ductive cells. Xnesc spores, too tiny to be readily seen witn tne naked eye, 

 are enclosed in little spore-cases, or sj^orangm, which grow in clusters or 

 lines called fruit dots, or sor}\ on the hacks oi the leaves. The son turn 

 hrown when ripe, and then they become a conspicuous character. They 

 may be naked, as in the common polypody, or covered by the rerlexed edge 

 of the leaf, as in the miidenhair, or they may have a special covering, the 

 I'ndusium, as in the shield ferns. The position and character of the son 

 determine largely the relationships or the ferns, and hence their classification. 



In the simpler ferns, all of the leaves bear spores; but in many species 

 there is a division of labor, only a part of the fronds producing them. The 

 latter are then distinguished as the fertile, the other as the sterile leaves. 

 Fertile leaves may differ only slightly from the sterile, or they may be- 

 come so modified and specialized, as in the sensitive fern, that they have a 

 very different appearance from the sterile. In some cases a few pinnae only 

 are thus modified, as in the flowering fern. 



In the nomenclature and the sequence of the species Gray s Manual 

 (revised, 1908), has been used throughout. 



Pu^e Sevei 



