DEPARTMENT OF BOTANICAL RESEARCH. 65 



blue rays are more intense at the Desert Laboratory (2,700 feet) than on 

 Mount Lemmon (9,000 feet). 



Acidity, Gaseous Interchange, and Respiration of Cacti, by H. M. Richards. 



In continuation of the investigation on the respiration of cacti and of the 

 phenomena related thereto, a considerable number of series of determina- 

 tions of carbon-dioxide evolution by Opuntia versicolor under various ex- 

 ternal conditions were made during the winter of 1911-12 in New York, and 

 exclusive attention was given to this work at Tucson, July and August 191 2. 



The effects of temperature changes were studied somewhat extensively. 

 At temperatures above 50° C. the respiration curve falls rapidly and the 

 maximum appears to lie between 45° and 50° C. At low temperatures, in 

 the neighborhood of 10° C, the CO2 evolution almost ceases. The range 

 from 10° to 50° C, while great, is not far from that to which the plant 

 might be subjected during the year. The daily course of respiration under 

 temperature conditions representing an average Tucson day of the late 

 spring was determined at hourly intervals over the whole 24 hours. The 

 most interesting fact which developed was the lag in response to temperature 

 changes. This amounts to about an hour both at the maximum and mini- 

 mum. Other experiments were also tried to further establish this and, as 

 might be expected, it was found that where a considerable change, say of 

 10°, was made suddenly the lag in response was greater than in the cases 

 cited above where the rise or fall had been gradual. 



As a check on all the respiration determinations the wound reaction was 

 studied, since it is necessary to mutilate the plants to the extent of separating 

 the joints. There is no appreciable increase in COo evolution until the 

 wounding has been considerable, as when the joints are slit lengthwise or 

 cut into five or six pieces transversely. In such cases the usual traumatic 

 response is to be noted. The maximum is attained about 24 hours after 

 wounding and subsequently the respiration returns to approximately the 

 normal in about 3 to 33^ days. No appreciable increase follows the mere 

 breaking of the joints apart, certainly not when the small wound surfaces 

 were properly protected with clay or vaseline. Intramolecular respiration in 

 atmospheres of both pure hydrogen and pure nitrogen is considerable and at 

 low temperatures practically the equivalent of the normal evolution of CO2. 



Coincident and parallel with these experiments, series were run for the 



COo 

 determination of the . -" equivalent. The work in gas interchange was 



done by Miss M. E. Latham, both in New York and Tucson. Not all of 

 the very considerable data gathered have been reduced ; but the calculations 

 show that there is an increased oxygen consumption in comparison Avith 

 carbon-dioxide evolution as the temperature decreases. At 40° C. it is more 



nearly _, but below 20° C. the carbon-dioxide evolution rapidly decreases 

 I 



while the oxygen consumption remains nearly stationary. 



5— YB 



