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 
Fic. 18. Cypress trees (Taxodium) growing in a swamp 
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 
