242 THE LIVING ORGANISMS OF THE SOIL [chap. 



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, 

 Solanacese, 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 

 by transpiration less readily. Though many excep- 

 tions can be observed, there seems to be a very 

 general association of the development of mycorhiza 

 with a diminished transpiration and the absence of 

 starch from the leaf, especially among plants like the 

 orchids, lilies, iris, etc., which often grow in dry or 

 shady situations, such plants being further distinguish- 

 able by a shiny, glossy leaf surface. Stahl has again 



