40 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



The utility of the wind to vegetation is especially shown in the con- 

 veyance of fresh supplies of carbon dioxide. The transport of pollen to 

 anemophilous plants, such as coniferous and dicotylous trees of 

 North and Central Europe, and in the dispersal of seeds ; many of our 

 common trees have their seeds scattered by the wind.^ 



CHAPTER IX. NATURE OF THE NUTRIENT SUBSTRATUM 



The nature of the nutrient substratum, or edaphic ^ conditions, 

 largely determines the habitats of plants and their topographical 

 distribution ; and among all characteristics of the substratum the most 

 important is the amount of water contained. 



There are two different forms of nutrient substratum available to 

 autophytes : water and soil. Both these have to provide the plant 

 with space and nutriment, as well as with external conditions suitable 

 for the absorption and preparation of nutritive material : they make 

 these provisions by entirely different methods and must, therefore, be 

 treated separately.^ 



The air is not in itself a nutritive medium in which plants habitually 

 live and feed, but is merely a temporary resort for organisms that are 

 nearly all of them microscopic but are present in countless numbers, 

 which vary according to time and space, being greatest in the vicinity 

 of human dweUings, particularly in large towns, and being least over 

 oceans, high mountains, and in forests. The weightiest geographical 

 role of the air is that by its currents it affords ways and means for 

 the transport of countless organisms from one place to another. 



The discussion on water and its characters that are of greatest import 

 to oecological plant-geography will be deferred until hydrophilous com- 

 munities are treated in Section IV. But the characters of soil will be 

 dealt with at once : they depend upon the physical and chemical attributes 

 of the soil-constituents. 



CHAPTER X. STRUCTURE OF SOIL 



The term soil is used here in a wide sense and includes 



1. Solid rock ; 



2. Loose soil produced in situ by weathering ; 



3. Secondary, loose, transported soil that is the product of weather- 



ing at some other spot. 



Solid rock. The characters of solid rock depend upon its miner alogical 

 nature. It varies greatly in hardness, porosity, specific heat, and power 

 of radiation, as is shown by contrasts such as granite, shale, and limestone. 



Loose soil. By the mechanical disintegration and chemical decom- 



Friih, 1901 ; Norton, 1897; Ganong, 1899; L. Klein, 1899, 1905; Kraus, 1905; 

 Klinge, 1890; Schimper, 1898; see Schenck, 1905. 



^ Cp. Warming, 1887 ; Sernander, 1901 ; P. Vogler, 1901. 



* TO Sa0os, the soil. This term was introduced by Schimper (1898). 



= See Chapters XXVII-XXXIII. 



