SRCT. II 



PHYSIOLOGY 



207 



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. 



More accurate consideration shows 

 that it is not correct to speak of definite 

 elements which are indispensable to 

 the plant. Just as a mixture of the 

 elements H and is not a substitute 

 for water, it is not sufficient to supply 

 the plant with the elements contained 

 in the nutrient salts either as elements 

 or in any of their combinations. Thus 

 metallic potassium or pure sulphur are 

 of no use. The plant requires particu- 

 lar salts or, since these in part dissociate 

 in water, particular ions. Necessary 

 kations are K+, Ca++, Mg++, while 

 So^"", H^Po^" and N03" are neces- 

 sary anions. While phosphorous and 

 sulphur can only be utilised in these 

 combinations, the nitrogen can also be 

 obtained, although not always so use- 

 fully in the form of the kation NH4+. 



The method of water-culture has 

 not only shown the necessity for certain 

 salts, but also that many substances 

 which the plant usually absorbs can 

 be done without. 



This is true, for instance, of sodium, which, 

 in combination with chlorine, actually pre- 

 dominates in some plants, and occasions the 

 characteristic development of many of the 

 succulent salt-plants (halophytes) (Fig. 192) ; 

 and also of silicon, which, as silica, is so 

 abundantly deposited in the cell-walls of 

 many plants — Equisetaceae, Grasses, Sedges 

 (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, preserv- 

 ing the structure of the cell-walls. The hardness and firmness of the cell-walls 

 are greatly increased by these siliceous deposits ; they themselves have about the 

 degree of hardness 2, but the silicified epidermis of Equisetum has the hardness 4 

 ( = Fluorspar), and that of the fruits of Coix 7 ( = Quartz). Some of the Equise- 

 taceae are even used for polishing and scouring. The silicified cell- walls of Diatoms 

 occur as fossils, and form deposits of siliceous earth (Kieselguhr) in some 



/• >V^vXt<.iAW. 



Fio. 192. — Salicomia herbacea, a character- 

 istic halophyte. (From Schimper's 



Plant- Geography. ) 



