Minnesota Plant Life. 



171 



trich-ferns, the sensitive fern, one of the flowering- ferns, the 

 four-leaved water-fern and some other forms which have not 

 been mentioned, there is a division of labor and the leaf which 

 makes starch is not also designed to produce spores. 



In comparing the true ferns with the adder's-tongue ferns, 

 it would appear that the condition of things is somewhat pe- 

 culiar. Since it bears the spore-cases it would seem that the 



Pf\ 





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Fig. 64. Spore -cases of the common fern, much magnified, showing how the spring back 

 reverts and then snaps shut again, throwing the spores as from a sling. After Atkin- 

 son. 



ordinary fern-leaf compares only with the fertile lobe of the 

 adder's-tongue fern-leaf. This fertile lobe has greatly enlarged 

 and assumed the function of starch-making. At the same time 

 the spore-cases have come to be developed from special cells at 

 its surface, not from mounds of cells as in the lower form. 

 What, now, has become of the sterile segment of the adder's- 

 tongue fern-leaf? The most reasonable reply to this question 

 that can be offered is that it has undergone steady reduction and 



