314 



THE PTERIDOPHYTES 



307. Fronds, vegetative leaves, and spore leaves (sporo- 

 phylls). Most fronds are vegetative, that is, perform chloro- 

 phyll work (photosynthesis) during the early part of the season 

 and develop sporangia later. However, some types, as the royal 

 and cinnamon ferns (Osmunda) and the sensitive and ostrich 

 ferns (Onoclea), devote the whole of certain leaves or portions of 

 them entirely to the work of spore production. The blades of 

 these fronds or portions of fronds never become expanded, but 

 remain somewhat rolled up, forming pod-like structures in which 

 the sporangia are developed (Fig. 276). Other fronds on these 



FIG. 275. The sporangium of a common fern (Aspidium Filix-mas) 



A, closed sporangium; B, sporangium opening; C, fully opened and discharging 

 the spores. After Kerner 



ferns are devoted entirely to vegetative activities and never de- 

 velop sporangia. There is thus in some ferns a division of labor 

 among the fronds, certain of them becoming strictly vegetative 

 leaves, while others are spore leaves, called sporophylls. 



There is a constant tendency in the pteridqphytes to give all 

 the work of spore production to the specialized spore leaves 

 (sporophylls), which means that all the other fronds on the 

 plant become entirely devoted to vegetative activities and may 

 then be called vegetative leaves or simply leaves in the sense in 

 which this term is generally used in the seed plants. This 

 differentiation of the frond into leaves arid sporophylls reaches 



