136 VEGETABLE PHYSIOLOGY 
captured insects are digested, and the nitrogenous material 
is subsequently absorbed by the leaf surface. A fuller 
discussion of these mechanisms will be found in .a subse- 
quent chapter. These plants are generally found growing 
in such a situation that they are not brought into contact 
with inorganic compounds of nitrogen, and hence are cut 
off from the supplies which are afforded to the roots of 
ordinary terrestrial plants. The mechanisms described 
afford instances of special adaptations to particular environ- 
ments, and will therefore be considered in more detail 
later. 
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 
probably 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 combi- 
nations, 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 
prominent in the grasses and the horsetails. It is taken 
up from the soil in the form of soluble silicates, and 
possibly to some extent in that of soluble silicic acid. 
