DESERT PLANTS 



543 



ods in which the strength of a solution, such as cane sugar or potassium 

 nitrate, which will balance the solution in the cell is measured, or by ex- 

 tracting a certain measured amount of living material which has been 

 crushed with distilled water and after the freezing point of this ex- 

 tract has been found the original pressure may be calculated. The 

 simpler process of squeezing out sap and testing its freezing point can 

 not be used in a large number of instances since the highest pressures 

 that can be applied fail to bring out the scanty sap from some species. 

 The use of such methods at the Desert Laboratory demonstrates that 

 the leaves of the creosote bush (Covillea, or Larrea) (Fig. 1) may have 



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Fig. 1. Covillea (Larrea), the Creosote Bush, the most widely distributed 

 Shkub with Restricted Surfaces in American Deserts. The leaves show osmotic 

 pressures equivalent to over 75 atmospheres. 



osmotic pressures of 75 atmospheres, the upper parts of the stems 35 to 

 60 atmospheres, and the basal portions of the stems 35 to 50 atmos- 

 pheres. Fitting, by the use of plasmolytic methods on plants in the 

 Algerian deserts at Biskra, found pressures in leaves of plants of this 

 type of over a hundred atmospheres. These pressures would support a 

 column of water 250 to 300 feet high. 



It is notable that plants of this type are constantly in absorbent con- 

 tact with the soil, and apparently continue to derive some water from 

 it even in the driest times, as evinced by the fact that they wilt quickly 

 when taken up. Such forms are very difficult to transplant. A misap- 

 prehension as to the influence of concentration of sap upon transpira- 



