54 LECTURE IV. 



the cell-sap is smaller than that in the external fluid. In 

 the case of root-hairs, this inequality is maintained not only 

 by the metabolism of the absorbing cell, as in Yeast, but, 

 inasmuch as the plants now under consideration are multi- 

 cellular, the substances in solution in the cell- sap of the root- 

 hairs are being constantly withdrawn by the more internal 

 cells of the root and are transmitted throughout the plant. 

 In fact, as far as the process of absorption is concerned, the 

 cell-sap of one of these internal cells stands in the same 

 relation to the cell-sap of the root-hair as the cell-sap of the 

 root-hair does to the external fluid. This relation exists 

 likewise between the successive layers of cells of the root, 

 and thus there is set up a current of water holding substances 

 in solution, passing from the surface towards the interior. 



Although the root-hairs can only absorb salts in solution, 

 it must not be thought that the root-hairs of land-plants are 

 immersed in water like a yeast-cell in Pasteur's solution. This 

 is true indeed of floating water-plants, but in the case of 

 land-plants the root-hairs are imbedded in soil which fre- 

 quently contains a relatively small amount of water, and 

 in which many of the salts to be absorbed are present in an 

 insoluble form. The salts, soluble in water, which the par- 

 ticles of the soil may contain, are dissolved by the hygroscopic 

 water, and are thus prepared for absorption by the root-hairs. 

 That salts which are insoluble in water, are, however, absorbed, 

 is quite certain, and we will now proceed to enquire into the 

 means by which their solution is brought about. 



In the first place, a soil which is rich in humus contains 

 a considerable quantity of carbon dioxide, and this gas is also 

 given off by the roots of living plants ; and it is well known 

 that water charged with carbonic acid is capable of dissolving 

 substances, such as calcium carbonate and certain silicates, , 

 which are insoluble in pure water. In the second place, the 

 presence of certain soluble salts in the soil, involving as it 

 does chemical decompositions, brings into the soluble form 

 substances which were originally insoluble, and increases 

 in some cases the solubility of those which are only slightly 

 soluble, v 



