62 CARNEGIE INSTITUTION OF WASHINGTON. 



of efficiency indices here emploj^ed indicates that the growth-rate is 

 the same for a temperature of 40° as for one of 116°, and that it is about 

 122 times as great for a temperature of 89° as it is for one of 40° F. 



A Sim-pie Climatic Index, by B. E. Livingston. 



Various studies heretofore reported, bearing on the relation of 

 climatic conditions to plant growth, have been aimed toward the expres- 

 sion of all climatic influences, for any given station, by a single numeri- 

 cal value. While this aim is a sort of ideal abstraction that must long 

 remain quite out of reach, it appears that progress enough has now 

 been made toward the dynamic analysis of plant relations, so that a 

 first tentative approximation may be proposed. The present proposal 

 involves the use of the precipitation-evaporation ratio of Transeau 

 (which may be taken as a roughly approximate index of the water- 

 relations of plants, so far as these are determined by cUmatic conditions) 

 and the summation of the daily physiological indices of temperature 

 efficiency (which is similarly considered as a measure of the tempera- 

 ture condition) . The precipitation-evaporation ratio is determined for 

 the period of the average frostless season, and this value is multiplied 

 by the temperature efficiency summation for the same period. The cli- 

 matic product thus derived is proposed as an approximate index of the 

 normal climatic efficiency for plant growth, for the station in question. 

 This value may be derived for a large number of stations and a climatic 

 chart may be constructed from the data thus obtained, and such a 

 chart exhibits several fundamental features not shown by the charts of 

 any of the simple chmatic indices. From the method of derivation 

 it is seen at once that this compound climatic index becomes larger 

 (1) with increase in the values of the normal daily temperature efficiency 

 indices for the average frostless season; (2) with increase in the length of 

 the average frostless season; (3) with increase in the total precipitation 

 for the average frostless season, and (4) with decrease in the total evapo- 

 ration for that season. Conversely the value in question becomes smaller 

 with lower daily efficiency indices, with a shorter average frostless sea- 

 son, with decreased rainfall, and with increased evaporation. According 

 to this compound climatic index, northern humid and southern arid 

 regions are represented as lying in the same cUmatic zone. 



Root Growth of Opuntia versicolor at Constant Soil Temperatures, 

 by W. A. Cannon, 



The temperature of the soil changes relatively slowly during the 

 course of the day, and also the range is comparatively small, at the 

 depths attained by the roots of most perennials of the Tucson region, 

 including the cacti. Both of these conditions vary with the depth, 

 among other features, and are accentuated in seasons of cloudiness and 

 storm. In order to estimate the actual root growth of a plant, there- 



