DEPARTMENT OF BOTANICAL RESEARCH. 83 



It is thus seen that the roots of Prosopis continue growth for a 

 longer time in a soil-atmosphere poor in oxygen, with no carbon dioxide, 

 than is the case in Opuntia. 



The oxygen, as well as the carbon-dioxide response, is thus known 

 to be a specific one, at least as far as Opuntia and Prosopis are con- 

 cerned, and thus it is a matter of ecological importance. The relative 

 indifference of the roots of Prosopis to soil aeration permits them to 

 grow deeply as well as shallowly, and permits the species to range 

 widely as regards soils, although typical development of the species 

 appears to occur along the river-floor plain, where the superficial soils 

 at any rate are of a fine texture and where deep penetration is possible. 

 In a similar manner the relative dependence of the roots of Opuntia 

 versicolor on good soil aeration makes it impossible for the roots of 

 the species to attain to any considerable depth and especially pre- 

 cludes the species from inhabiting soils that possess a fine texture 

 and which in seasons of most active growth are inadequately aerated. 



Modification of Root Habits by Experimental Means, byW.A. Cannon. 



Among the questions associated with a study of root and root 

 responses, the fixedness of habits in roots is a matter that attracts 

 the attention of the worker who concerns himself with experimental 

 modifications. In roots there is an especially good opportunity to 

 observe the immediate influence of the soil environment upon inher- 

 ited tendencies, and certain features of such studies have been reported, 

 from time to time, in the Year Books of the Carnegie Institution of 

 Washington. 



The roots of all species are not equally adjustable to their environ- 

 ment, and such adjustability is directly dependent on soil aeration 

 and temperature. An adequate moisture-supply is a condition sine 

 qua non for root-growth of whatever type, and it does not figure to 

 the same degree or in the same way as the feature mentioned as a 

 character-determining factor. 



So far as the observations show, it appears that roots which are 

 capable of deep penetration of the soil, or that customarily grow in 

 soils of fine texture, are less responsive to relatively small changes 

 in soil aeration and may be less dependent on temperature than is 

 the case with roots that are essentially superficial and which occur 

 in relatively well-drained soils only. However, roots of the former 

 type are apparently not all non-responsive to such changes. For 

 example, Pistacia atlantica, which is native in the dayas of southern 

 Algeria, has been found capable of making a slow growth-rate in the 

 garden of the Coastal Laboratory, although the temperature of the 

 soil is relatively low. In the growth and development of the root- 

 system, of the species in the garden it was observed that a pronounced 

 taproot was at first formed and the formation of laterals was not at 



