PHYSIOLOGY 191 



nutrient solution containing all the essential food elements except 

 iron, the new leaves developed are no longer green, but are of a pale 

 yellow colour ; they are " CHLOROTIC," and not in a condition to 

 decompose the carbonic acid of the atmosphere and nourish the 

 plant. Upon the addition, however, of a mere trace of iron to the 

 solution the chlorotic leaves in a very short time acquire the normal 

 green colour. The chlorophyll, however, does not contain iron, but 

 always contains magnesium. 



So long as the necessary nutritive substances are provided, the form in which 

 they are ottered to the plants, as well as the proportionate strength of the nutrient 

 solution (if not too concentrated), may vary. Plants have the power to take up 

 these substances in very different combinations, and are able to absorb them in 

 other proportions than those in which they occur in the soil. In concentrated 

 nutrient solutions the absorption of water is increased ; conversely, in very dilute 

 solutions it is the salts that are chiefly taken up. The presence also of certain 

 substances often exerts an active and generally beneficial influence upon the 

 capacity for absorbing other substances : thus, calcium salts increase the absorp- 

 tion of potassium and ammonium salts. Calciphobous plants do not succeed on 

 soil rich in lime either because the absorption of potassium, magnesium, and iron 

 is greatly interfered with (Castanea, Sarothamnus), or because the calcium exercises 

 a poisonous effect (Pinus pinaster , Sphagnum, Drosera). Other plants are calcicolous 

 and succeed best with a high percentage of lime in the soil. 



In order to avoid the poisonous effects of phosphates and salts of iron, when 

 supplied in a soluble form, v. r>. CUONE recommends the following nutritive 

 solution : 



Distilled water 1-2 litres. 



Potassium nitrate . . . . . 1 '0 gramme. 

 Ferrous phosphate . . . . . 0'5 ,, 

 Calcium sulphate ..... 0'25 ,, 

 Magnesium sulphate . . . . 0'25 ., 



A mixture of equal parts of ferrous phosphate and tri-calcic phosphate may be used 

 in place of the ferrous phosphate in the above formula. The phosphates which are 

 present as a fine powder in the solution become deposited on the surface of the 

 roots of plants growing in the fluid. Plants are found to grow better in the above 

 solution than in those used by KNOOP and SACHS. The growth of Algae is hindered 

 fn this solution ( 14 ). 



As a most important result of such culture experiments, it has 

 been demonstrated that only the ten elements already named are 

 necessary for the growth of plants ; all other elements, although 

 present in plants in large quantities, are of subordinate value to 

 plant life. This is true, for instance, of SODIUM, which, in combina- 

 tion with CHLORINE, actually predominates in some plants, and 

 occasions the characteristic development of many of the succulent 

 salt-plants (halophytes) ; and also of SILICON, which, as silica, is so 

 abundantly deposited in the cell walls of many plants Equisetaceae, 

 Grasses, Sedges, Diatoms (in the ash of Wheat-straw 70 per cent, 

 and of Equisetaceae 70-97 per cent) that, after combustion of their 



