DIRECT ACTION* OF HOOTS UPON TIIE SOIL. 3^27 



moist. The roots jienetrated 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 plain- 

 ly corroded Avhere they had been in contact with the 

 roots, so that tlie 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 tlie path of the root. Gypsum and glass 

 were not perceptibly acted on. 



Dietiich has m:ide a series of experiments {SoffmanrCs 

 tTahreshericlit^ 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 

 lbs. ; 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 

 Nvater, to which a hundredtli 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 washin<j:s of the rock-soils the ash 



