42 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



sand, grit, and stones, which can be further sub-divided into groups by 

 the aid of the sieve) ; the remaining constituents are termed fine earth. 

 The fine earth plays a special part on plant-life, directly as food material, 

 and indirectly because of its power of absorbing important nutritive 

 substances and because of its purely physical attributes. An admixture 

 of stones and grit nevertheless considerably modifies physical relations 

 in the soil. 



Pore-volume. The commixture, the relative amounts and the arrange- 

 ment of the solid constituents enumerated is very different in different 

 soils. Between the solid constituents there are small cavities termed 

 pores. The sum of these spaces not occupied by solid constituents in a 

 given volume of soil is termed its pore-volume. vSoil is very rich in con- 

 tinuous spaces which become the more capillary the narrower they are. 



These pores are filled with air and water, the relative proportions 

 of which depend upon the size of the pores and other circumstances. 

 In the region of the ground-water, the pores are nearly completely filled 

 with water ; at the surface of a sand-dune that has been exposed to 

 prolonged drought we find the converse, a maximum of air and a 

 minimum of water. 



Some kinds of soil are more or less crumbly, or capable of becoming 

 so, that is, their individual particles do not remain separate, but combine 

 to form larger particles, which may be termed compound particles or 

 compound grains. Compound particles are especially found in humus ; 

 according to Darwin,^ P. E. Miiller,^ and others, they are often the excre- 

 menta and casts of subterranean animals, especially earthworms and 

 insect-larvae.^ Soil having these compound particles acquires characters 

 other than those of a soil consisting of simple particles : it is looser, more 

 easily aerated, takes up water more readily, and allows roots to penetrate 

 more freely. In the practise of horticulture and agriculture an endeavour 

 is made to promote the formation of compound particles in the soil by 

 turning over and ploughing the soil so that its bulk is easily changed 

 by physical factors (especially frost), and by adding other kinds of 

 soil or substance, such as sand, humus, marl, so that its tenacity is 

 changed. 



Tenacity of the soil. The force with which particles of soil are held 

 together varies greatly. As contrasts may be mentioned the dune, whose 

 grains of sand are quite loose in a dry condition, and clay ; humusjike- 

 wise, has little tenacity. Soils may be distinguished into such as are 

 rigid, stiff {heavy), mellow (mild), lax, loose, shifting. Rigid soil on drying 

 becomes hard, fissured, and crustaceous, so that the subterranean parts 

 of plants may be ruptured ; the particles of shifting soil on drying become 

 separated from one another, and are so light that they may be carried 

 away by the wind. Tenacity depends, inter alia, upon the size and chemical 

 constitution of the particles ; the smaller the particles are, the greater in 

 general is the tenacity. 



Plant-form and vegetation as a whole are clearly influenced by the 

 tenacity of the soil. In loose soil (sand, mud, humus in the forest, bog- 

 moss, and so forth) the production of long, richly-branched roots and 

 long, horizontal, subterranean stems (runners and rhizomes) with long 

 internodes is favoured, doubtless because the resistance to be overcome 



^ Darwin, 1881. ^ P. E. Miiller, 1887a. ' See Chapter XX. 



