ABSORPTION OF FOOD MATERIALS 141 
to enter into some form of combination, and is then absorbed 
by the root, probably through’the tissue of the fungus. It 
is not absorbed by the leaves of the plant. 
Organic compounds of nitrogen are seldom presented to 
the roots of plants, so that the amount of the elenient which 
is absorbed in such a way is very small. Indeed, it may 
be said that such an absorption is almost entirely excep- 
tional. It has been found that plants are able to utilise 
urea and other amides when these are present im the soil. 
In very rich soils, or those containing a large quantity of 
humus, such compounds are to be met with, and there is a 
probability that they are more easily worked up into actual 
nutritive substance than the inorganic compounds which 
have been spoken of. 
Besides compounds of nitrogen, the materials absorbed 
by the roots of normal green plants include the constituents 
of the ash. Of these the more prominent are the com- 
pounds of potassium, sodium, magnesium, calcium, and 
iron. The sulphur and phosphorus which enter into the 
composition of the protoplasm are also taken in by the 
roots, in combination with the metals mentioned, and 
with others whose occurrence is not so general. The 
sulphur is absorbed in the form of sulphates, and the 
phosphorus in that of phosphates, of these metals. 
Potassium is present in the soil in various combinations, 
principally as the sulphate, phosphate, chloride, and pro- 
bably the silicate. After the nitrate the chloride appears 
to be the salt which is the most advantageous to plants. 
Calcium and magnesium exist in similar combinations, all 
of which, except the chloride, appear to be suitable for 
absorption. The chloride is, on the whole, deleterious. 
Tron can be absorbed in almost any inorganic combination. 
Sodium is absorbed in similar forms to those of potassium, 
the nitrate being the most valuable. Sodium chloride is 
frequently present in considerable quantity in the plants 
which are found on the sea-shore. 
Silicon is present in many plants, being especially 
