THE FERN BULLETIN 



Vol. XVI 



JULY, 1908 



No. 3 



ON CHANGES IN FUNCTION IN DIMORPHIC 

 FRONDS. 



By Willard N. Clute. 



LIBRARY 

 NEW YORK 



BOTANICAL 

 GARDEN 



By far the greater number of ferns throughout the 

 world produce their asexual spores on the backs of 

 fronds that do not differ materially from other fronds 

 devoted to purely vegetative functions. A few species, 

 however, such as the cinnamon, sensitive and ostrich 

 ferns have sharply separated the work of reproduction 

 from that of food making, at least so far as this relates 

 to the chief or sporophyte generation, and as the 

 spore-bearing fronds have no longer use for a large 

 expanse of chlorophyll tissue, we find them variously 

 reduced and quite unlike the sterile fronds, or as we 

 say, they have become dimorphic. Occasionally, too, 

 we mid this tendency to separate the two functions dis- 

 played in the same frond, as in the royal, the inter- 

 rupted and the grape ferns. Usually the plants which 

 show this latter tendency are those which commonly 

 produce a single frond each year, as Ophioglossum, 

 Anemia and Botrychium , but the phenomenon is not 

 restricted to them as is illustrated by the Osmundas 

 cited as well as by the Christmas fern and others. 



That the law which determines whether a frond shall 

 be fertile or sterile is not irrevocably fixed is seen in 

 the instances which are steadily accumulating of fertile 

 co parts made sterile or partly sterile and what would nor- 

 § mally be sterile parts turning fertile. We see this in that 

 striking form obtusilobata of the sensitive fern, which 



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