CHAPTER YIII. 



THE BIOLOGY OF A PLANT. 



The Common Brake or Fern. 



{Pteris aquilina, Linnaeus.) 



Foe the study of a representative vegetal organism some 

 plant should be chosen which may be readily procured and is 

 neither very high nor very low in the scale of organization. 

 Such a plant is a common fern. 



Ferns grow generally in damp and shady places, tlioiigli 

 they are by no means confined to such localities. Some of tlie 

 more hardy species prefer dry rocks or even bold cliffs, in the 

 crevices of which they find support ; others live in open tiekls 

 or forests, and still others on sandy hillsides. In tlie northern 

 United States there are altogether some fifty species of wild 

 ferns, but those which are common in any particuhir locaHty are 

 seldom more than a score in number. Throughout the \vh(>lc 

 world some four thousand species of ferns are known, l)ut by 

 far the greater number are found only in tropical regions, where 

 the climate is best suited to their wants. At an earHer j)eriod 

 of the earth's history ferns attained a great size, and formed a 

 conspicuous and important feature of the vegetation. At 

 present, however, they are for the most part only a few feet in 

 height. Nearly all are perennial; that is, they may Hve for ;in 

 indefinite number of years. Most of tliem liave creeping or 

 subterranean stems; but some of the troj)ical species have eri'ct, 

 aerial stems, sometimes rising to a height of fifty feet or more 

 and forming a trunk which is cylindrical, of equal diametei 

 throughout, and bears leaves only at the sunnnit, like a palm 

 (tree-ferns). 



Of all the ferns perhaps the commonest and most widely 



distributed is the " brake" or " eagle-fern," which is known to 



botanists as Pteris aquilina^ Linnaeus, or rtevldlum aquUinuni^ 



105 



