DEPARTMENT OF BOTANICAL RESEARCH. 75 



estimating their influence as determinants of the root-system types 

 themselves. It is beheved that such unHke root development is not 

 directly due to differences in the soils or variation in the soil-moisture 

 as the various root-types may be developed in soils that are uniform 

 and suitably moist throughout. It is concluded, however, that the 

 widely different root-types are in the main the direct response of the 

 roots to temperature, and to a less degree perhaps to the variation in 

 the composition of the soil-atmosphere wliich is associated with 

 differences in depth. To test the latter hjrpothesis, experiments have 

 been carried out on the roots of Opuntia and Prosopis, in which they 

 were exposed, at known soil- temperatures, to atmospheres of known 

 but different composition. The atmospheres used were composed of 

 pure carbon dioxide, or of carbon dioxide to which atmospheric air 

 had been added in known amounts. 



The experunents showed that the composition of the soil-atmosphere, 

 as well as the length of the exposure, were alike important. Exposure 

 of both Prosopis and Opuntia for a period of 15 minutes to pure car- 

 bon dioxide did not alter the growth-rate as observed hourly. An 

 exposure of the roots of both species to pure carbon dioxide to periods 

 over 30 minutes, however, inhibited growth during the time of the 

 exposure. Exposures were made up to 3 hours. Growth of the roots 

 was renewed soonest after the shortest exposures, sooner in Prosopis 

 than in Opuntia, and sooner in both species at high than at low soil- 

 temperatures. In general, a longer time was required to bring about 

 cessation of growth in the atmospheres with an admixture of air than 

 in oxygen-free atmospheres, and recovery was soonest in atmospheres 

 most rich in oxygen. Also, the roots of Prosopis recovered their usual 

 growth-rate sooner than did those of Opuntia. It would appear, 

 therefore, from the experiments, that the response of the roots of 

 Opuntia to a diminished oxygen-supply of the soil is such as would 

 tend to prevent deep penetration, and thus to support the effects of the 

 relatively high temperature characteristic of the upper soil-layers in 

 bringing about the formation of a superficial type of root-system. 

 On the other hand, the reaction of the roots of Prosopis to a diminished 

 oxygen-supply in the soil-atmosphere indicates that poor aeration, or a 

 comparatively small proportion of oxygen in the soil, does not operate 

 as a factor limiting the penetration of the roots of the species. 



Rate of Root-Growth of Covillea tridentata in relation to the Temperature of the 



Soil, by W. A. Cannon. 



It has already been shown (Carnegie Institution Year Book for 

 1914, p. 93), that there appears to be a causal relation between the depth 

 of root penetration in Prosopis velutina, Fouquieria spendens, and 

 Opuntia versicolor and the response of the roots of these species (as 

 shown by the rate of growth) to the temperature of the soil. It will be 



