

 ferns. Here, as in the sensitive fern (Onoclea 

 sensibilis) , there are certain leaves with large, 

 expanded green blades. These perform the 

 function of photosynthesis. Other leaves have 

 stalks (or petioles) equally long, but the blade 

 of the leaf is very much contracted and the 

 pinnae inrolled. These are the fertile leaves, 

 and bear the spore cases in crowded sori within 

 the roll of the pinnae. The vegetative leaves 

 arise early in the season, while the spore-bear- 

 ing leaves (sporophylls) are developed later, 

 some time in June. Cutting off the vegetative 

 leaves as fast as they appear in the spring will 

 change the spore-bearing leaves to vegetative 

 ones > an d many of them will be intermediate 

 between the two, if the vegetative leaves are 



to prothallium. 



Fig. 322. 

 Staghorn fern (Platycerium alcicorne). 



allowed to get about 20-30 cm. (8-12 inches) high before they 

 are cut off. The vegetative leaves will need then to be cut twice. 



