Ch. XIV] FORMATIONS AND ASSOCIATIONS 579 



their supply of combined nitrogen, which is usually scanty 

 in places where those plants occur (page 246). The method 

 by which the insects are taken determines the vegetation 

 forms as follows. 



Pitcher Plants; typified by Nepenthes (Fig. 49) and 

 Sarracenia (Fig. 48) with true pitchers containing secreted 

 water and digestive enzymes, often with remarkable adapta- 

 tions for attracting and holding the insects. Allied is the 

 form of the Bladderwort (Fig. 399). Leaf Traps, typified • 

 by the Venus Fly-trap (Fig. 50), with hinged leaf closing sud- 

 denly on contact. Adhesive Traps ; typified by Sundew (Fig. 

 392), with viscid glands ; leaf incurling and digesting the in- 

 sect. Allied is the form of the Pinguicula or Butterwort, the 

 leaves provided beneath with wax to which insects adhere. 



In some measure allied is the association between ants and 

 plants called myrmecophily. The former belief that the 

 constant presence of ants on certain tropical trees is symbiotic, 

 the trees providing the insects with residence and food, 

 and receiving in return protection against leaf-cutting ants, 

 seems now doubtful. But there is no question as to the 

 cultivation of Mushrooms by ants in Brazil (page 473). 



3. Plant Formations and Associations 



The vegetation of the earth, considered comprehensively, 

 is presented to the eye in areas and zones having each its 

 characteristic aspect and distinctive place as a feature of the 

 landscape. Usually also a connection is obvious between 

 these areas and zones and the physical features of the country, 

 that is, the principal habitats. Such areas or zones of vege- 

 tation are called plant formations, and a formation is the 

 totality of vegetation in any given habitat. Their reality 

 and prominence are attested by our common nomenclature, 

 which recognizes woods (of different kinds), marshes, bogs, 

 prairies, etc. These words, by long usage, have come to 

 involve something of the idea of both the vegetation and the 

 habitat. It is because of this close correlation between vege- 



