CHAPTER XVII 

 THE PTERIDOPHYTES 



256. Introductory statement. This division of the plant 

 kingdom is usually spoken of as the ferns. It must be under- 

 stood at the beginning of the study, however, that the true 

 ferns constitute but one class of pteridophytes. Two other 

 classes are the scouring rushes, or horsetails, and the club 

 mosses. There are other classes of pteridophytes, but since 

 they are represented by only a few highly specialized plants, 

 and since these are not often observed by general students, 

 they are not of great importance in an elementary treatise. 

 There is abundant evidence that ferns were formerly more 

 numerous upon the earth, and some of them of much larger 

 size than those that now exist. Some of these ancient forms 

 doubtless represented classes that are now extinct, and others 

 were the older members or the ancestors of the classes which 

 we now have, and which in some cases are now represented 

 by only a few kinds of plants. 



THE TRUE FERNS (FILICINE^E) 



Those plants which are ordinarily regarded as ferns belong 

 to this class. They may grow in almost any region where any 

 plants are found. Most ferns grow in moist regions, but some 

 species occur in peculiarly dry situations. Although they show 

 considerable variation in form, they can, in nearly all cases, 

 be distinguished from other plants by their greatly divided, 

 feather-like leaves (Fig. 228). There is much range in size of 

 ferns, from very small, lowly plants to those as high as a man's 

 head, and to tree ferns that may be forty feet or more in height 

 (Fig. 229). In some regions dense thickets of ferns are formed. 



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