76 CARNEGIE INSTITUTION OF WASHINGTON. 



suggested in this place that similar relation is probably to be found in 

 Covillea tridentata, the depth of penetration of the roots of which is 

 fairly intermediate between that of the roots of Prosopis and of the 

 other two species mentioned. 



The roots of Prosopis have a comparatively rapid growth-rate and 

 those of Opuntia a comparatively slow growth-rate at relatively low 

 soil-temperatures. For example, at soil-temperatures ranging between 

 12° and 19° C, in 72 hours the root of a young Prosopis increased 22.5 

 mm. in length, while a root of a young Opuntia grew only 6 mm. 

 under the same conditions. Relatively low soil-temperatures, there- 

 fore, do not constitute a factor limiting root-penetration in Prosopis, 

 while they do in Opuntia. A study of the reaction of the roots of 

 Covillea to soil-temperatures shows that the root-temperature relation 

 in this fonn is unlike that in either of the other species mentioned. 



The rate of growth of the roots of Covillea tridentata is relatively slow 

 at all soil-temperatures, particularly at those that are comparatively 

 low. Thus, at soil-temperatures between 15° and 20° C, the average 

 rate of root- growth in young plants is about 0.12 imn. hourly; between 

 20° and 25° C. it is about 0.16 mm. an hour; between 25° and 30° C, 

 the hourly rate is about 0.31, and between 30° and 35° C. it is approxi- 

 mately 1.6 nrni. It appears, therefore, that at parallel soil-tempera- 

 tures the growth-rate of the roots of Covillea is intermediate between 

 that of the roots of Prosopis on the one hand and of Opuntia on the 

 other. It is therefore probable that in the case of Covillea, low soil- 

 temperatures do constitute a factor which limits the penetration of 

 the roots. 



Root-Growth of Introduced Desert Plants at Carmel, by TV. A. Cannon. 



Species native to the daya region of Algeria, the Mohave Desert of 

 CaUfornia, and the vicinity of the Desert Laboratory in southern 

 Arizona have been growing in the experimental plots at the Coastal 

 Laboratory from one to five years for the purpose of placing under 

 observation the effects of the cool, equable chmate of Carmel on their 

 growth, especially on the growth and development of the roots. The 

 plants studied in this connection consisted of Opuntia basilaris and 0. 

 ramosissima from the Mohave, 0. versicolor and Prosopis velutina from 

 Tucson, and Pistacia atlantica from Algeria. 



The three regions from which the species came may be characterized 

 as warm temperate with rain in winter only (Mohave and Algeria), 

 or rain both in winter and in summer (Tucson) . Such climatic condi- 

 tions are to be contrasted with the fairly humid climate of Carmel, 

 with relatively little difference in air-temperature between summer and 

 winter and with rains in winter only. In the regions with winter 

 rains the growing-season is in late winter or early spring, when the 



