in the notoriously impure water of the River Thames.  Sachs, in 1859, carried on an elaborate series of experiments on transpiration in which he showed that the addition of potassium nitrate, ammonium sulphate or common salt to the solution in which plants grew reduced the transpiration; in fact, the reduction was large, varying from 10 to 75 per cent.  This was confirmed by a number of later workers, among them, for instance, Buergerstein, who, in 1875, showed that whenever acids were added to a soil or to water in which plants are growing, the transpiration is increased greatly; but when alkalies of any kind are added, transpiration decreases.  This is of special interest in the development of dry-farming, since dry-farm soils, as a rule, contain more substances that may be classed as alkalies than do soils maintained under humid conditions.  Sour soils are very characteristic of districts where the rainfall is abundant; the vegetation growing on such soils transpires excessively and the crops are consequently more subject to drouth.

The investigators of almost a generation ago also determined beyond question that whenever a complete nutrient solution is presented to plants, that is, a solution containing all the necessary plant-foods in the proper proportions, the transpiration is reduced immensely.  It is not necessary that the plant-foods should be presented in a water solution in order to effect this reduction in transpiration; if they are added to the soil on which plants are growing, the same effect will result.  The addition of commercial fertilizers to the soil will therefore diminish transpiration.  It was further discovered nearly half a century ago that similar plants growing on different soils evaporate different amounts of water from their leaves; this difference, undoubtedly, is due to the conditions in the fertility of the soils, for the more fertile a soil is, the richer will the soil-water be in the necessary plant-foods.  The principle that transpiration or the evaporation of water from the plants depends on the nature and concentration of the soil solution is of far-reaching importance in the development of a rational practice of dry-farming.

Transpiration for a pound of dry matter

Is plant growth proportional to transpiration?  Do plants that evaporate much water grow more rapidly than those that evaporate less?  These questions arose very early in the period characterized by an active study of transpiration.  If varying the transpiration varies the growth, there would be no special advantage in reducing the transpiration.  From an economic point of view the important question is this:  Does the plant when its rate of transpiration is reduced still grow with the same vigor?  If that be the case, then every effort should be made by the farmer to control and to diminish the rate of transpiration.