OTHEK FACTOKS INFLUENCING DISTRIBUTION. 33 



TROPICAL REGION. 



This zone embraces in eastern North America a narrow belt along 

 the coasts of the southern extremity of peninsular Florida and the 

 surrounding keys, and contains part of the West Indian or Antillean 

 tlora. 



SECONDARY FACTORS INFLUENCING PLANT DISTRIBUTION. 



Besides the primar}^ factors, heat, light, and atmospheric moisture, 

 which regulate the distribution of life within the wide areas recog- 

 nized as life zones, there are others which exercise a potent influence 

 upon the distriliution of plants in confining them to narrower l)ounda- 

 ries. The former are due to geographical position, latitude and longi- 

 tude, extent of land area, presence and position of mountain chains, and 

 prevailing winds and oceanic currents, while the latter are dependent 

 on ndnor local conditions, such as altitude, topograph}^, proximity to 

 large bodies of water, all of which influence the lesser fluctuations of 

 temperature, precipitation, occurrence of frosts or other extremes, 

 secondary currents of the air, and exposure to sunlight. In addition 

 to these and of equal importance are the stratigraphical relations deter- 

 mining the chemical and physical character of the soil and sul^soil, as 

 well as the drainage of the surface and the soil l)eneath, which in itself 

 must be considered as one of the most important of the secondary 

 factors influencing distribution. 



To this class of factors belong also the facilities possessed by 

 different species for dissemination. These consist chiefly of biological 

 agencies, special adaptations with which the fruits and seeds of man}' 

 species are provided, to secure their dispersion. This is effected in 

 various ways, particularly through the agency of animals. One means 

 consists of fleshy, sweetish fruits tempting animals to feed upon them. 

 In most instances the seeds are discharged with their vital power unim- 

 paired and thus are transferred to distant localities, particularly when 

 the fruit has served as food for birds. A second means consists of the 

 prickles, hooks, or stifl", hooked hairs covering the surface of the hard 

 fruits, by which they attach themselves to wandering animals. 



Distribution of seeds is also effected l)y atmospheric agency, the 

 fruits or seeds being very minute or provided with l)road, wing-like, 

 or plumose attachments, which enable them to be wafted through 

 the air. In some cases the sudden tearing asunder of the parts of 

 the fully-matured hard fruit when touched causes the scattering 

 of the seeds. Finally, some seeds are disseminated through their 

 ability to float about in fresh or saline water without losing the power 

 of genuination.^ 



• O. Drude, Handbuch der Pflanzengeographie, 121 (1890) . 

 15894—3 



