INTRODUCTION 17 



3. The Factors which Regulate the 

 Activities of the Plant. 



{a) Soil. 



Plants live in soil or water^ or, rarely, on other 

 plants. In either case they are subjected to the 

 influence of air, light, and heat. It is by aid of the 

 three last, and of water derived from the soil, or, in 

 the case of aquatic plants, from the surrounding 

 medium, that they make their main non-nitrogenous 

 organic substances, which consist of carbohydrates. 

 From the soil (or water), however, they obtain the 

 bulk of the inorganic and some organic substances 

 which are contained in nutrient solutions. These 

 will be described in section 9 (post). 



These constituents are derived from the oxidation 

 in the last instance of the mineral or organic sub- 

 stances found in the rocks. Primarily the former 

 are made accessible to plants for nutrition by the 

 action of various physical forces upon them, collec- 

 tively included in the term *' weathering." 



These are water, which acts by chemical means 

 upon the rocks, e. g. when carbonic acid in gaseous 

 form in water dissolves such substances as calcium 

 carbonate in the state of chalk or limestone or 

 oolite. Felspars are affected in the same way by 

 the decomposition of the silicates of potash and 

 lime by the action of rain-water. Granite thus 

 becomes clay and sand in course of time. 



In another way water wears down rocks and 



2 



