70 



NUTRIENT SALTS. 



protoplasm in construction, and in the storage of materials. The annexed table, 

 which gives side by side analyses of the ash of branches of the Yew (Taxus baccata) 

 with their leaves attached, illustrates the replacement of calcium by magnesium : — 



The Yew occurs in Central Europe on very various mountain formations, chiefly on 

 limestone, but not infrequently on gneiss, and occasionally on serpentine rocks. 

 On comparing the quantities of calcium and of magnesium in the ash of yews, grown 

 on lime and on gneiss respectively, with those yielded in the case of serpentine for- 

 mation, we find that magnesia preponderates considerably in weight over lime in a 

 yew from serpentine rocks (which are in the main a compound of magnesia and 

 silicic acid), whilst the proportion between these two salts is reversed in a yew 

 grown upon limestone. The obvious inference from the table is that, in plants from 

 a serpentine ground, lime is to a great extent replaced by magnesia. This is fur- 

 ther supported by the circumstance that if lime and magnesia are counted together 

 the resulting numbers are very near one another, namely 41 '2 per cent of the ash 

 for limestone, 38"8 per cent for serpentine rock, and 36'3 per cent for gneiss. 



But all these phenomena observed in connection with the selection of food-salts 

 are not nearly so surprising as the fact that plants are also capable of singling out 

 from an abundance of other matter particular substances, which are of impor- 

 tance to them, even from a soil containing them in barely perceptible quantities, and 

 of concentrating them to a certain extent. As has been shown above, nearly a 

 third of the ash of the white water-lily is composed of common salt. One might, 

 therefore, suppose that the water in which water-lilies flourish contains a particu- 

 larly large quantity of common salt. But nothing of the kind is the case. The 

 boo- water which bathed the stem and leaves of this specimen only contained 0'335 

 per cent of common salt, and the mud through which the roots straggled contained 

 only O'OIO per cent. 



No less astonishing is it to find Diatomacese, with cell-membranes, as above 

 mentioned, sheathed in silicic acid, existing in water which contains no trace of 

 silicic acid. Above the Arzler Alp, in the Solstein chain near Innsbruck, there is a 

 spring of cold water which falls in little cascades between blocks of rock. The 



