Abstract
The keys to be found in the Manuals used for taxonomic study often require the student using them to have a greater knowledge of the species under than is obtainable from the specimen at hand. For example, the indusium regularly drops from the sorus in some genera when the fronds are young, while in other genera, indusia are never present. Examples of these are found, in the case of the former condition, in Cystopteris and Woodia, and in the case of the latter condition, in Polypodium and Phegopteris. How is the student just beginning taxonomic work on the ferns to know, when he encounters a fruiting frond with no indusia, whether it is a case of entire absence of indusia of a species from which the indusia drop off early? He should, of course, examine a number of specimens, but the working collection available for his study does not always have both young and old specimens of each species. It should have, to be sure, but it is not always expedient for the instructor to have material representing all ages of fronds. Further, suppose the student is identifying material of his own collection instead of a set prepared for him by some one else, and he wishes to know today whether the absence of an indusium is a fixed character. He cannot always wait a year to determine his point by observation on younger fronds.
