104 GENERAL BOTANY 



pots and beds by the roots of neighbouring trees 

 (see p. 124.) 



4. We learnt in chapter iii that, like every other 

 living part, roots respire and give out carbon-dioxide. 

 This gas must be got rid of because too much of it 

 is injurious to living tissues, and it has been proved 

 that a root will grow away from a region rich in 

 carbon-dioxide, and towards one rich in oxygen. This 

 reaction is however much less powerful than the others 

 mentioned. 



The necessity of comparatively free interchange of 

 air so as to prevent an injurious accumulation of 

 carbon-dioxide is partly, though not wholly, the reason 

 why plants grow as a rule much better in soil which 

 is kept constantly stirred and loose than in one al- 

 lowed to get stiff, and why a stiff clay is often sterile. 

 Another reason is the greater quantity of water which, 

 it has been proved by experiment, is lost by evapora- 

 tion from a hard smooth surface than from loose soil. 

 See p. 345 on the peculiar upright roots of certain 

 mangroves. 



