ABSORPTION OF FOOD MATERIALS 187 



place, their concentration is approximately uniform in any 

 particular soil. Other salts are insoluble in pure water, 

 and their absorption presents more difficulty. Many are 

 soluble in water which contains carbon dioxide, and as 

 considerable quantities of this gas are continually being 

 generated in the soil, the water there is charged with it, 

 and bodies, otherwise intractable, are thereby brought into 

 solution and absorbed. 



The power of water containing carbon dioxide to effect 

 the absorption of such substances is capable of easy demon 

 stration. One of these salts is calcium sulphate or gypsum. 

 If a plate of this substance is placed at the bottom of a 

 flower-pot and the pot then filled with moist earth, a plant 

 caused to grow in it till its root system is well developed will 

 have some of its roots closely adpressed to the gypsum 

 plate. After a time, examination will show the surface of 

 the plate eaten away at all points except where the roots 

 have become adpressed to it, and the regions covered by 

 the latter will stand out in slight relief. The whole sur- 

 face will have been subjected to the action of the water 

 and the carbon dioxide it contains, except where it has 

 been covered by the roots, and the solvent action will 

 consequently be recorded. 



A third factor which must be considered in the process 

 of absorption is the acid sap which the root-hairs contain. 

 Not only does the acid cause water to enter the hair 

 osmotically, but a little of the sap exudes in the same way, 

 and this has a certain solvent action upon the particles 

 to which the root-hairs cling. Thus certain salts can be 

 absorbed, though they may be soluble neither in pure 

 water nor in water containing carbon dioxide. 



A similar experiment to the one just described will 

 demonstrate this property of the acid sap. If, instead of 

 gypsum, a polished plate of marble is inserted into the 

 flower-pot, after a certain time of growth of the plant con- 

 tained in it, the plate will exhibit a tracing of the course 

 of the roots which have come into contact with it, but, 



