34 THE STORY OF PLANT LIFE 



amount of water present. On dry soils plants are 

 small or dwarfed, on wet soils they are more luxuriant. 

 Flowering and fruiting, however, go on independently 

 of water, and are accelerated by drought. 



Unless there be oxygen the plant cannot respire, 

 hence well-aerated soils and a clear atmosphere are 

 the best conditions for growth. A smoky city atmo- 

 sphere is deleterious. 



The light conditions must be properly adjusted to 

 the plant's requirements. Variation causes etiolation 

 when plants grow in the dark or shade, and hence 

 the division into sun plants and shade plants. 



5. Absorption. 



Reference has already been made to the fact that 

 plants cannot take up substances from the soil 

 except in solution. The mineral salts in the soil 

 are either in the film of water around the particles 

 which are adherent to the root-hairs (Fig. i), or are 

 in a solid state not soluble in water. In the latter 

 case the root-hairs, which are outgrowths of the 

 epidermal cells (Fig. 2), with vacuoles and a nucleus, 

 dissolve such substances in an acid, which they 

 excrete for the purpose. Marble or limestone is 

 attacked in this way, and the effect of roots upon 

 soil of a dark colour in which calcium occurs may 

 be seen where it is bleached a light colour, by the 

 action of such humous acids. 



Where the root-hairs occur the epidermis is not 



