150 VEGETABLE PHYSIOLOGY 



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. 



The other occasional constituents of the ash, which 

 have not so general a distribution as those already 

 mentioned, include a number of metals which play no part 

 in the nutritive processes. They are usually present in 

 very small amount, and appear to be of accidental occur- 

 rence, being absorbed by reason of the solubility of their 

 salts and their power of entering the root-hairs by the 



