ABSOKPTION OF FOOD MATEKIALS 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 element 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 in 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. 

 Iron 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 



