Roots 135 



It is, of course, possible for the soil particles to be 

 too fine, as they are in pure clay, which is so close in 

 texture as to exclude what is as necessary to the plant 

 as food and water, namely, air. A plant's roots need 

 air as much as any other part of it, for they are con- 

 stantly taking up oxygen, and hence all specially fertile 

 soils contain a large proportion of sand, which makes 

 them light and porous ; for, though fine, it is not nearly 

 as fine as the particles composing the clay, and does 

 not turn into a stiff paste when mixed with water. 



In a stiff, heavy soil roots make their way with less 

 ease and with less rapidity than in a light, loamy one 

 containing a large proportion of sand, and therefore 

 they must needs collect food less rapidly. Moreover, 

 both air and water penetrate a heavy soil less easily 

 than they do a light one, and hence not only is the air 

 which the roots need less able to reach them, but less 

 water can enter also, and consequently less soil is dis- 

 solved and made ready for their use. The 'loamy 

 soil' which a farmer loves contains from forty to 

 seventy per cent, of sand. 



Here, again, we see the great usefulness of earth- 

 worms. In sand they, like the roots, can make their 

 way so easily that they have little need to remove the 

 soil by swallowing it, the only means at their disposal. 

 But in a stiffer soil they are obliged to do this, and 

 thus they let in both air and water, to the great 

 advantage of the plants, while they also spare the roots 

 much labour by preparing for them airy passages, 

 down which they can run with ease. 



But though roots take advantage of these ready- 

 made channels, and are evidently all the better for 

 them, they do not let go their hold on the soil, 



