DISINTEGRATING WORK OF PLANTS. 445 



exert a powerful effect in the disintegration of the rocks and help to form a 

 thin soil, which is gradually taken advantage of by the larger plants. 



cacti. — Cacti and similar plants may be very effective in breaking up 

 the rocks and producing a soil, because able to exist with a small amount 

 of moisture. Storer states that the lava beds of Etna are planted with a 

 prickly pear. The roots of the cacti soon crack the lava and in a few 

 years it breaks up to a sufficient depth to allow vineyards to be planted. 

 Bv the same sort of action cacti are breaking up the amygdaloidal rocks of 

 the pyramids of old Mexico." The cacti are very abundant in the semi- 

 arid and arid regions, and consequently in such countries produce their 

 most important effect. 



Grasses, grains, and vegetables. — Grasses and grains may exist on a very thin 

 soil, but under favorable conditions the roots penetrate to considerable 

 depth Many of the grasses send their roots into the soil several meters. 

 The roots of the red clover are known to extend a meter into the ground; 

 those of barley, wheat, and oats may penetrate 2 or more meters. In 

 the case of corn, careful experiments have shown that for the depth of 2 

 meters or more the soil may be permeated with roots, so as to constitute 

 a remarkable tangle, fully occupying all the distance between the hills. 

 But as to size and extent of roots probably the most remarkable of the 

 plants cultivated is alfalfa. It is well known that the roots of this plant 

 often extend to a depth of 6 meters, and the soil is commonly a tangle of 

 alfalfa roots and rootlets to the depth of 3 to 5 meters. Cases are recorded 

 where the roots have been found in sandy soils at river banks at a depth 

 of 15 to 18 meters below the surface. 6 



shrubs and trees. — Shrubs and trees may extend their roots several or many 

 meters from their base, both laterally and vertically. Of the shrubs, 

 mesquite is one of the more remarkable. This plant in the arid regions 

 sends a tangle of roots several meters below the surface; indeed, the amount 

 of vegetable fiber below the surface is so much greater than that above the 

 surface that the roots are frequently dug out for fuel, as furnishing a better 

 supply than the growth above the surface. Second-growth black oak has 

 been known to extend its roots laterally more than 9 meters beyond its 



a Storer, F. H. , Agriculture in some of its relations with chemistry, Charles Scribner's Sons, 

 New York, vol. 1, 1887, pp. 130-131. 



& Smith, J. G., Leguminous forage crops: Yearbook of the Dept. of Agric, 1897, p. 494. 



