262 LECTURE XV. 



surfaces of the small particles of soil, forming extremely fine coats on them. 

 But it is these substances which must be taken up by the root-hairs, and it is obvious 

 that this is only possible by the root-hairs coming into the closest and most 

 extensive contact with the particles of soil. Since the nutritive materials clinging 

 to the particles of soil are not soluble — or but very slightly so — in the layers of 

 water, the root-hairs, as they apply themselves fast to the surfaces of the particles, 

 must themselves effect the solution of the absorbed materials. This they ac- 

 complish by means of the extremely thin membrane of the root-hair being 

 permeated with an acid fluid, which, coming in contact with the surfaces of the 

 particles of soil, renders soluble the molecules of absorbed materials adhering there : 

 it thus becomes possible for these substances to penetrate into the root-hairs accord- 

 ing to the laws of diffusion, and thence to pass over into the stream of sap, to be 

 carried finally to the organs of assimilation. That the root-hairs are, as a matter 

 of fact, able to bring the whole of the potash salts and phosphates necessary 

 to vigorous vegetation from the absorbed condition into the plant, may be easily 

 demonstrated by repeating an experiment first proposed by Naegeli in 1861. 

 I am accustomed to make the experiment in the following way: bits of peat 

 which, according to chemical analysis, contain practically no potash salts and 

 phosphates, are laid for some days in a 1-2 per cent, solution of any potassium 

 salt (e.g. potassium phosphate), or even in a solution of a complete nutritive 

 mixture such as we shall learn more about later on, until they are completely 

 soaked. The bits of peat are now lixiviated for some days in pure water, fre- 

 quently renewed, until the latter no longer contains any traces of potassium and 

 phosphoric acid: they are then broken up into small pieces, and a large flower 

 pot filled with them. Seeds of Maize, Wheat, Tobacco, Hemp, Gourd, Beans, or 

 other plants are now sown in this medium. It suffices to water in the usual 

 manner with ordinary spring water, which is always rich in sulphates of lime and 

 magnesia, or with a very dilute solution of gypsum, magnesium sulphate, and calcium 

 nitrate in distilled water. On the completion of germination the plants then go 

 on growing as in good garden soil, and attain to complete development and 

 vigorous production of seeds; while plants of the same species cultivated in a 

 similar manner, except that the peat soil had not previously absorbed any phosphoric 

 acid and potassium salts, remain exceedingly feeble, and languish. Such experi- 

 ments prove that the quantities of phosphoric acid and potash necessary to 

 vigorous development are taken up by the root-hairs from the surfaces of the 

 pieces of peat. 



By means of the acid fluid which permeates the walls of the root-hairs, the 

 latter are able to dissolve even solid and crystallised minerals. In the year 1859 

 I showed^ that roots which become closely applied to the polished surfaces of 

 marble plates, corrode them, so that after some time a corrosion figure of the roots 

 is obtained on the marble surface. Of course it is not to be supposed that these 

 corrosions penetrate to, any considerable depth ; they are evident rather only 

 as extremely fine rugosities on the polished surface. I subsequently showed that 



' I fir$t drew attention to the corrosion of polished stone plates, ' Bof. Zeitung^ i860, p. 118, 

 &c. More details in my 'Experimental-physiologies p. 188. 



