60 LECTURE IV. 



In answer to this question it may be at once stated that 

 it has been found that different plants growing in the same 

 soil or water and under the same conditions contain very 

 different quantities of the substances which they must have 

 absorbed from the soil or the water. This has been ascer- 

 tained by a great number of analyses of the ash of plants, 

 that is, of the mineral residue which is left after the plant has 

 been incinerated. Since there is no evidence to prove that 

 a plant loses any of the mineral substances which it absorbs, 

 excepting of course when it throws off parts of itself, such as 

 leaves, bark, and seeds, it is obvious that a knowledge of the 

 composition of the ash must afford valuable information 

 as to the specific absorbent capacity of the plant. Besides 

 this mode of investigation there is the method of water- 

 culture (see Fig. 21), to which allusion was made above, 

 which consists in growing plants with their roots immersed 

 in water holding known quantities of salts in solution. By 

 analysis of the plant, or of that portion of the solution which 

 remains unabsorbed at the close of the experiment, the 

 amount of the various salts absorbed can be determined. 



A familiar and striking illustration of the difference in 

 the composition of the ash of different plants grown under 

 the same conditions is afforded by a comparison of the 

 amount of silica present in the ash of equal dry weights 

 of gramineous and leguminous plants : thus, according to 

 Wolff, 



ioo parts of Meadow-hay contain 27-01 of silica 

 Wheat-straw 67-50 



Red clover 2-57 



Lucerne 6-07 



Pea-straw 6-83 



It may be further illustrated by citing Trinchinetti's 

 observation that Mercurialis annua and Chenopodium viride 

 took up more nitre than common salt from a solution con- 

 taining these two substances, whereas Satureia hortensis and 

 Solamim Ly coper sicum took up more common salt than nitre, 

 and again by analyses of water plants. 



