244 THE CULTURE OF FARM CROPS. 



the absence of a sufficient supply to meet their wants. It 

 is known that the roots of plants exert a corrosive action . 

 upon the stones in the soil, and upon rocks with which they 

 come in contact. This they do by the excretion of acetic 

 acid, and this acid is found in the soil in which young plants 

 have been grown for experimental purposes. Roots of 

 plants have been known to form a network of lines upon 

 stones and rocks against which they have grown, and to 

 have caused the solution of the mineral matter for their own 

 use. Lichens constantly exert this effect upon the rocks 

 upon which they grow. Roots of grape vines have been 

 known to wholly envelop a bone with a mass of fibers, and 

 to have caused the decomposition of the bony matter for 

 their own support. 



Still another function of roots is the power to excrete 

 useless matter from their substance. This is shown by the 

 fact that at various periods of growth plants contain differ- 

 ent proportions of certain mineral matters. Thus a wheat 

 plant contains about 8 per cent, of ash previous to the bloom- 

 ing period; 5J per cent, when in flower; and but 3i per 

 cent, when fully ripe. It may be supposed that this dimin- 

 ution of the ash may be caused by the increase in organic 

 matter which affected the ratio. But there is a very im- 

 portant change in the character of the ash at these periods; 

 for instance, the silica in it varies from 12? to 26 and 51 

 per cent, according to the variation of time mentioned. 

 Thus while the silica is increased 4 times, the total ash is 

 reduced nearly one-third. There must then have been a 

 diminution of other parts of the mineral matter, which can 

 only have taken place by their rejection from the plant 

 through the roots. 



This process of rejection of useless matter however is not 

 of sufficient importance to affect methods of culture of dif- 

 ferent crops. It was formerly believed that the matter re- 

 jected or excreted by one crop was of great use as food to a 

 succeeding one; and this was made the basis for explaining 

 the beneficial result from a rotation of crops. But this 

 theory is now obsolete, and the advantage of a rotation of 



