PHYSIOLOGY 175 



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. The following are the proportions of one 

 among the many nutrient solutions recommended : 



Distilled water . . 1000 to 1500 grams. 



Potassium nitrate l'O „ 



Magnesium sulphate . . 0'5 ,, 



Calcium sulphate . . 0'5 ,, 



Calcium or potassium phosphate . 0'5 ,, 



To this solution a trace of some iron salts should be added. 



The solution should be kept in the dark to prevent the development of algoid 

 growths, and occasionally aerated during the culture experiment. 



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 combination with 

 CHLORINE actually predominates in some plants, and occasions the 

 characteristic development of many of the succulent salt-plants ; 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 organic substances, it remains as a firm 

 siliceous skeleton, preserving the structure of the cell walls. The 

 hardness and firmness of the cell walls are so greatly increased by 

 these siliceous deposits that some of the Equisetaceae are even used for 

 polishing and scouring; while the margins of grass blades, from a 

 similar deposition of silica in their cell walls, are often rendered sharp 

 and cutting. The silioified cell walls of Diatoms occur as fossils, and 

 form deposits of SILICEOUS earth (Kieselguhr) in some geological for- 

 mations. The value of the siliceous concretions, termed " Tabasheer," 

 that are found within the joints of Bamboo has not, as yet, been 

 satisfactorily explained. Aluminium, although like silica everywhere 

 present in the soil, is only in exceptional instances taken up by plants. 

 Aluminium has been detected in the ash of Lycopodiaceous plants ; 

 \Lycopodium complanatum contains a sufficient quantity of acetate of 

 aluminium to render the sap useful as a mordant. The same salt is 

 found also in Grapes. On the other hand, although scarcely a trace of 

 iodine can be detected by an analysis of sea-water, it is found, neverthe- 

 less in large quantities in sea-weeds, so much so that at one time 

 they formed the principal source of this substance. 



