ABSORPTION OF FOOD MATERIALS 137 
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 containg 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, 
