34 Introduction to Botany. 



and animal remains which, on account of the large percent- 

 age of carbon contained in them, impart to the soil its 

 dark color. 



The process of soil formation from the disintegration of 

 rocks can be seen to advantage in any abandoned quarry. 

 The newly uncovered rocks are hard throughout, but those 

 which have lain for some time exposed to the weather 

 become so soft at the surface that they may easily be 

 scratched, or a considerable amount of material may be 

 scraped from them by the finger nail. After longer expos- 

 ure, and particularly after the water imbibed by them has 

 been frozen, the rocks begin to crumble into pieces of vari- 

 ous degrees of fineness. 



In whatever way rocks become broken down — whether 

 by the solvent effect of water, the expansive force of freez- 

 ing water, or the beating of storms ; by abrasion when 

 carried along by torrents, or when hurled to and fro by 

 the surf, or when ground as in a mill by glaciers — the ac- 

 cumulated particles in time form a soil for the growth of 

 plants. But long before the new soil is occupied by the 

 higher plants it becomes the home of myriads of micro- 

 scopic forms whose remains contribute to its richness and 

 put it in a physical condition better adapted to the recep- 

 tion of the larger and more exacting plants. 



21. Soil a Resei'voir for Water. — The capacity of the soil 

 to hold water is dependent on the fineness of its particles ; 

 for the finer the particles, the greater the number of small 

 capillary spaces and the larger the surface exposed for 

 holding water by adhesion. To take a concrete example: 

 A cubic foot of round soil particles having a diameter of 

 one inch would expose a total surface of 37.7 square feet, 

 while a cubic foot of such particles one one-thousandth of 

 an inch in diameter would present an aggregate surface 



