HOW PLANTS FEED. 243 



in the soil, precisely as fowls will select grains of wheat from 

 among sand or gravel or sawdust cannot be doubted. If 

 any necessary substance required for the growth of plants 

 is absent from the soil, the crop will refuse to grow. If a 

 seed is sown in pure quartz sand the young plant will per- 

 ish as soon as the nutriment of the seed is exhausted. If 

 the plant is fed with a mixture of lime, phosphoric acid, 

 and the other elements of its composition, it will grow to 

 maturity; but if one of these are absent it will not survive. 

 If other substances are offered to it in place of its needed 

 food it will not take up those instead of these. It will not 

 appropriate magnesia in place of lime; nor soda instead of 

 potash. If various plants are grown side by side in the same 

 soil, each will still extract from the soil its own peculiar 

 food and will leave in its ash its own peculiar propor- 

 tions of various mineral matters. If a bean be grown 

 near a stalk of corn, the ash of the corn will contain a large 

 proportion of silica, but that of the bean very little. Abun- 

 dant proof is not wanting to show that plants select their 

 food from the soil, according to their own necessities, and 

 exhibit in this way an instinct much like that of animals. 



Moreover plants refuse to absorb useless matter, or if it 

 is necessarily taken into the roots, it is returned to the soil; 

 it is not stored up in their tissues; and they perish when 

 noxious matter is absorbed by which the chemical action 

 and assimilation of their proper food is interfered with. 



On the whole, the conclusion seems to be reasonable, that 

 the roots of plants select from the soil, in preference, those 

 substances which their nature and composition render nec- 

 essary for them, and in certain proportions; that to a cer- 

 tain and very narrow extent they have the power to substi- 

 tute other substances in place of those which they would 

 prefer naturally ; and that they refuse admission to certain 

 useless or injurious substances, although they are unable 

 certainly to discriminate against and reject everything that 

 may be hurtful or useless to them. 



Another function of roots is the power to prepare food 

 for themselves from the store of inert matter in the soil, iu 



