DIRECT ACTION OF ROOTS UPON THE SOIL. 327 
moist. The roots penetrated the sand and came in con- 
tact with the plates below, and branched horizontally on 
their surfaces. After several days or weeks the plates 
were removed and examined. The plants employed were 
the bean, maize, squash, and wheat. The carbonates of 
lime and magnesia and the phosphate of lime were plains 
ly corroded where they had been in contact with the 
roots, so that the course of the latter could be traced with- 
out difficulty. Even the action of the root-hairs was mani- 
fest as a faint roughening of the surface of the stone 
either side of the path of the root. Gypsum and glass 
were not perceptibly acted on. 
Dietrich has made a series of experiments (Hojfmann’s 
Jahresbericht, VI, 3) on the amount of matters made solu- 
ble from basalt and sandstone, both coarsely powdered, 
and kept watered with equal quantities of distilled water, 
when supporting and when free from vegetation. The 
crushed rocks were employed in quantities of 9 and 11 
Ibs.; they were well washed before the trials with dis- 
tilled water, and access of dust was prevented by a layer 
of cotton batting upon the surface. After removing the 
plants, at the termination of the experiments, each sam- 
ple of rock-soil was washed with the same quantity of 
water, to which a hundredth of nitric acid had been 
added. It was found that the plants employed, especially 
lupins, peas, vetches, spurry, and buckwheat, assisted in 
the decomposition and solution of the basalt and sand- 
stone. Not only did these plants take up mineral mat- 
ters from the rock, but the latter contained besides, a 
larger amount of soluble matters than was found in the 
experiments where no plants were made to grow. The 
cereal grains had the same effect, but in less degree. In 
the subjoined table we give the total quantities of sub- 
stances dissolved under the influence of the growing 
vegetation. These figures were obtained by adding to 
what was found in the washings of the rock-soils the ash 
