362 



SCIENCE 



[N. S. Vol. XLIII. No. IIOS 



pacity of the internal tissue controls the second- 

 ary absorbing power of the roots and probably 

 also the transpiring power, since a greater absorb- 

 ing capacity would mean also a greater resistance 

 capacity to the loss of water. It seems quite pos- 

 sible that in the forenoon the increasing absorbing 

 capacity of the internal tissues may take water 

 from the guard cells, thus causing them to close, 

 and at night the decreasing water-holding ca- 

 pacity may allow the guard cells to take water 

 from the internal tissues, and thus open. The ef- 

 fects exerted by light intensity and air tempera- 

 ture, together with their duration, show that the 

 variations in absorbing capacity are due, at least 

 in part, to chemical changes brought about by the 

 metabolic processes. Many tests showed that the 

 changes in the water-absorbing capacity of the 

 tissues parallel acidity changes in the plants in 

 such a way that when the acidity is highest the 

 absorbing capacity is lowest, and vice versa. How- 

 ever, certain exceptions which occur under con- 

 trolled conditions show that the relation can not 

 be so simple as the influence of mere changes of 

 H-ion concentration. Consequently other factors 

 must be taken into consideration, including the 

 accumulation and disappearance of the salts of 

 organic acids. It is impossible to state yet 

 whether the absorbing capacity of the internal 

 tissue is due to colloidal absorption or to osmotic 

 forces or to both. Although the concentration 

 changes in the total sap which are indicated by 

 the- known changes caused by the metabolic 

 changes and by the changes in water-content seem 

 to show that osmotic forces are not of prime im- 

 portance in controlling the water-absorbing ca- 

 pacity of tissues, still there is no direct proof that 

 it is colloidal absorption which is the sole or the 

 controlling force. 



Physiological Temperature Indices for the Study 

 of Plant Growth in Helation to Climate: Bnu- 

 TON E. Livingston. 



The aim of this study is to obtain indices of 

 temperature efficiency for plant growth, by means 

 of which temperatures on the thermometer scale 

 may be weighted in evaluating the temperature 

 term of the environmental complex in physiolog- 

 ical, agricultural, forestal and ecological investi- 

 gation. The system of indices here brought 

 forward is based on the results of Lehenbauer's 

 recent study of the relation of temperature r.o 

 growth of young maize shoots exposed twelve 

 hours to maintained temperature. Lehenbauer's 

 graph is first smoothed mechanically, and then the 



ordinates for each degree of temperature are 

 measured. These are all expressed in terms of the 

 ordinate for 40° F. (4.5° C.) taken as unity. 

 The index values increase from zero (2° C.) 

 through unity (4.5° C.) to 122 (32° C), and then 

 decrease again to zero (48° C). This is the first 

 system of temperature efficiency indices that takes 

 account of the optimum and maximum in the 

 growth-temperature graph. A chart of the cli- 

 matic zonation of the United States with respect 

 to temperature efficiency for plant growth during 

 the period of the average frostless season is 

 shown, based on the new indices. 

 A Single Climatic Index to Bepresent Both Mois- 

 ture and Temperature Conditions as Belated to 

 Plants: Burton E. Livingston. 

 A method is described by which the indices of 

 precipitation, of atmospheric evaporating power, 

 and of temperature efficiency for plant growth, 

 for any period of time, may be combined into a 

 single index of moisture-temperature efficiency. 

 The new index is the product of the rainfall-evapo- 

 ration ratio and the summation index of tempera- 

 ture efficiency for the period. The method is only 

 a first approximation and improvements are of 

 course to be expected. By means of these mois- 

 ture-temperature indices a new climatic chart of 

 the United States is constructed, for the period of 

 the average frostless season. This chart shows 

 that, as far as moisture and temperature condi- 

 tions are concerned, peninsular Florida possesses 

 the best climate for plant growth, while the least 

 efficient climates of the country are those of the 

 extreme north and of the arid regions. The nat- 

 ural climate of the arid regions is modified hy 

 irrigation, and that of the cold regions is modi- 

 fied, on a small scale, by artificially heated green- 

 houses. 



A Living Climatologieal Instrument: B. E. Liv- 

 ingston and F. T. McLean. 

 While methods for interpreting instrumental rec- 

 ords of climatologieal conditions are being devised, 

 climates may be studied and compared in terms of 

 their actual effectiveness in promoting growth of 

 standard plants. In the first trial of this method 

 the plants were grown in plunged pots, always 

 filled with the same kind of soil, which was re- 

 newed after each culture. Ten different stations in 

 Maryland were employed. Soy bean proved very 

 satisfactory as a standard plant. Seeds were 

 soaked in water at a given temperature for a cer- 

 tain time before planting. Various measurements 

 were made on the plants after two weeks and 



