STOMATA AND THE HYDROLOGIC CYCLE

Evaporation and precipitation over land and sea, connected by streams and atmospheric exchange, constitute the vast hydrologic cycle. To alter the course of this great process, mall must apply his small efforts at a strategic juncture in some major component of the cycle. For example, the rainmaker hopes to change the income from rain by adding a few but sufficient crystals to clouds. Outgo, however, is our concern in this paper. We hope to show a modification of the hydrologic cycle caused by a decrease in the evaporation from plants. Certainly this evaporation is a major component of the cycle, for two thirds of the rainfall on this nation returns to the atmosphere by evaporation, mostly from leaves. The strategic juncture is provided by the microscopic pores, or stomata, in the leaves through which the transpired water diffuses. Here we report that spraying about 3 kilograms of substance per hectare upon 16-meter pine trees shrank their stomata enough to decrease significantly the evaporation portion of the hydrologic cycle locally. Before reporting the experiment, the nature of stomata, their role in evaporation, and means for shrinking them may warrant review. In red pine (Pinus resinosa Ait.) needles, stomata provide 5000 holes/cm2 (Fig. 1). Closure occurs at the bottom of a pit about 20 ,u deep when walls of the guard cells press together. When these walls draw apart, a pore as wide as 11 , may open (Fig. 2). Countless variations on the number and dimensions of the pores are, of course, found. Through these pores passes the carbon dioxide required for photosynthesis. When the pores have been shrunk, however, the loss of water has decreased relatively more than the gain of carbon dioxide.1 Different opinions still are held concerning the effect of stomuatal size upon evaporation from even a single leaf. Fortunately, the discovery of chemicals2 that can narrow the stomata without damaging the leaf now permits controlled experiments. These have clearly demonstrated that evaporation from a single well-ventilated leaf is decreased by stomatal shrinkage throughout the range of stomatal widths.1 Numerous substances will close stomata. Several do so by altering the ability of the stomatal guard cells to accumulate water and to open the pores.3 One of these, phenylmercuric acetate, is outstanding because it lasts and because it acts at low concentrations.3 Also, phenylmercuric acetate has been shown to decrease transpiration of small isolated conifers.4' It was therefore chosen for the experiment. The goal of the experiment was, of course, to test whether stomatal shrinkage or even natural differences among stomata affect the hydrologic cycle, not to test a particular substance that is a model of other substances. Thus, at the outset of the experiment, we had a means of shrinking stomata that had decreased the evaporation from a single well-ventilated leaf in a chamber and from small isolated trees. Things are different, however, in a forest. There evaporation is often limited by the supply of energy or by ventilation,6 and a multitude of leaves cover each land area. The following paragraphs report whether the