Page:Popular Science Monthly Volume 79.djvu/547

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543
DESERT PLANTS

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 extracting a certain measured amount of living material which has been crushed with distilled water and after the freezing point of this extract 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

PSM V79 D547 Creosote bush covilla larrea.png

Fig. 1. Covillea (Larrea), the Creosote Bush, the most widely distributed Shrub 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 atmospheres. 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 contact 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 misapprehension as to the influence of concentration of sap upon transpira-