26 



INTRODUCTION TO BOTANY 



and fall dandelion, pull their rosettes down tightly against the 

 surface of a lawn, kill the surrounding grass, and thus secure 

 for themselves a little clear space in which to grow. 



24. Effects of roots on the soil. If we dig up a spadeful of 

 earth from a well-grassed meadow or from a little inside the 

 circumference of the circle formed by the roots of a tree, we 

 shall find the soil bound together by the living roots or full 



FIG. 18. Cypress trees (Taxodium) growing in a swarnp 



The conical " knees" growing from the roots and nearly always above water are 

 thought to serve as channels to supply air to the roots 



of little, crooked, tubular channels left by the decay of dead 

 ones. Thus the soil is in the one case held together so as to 

 prevent its becoming gullied and washed away by rains, and in 

 the other case made more porous and more easily penetrated 

 by air and water. The latter effect is a very important one in 

 the case of stiff clay soils, which, when closely packed, are 

 almost waterproof. 



The extensive washing away of soils when they are unpro- 

 tected by a covering of plants, such as grass, shrubs, or forest 



