206 THE LIVING ORGANISMS OF THE SOIL [chap. 



a large evaporating surface, the root is well developed 

 and provided with root hairs to keep up the supply of 

 water to the plant. There are, however, a number of 

 plants in which transpiration is much less active, and 

 the leaf area is restricted or otherwise arranged to 

 diminish the loss of water, so that the proportion 

 previously stated as existing between the dry matter 

 produced and the water passing through the plants 

 is greatly diminished. A diminished supply of water to 

 the root would, however, necessitate a loss of nutri- 

 ment to the plant, as both nitrates and other mineral 

 salts enter the plant with the transpiration water. 

 Stahl has shown that, in general, these plants with a 

 small transpiration activity are furnished with mycor- 

 hiza, by means of which they obtain food of all kinds 

 from the soil ; whereas, on the contrary, the plants, 

 like the cereals, the cruciferous and leguminous plants, 

 Solanaceae, etc., which give off water freely, are never 

 associated with mycorhiza. Many of the conifers and 

 heaths which grow on dry soils show this correlation 

 of a low evaporation and restricted leaf development 

 with a root-system furnished with mycorhiza. 



Another interesting generalisation has also been 

 brought into line with the above facts by the observa- 

 tions of Stahl that the mycotrophic plants with a 

 feeble transpiration do not store starch in their leaves, 

 but contain instead considerable quantities of soluble 

 carbohydrates, chiefly glucose. In normal plants, though 

 sugar is the first tangible result of assimilation, it is 

 rapidly removed from the sphere of action by being 

 converted into starch, such withdrawal of the product 

 of the reaction being necessary if a rapid rate of 

 assimilation is to be maintained. Should, however, 

 sugar accumulate in the cells, the concentration of the 

 cell sap is increased, so that it parts with its water 



