ABSOEPTION OF FOOD MATERIALS 131 



culty if they are capable of passing, through the limiting 

 layers of the protoplasm of the root-hair. The solution 

 of the salts is always very dilute, and, on account of 

 the ready diffusion that takes 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 intract- 

 able, 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 con- 

 sequently 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 



