FOOD-MATERIALS AND THEIR ABSORPTION 205 



ticular material from its cell-sap so that more can enter by 

 osmosis. 



Two different species of plants growing in the same nutrient- 

 solution or with their roots in the same soil are generally found 

 to contain very different amounts of each of the various ash- 

 constituents. For example, the amount of silica in the ash of 

 the white water-lily is generally less than a ^ per cent., while that 

 of the common reed {Phragmiies communis Trin.) growing on the 

 same marshy soil contains more than 70 per cent, of silica ; and 

 while the ash of pea plants is found to contain not more than 

 about 7 per cent, of this substance, that of grasses growing on 

 the same soil contains over 20 per cent, of it. 



This different quantitive selective power is chiefly due to the 

 difference in the power of making use of silica by the two species 

 of plants compared; the substance from which the sihca is 

 derived probably diffuses with equal freedom through the cell- 

 walls of both, but whereas the reed continually removes the 

 compound from the cell-sap and deposits large quantities of 

 silica in its cell-walls, thus allowing more to flow in, the water- 

 hly uses very little and a state of osmotic equilibrium is soon 

 reached, after which no more enters the plant. 



The amount of any particular substance absorbed from the 

 soil by a plant is in direct proportion to the amount used in the 

 chemical processes carried on by the plant, so that a substance 

 present in abundance may be absorbed in very minute quantities 

 only, whereas a compound present in small amount may be 

 completely extracted from the soil. 



4. The nature of the various inorganic compounds from which 

 green plants obtain their supply of the elements essential for 

 complete nutrition, has already been mentioned in discussing 

 the composition of plants in chapter xii. 



Practically all these food-materials except carbon are absorbed 

 from the soil. 



Experience proves that the continuous growth and removal 



