ABSORPTION OF FOOD MATERIALS 131 
eulty 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 
