64 CARNEGIE INSTITUTION OF WASHINGTON. 



lie close to the surface of the ground. Studies on the possible imme- 

 diate causes of such characteristic differences in root habit indicate 

 that the temperature and the aeration of the soil may both play impor- 

 tant roles. For example, the roots of Prosopis velutina and of Opuntia 

 versicolor show unlike temperature relations. The roots of Prosopis, 

 in short, react to lower temperatures than do those of Opuntia. 

 Further, it appears from thermographic records that the temperature of 

 the soil at a depth taken by the roots of the former species does not 

 suit the more shallowl}^ placed roots of Opuntia. It is concluded from 

 these observations that the vertical distribution of the roots of these 

 species is directly related to the vertical differentiation of the tempera- 

 ture of the soil. It appears possible, however, that the relation to 

 soil aeration of shallowly placed roots may differ from that of roots 

 that penetrate deeply. To test this possibility and to determine the 

 immediate effect of a variable air-supply on the rate of root-growth, 

 several experiments have from time to time been carried out. The 

 results, which have not been entirely consistent, may be briefly given. 

 Increased shoot and root growth followed artificial aeration of the soil 

 in certain instances, while in others the aeration appeared to produce 

 no effect. When, however, the roots of Opuntia were kept at a constant 

 temperature, the results with this species were more consistent. For 

 example, in one series of experiments, the roots of Opuntia were held 

 at a temperature of 32° C. for 8 hours, during which time a slow current 

 of air passed through the culture in 2-hour periods, with the alternating 

 2 hours in which the roots were not aerated. At the same time obser- 

 vation was made as to the rate of root-growth of a control of the same 

 species which was not aerated. The leading results, in brief, were as 

 follows: the average rate of root-growth for 2-hour periods while 

 aerated, average of 13 observations, was 1.59 mm.; the average rate 

 of root-growth of the same root not aerated was 1.25 mm.; the average 

 rate of root-growth of the control was 1.3 mm. It would therefore 

 appear that artificial aeration shghtly increases the amount of root- 

 growth of Opuntia. Further observations on the reaction of the roots 

 of Opuntia, not here reported, indicate that an insufficient amount 

 of air may inhibit root branching. It seems probable, consequently, 

 that the superficial root habit assumed in Opuntia is due in part to 

 the favorable soil temperature, and in part to the relatively favorable 

 conditions of soil aeration. 



Periodic Variations of Respiratory Activity, by H. A. Spoehr. 

 The investigations in this subject have been continued with special 

 attention to controlling temperature. The plants used for study were 

 kept in an electrically controlled water thermostat, so that the tempera- 

 ture of the air surrounding the plants did not range more than 0.02° C. in 



