CHAPTER YIIL 

 THE BIOLOGY OF A PLANT. 



The Common Brake or Pern. 



(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, though 

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

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

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

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

 United States there are altogether some fifty species of wild 

 ferns, but those which are common in any particular locality are 

 seldom more than a score in number. Throughout the whole 

 world some four thousand species of ferns are known, but by 

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

 the climate is best suited to their wants. At an earlier period 

 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 live for an 

 indefinite number of years. Most of them have creeping or 

 subterranean stems ; but some of the tropical species have erect, 

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

 and forming a trunk which is cylindrical, of equal diameter 

 throughout, and bears leaves only at the summit, 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 Pteridium aquilinum, 



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