270 INTRODUCTION TO BOTANY 
of these ancient plants doubtless represented classes that are 
extinct, and others were the older members or the ancestors 
of the classes which we now have, which in some cases are 
represented by only a few kinds of living plants. 
Most ferns grow in moist regions, but some species are found 
in peculiarly dry situations, even growing like lichens on nearly 
Fic. 207. A group of maidenhair ferns 
Photograph by W. H. P. Huber 
bare rocks. 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. 207). 
Ferns have great range in size, from very small, lowly plants 
to those as high as a man’s head, and to the tropical tree ferns 
that may be forty feet or more in height. They may occur 
singly or in thickets so dense as to make it difficult to penetrate 
them. In all except the tree ferns the parts of the plant that 
we see are the leaves ; the stems and the roots are underground. 
