CHAPTER XXVIII. 



DIMORPHISM OF FERNS. 



563. In comparing the different members of the leaf series 

 there are often striking illustrations of the transition from one 

 form to another, as we have noted in the case of the trillium 

 flower. This occurs in many other flowers, and in some, as in 

 the water lily, these transformations are always present, here 

 showing a transition from the petals to the stamens. In the bud 

 scales of many plants, as in the butternut, walnut, currant, etc., 

 there are striking gradations between the form of the simple bud 

 scales and the form of the leaf. Some of the most interesting of 

 these transformations are found in the dimorphic ferns. 



564. Dimorphism in the leaves of ferns. In the common 

 polypody fern, the maidenhair, and in many other ferns, all the 

 leaves are of the same form. That is, there is no difference be- 

 tween the fertile leaf and the sterile leaf. On the other hand, in 

 the case of the Christmas fern we have seen that the fertile 

 leaves are slightly different from the sterile leaves, the former 

 having shorter pinnae on the upper half of the leaf. The fertile 

 pinnae are here the shorter ones, and perform but little of the 

 function of carbon conversion. This function is chiefly per- 

 formed by the sterile leaves and by the sterile portions of the 

 fertile leaves. This is a short step toward the division of labor 

 between the two kinds of leaves, one performing chiefly the labor 

 of carbon conversion, the other chiefly the labor of bearing the 

 fruit. 



565. The sensitive fern. This division of labor is carried to 

 an extreme extent in the case of some ferns. Some of our native 



273 



