356 PLANT-LIFE 



water, air, and soil. How essential light is to tlie healthy 

 life of green plants we already know, and we have, to 

 some extent, seen how plants behave in respect to hght. 

 A degree of heat is necessary to ensure growth, but both 

 light and heat are powerless in the absence of water, so 

 far as plant-hfe is concerned. From the air plants 

 obtain oxygen in respiration and carbon dioxide for the 

 formation of carbohydrates. Plants also are affected by 

 movements in the air — i.e., wind. Soil is a very im- 

 portant factor, and has to be studied in relation to 

 subsoil, and quantity and quality of humus. Various 

 soils differ in the amount of nutrient salts they contain, 

 the quantity of water they are able to retain, and the 

 extent to which they are permeated by air. Soils also 

 are affected by worms and bacteria. The exposure of 

 the habitat is also a factor worthy of note. In the 

 Edinburgh district wheat is ripened on ground with a 

 southern exposure up to the altitude of 700 feet above 

 sea-level, but on a north exposure the altitude at which 

 it can be successfully grown is 200 feet lower. 



Of all the factors in plant-ecology water is chief, and 

 the botanist is now in the habit of classing plants 

 according to their water environment. He divides them 

 into four great classes — Hydrophytes, Hygrophytes, 

 Mesophytes, and Xerophytes. 



Hydrophytes (Gr. hvdor, water; phyton, plant), or 

 aquatic plants, are specially adapted for life in water. 

 Some of them are free-floating, some are entirely sub- 

 merged, while others are rooted in mud, and have floating 

 leaves with their upper surface exposed to air. Light 

 reaches the true aquatic through the medium of water, 

 while oxygen, carbon dioxide, and nutrient salts, are 



