68 



ACIDITY AND GAS INTERCHANGE IN CACTI. 



TABLE 43. 



than that of the young plant at 31 to 36 C. The flaccid joints at the latter 

 temperature lose about the same percentage of their fresh weight as do the 

 turgid ones, but their dry-weight loss is considerably less. 



For comparison with these figures it is interesting similarly to calculate a 

 few of the results given by Aubert both of cacti and of some other plants that 

 are more typical. In two cactus forms, Opuntia tomentosa and MamiUaria 

 newmanniana, he finds the loss is respectively 0.14 and 0.08 per cent in a 

 24-hour period. In plants which are not succulents the following figures for 

 the loss in weight by the evolution of carbon dioxide, for a similar period, may 

 be given: Ritinus, 0.08 per cent; Hedera, 0.9 per cent; Triticum, 1.1 per cent. 

 In this connection it may be well to call attention to a mistake Avhich has 

 crept into the citations of the figures from which these calculations have been 

 made. Aubert's own results are given in cubic millimeters per hour, but they 

 have been cited as cubic centimeters". Comparison of Aubert's results with 

 our own indicate that the Opuntia versicolor was far more active than either 

 of the two cacti mentioned above. The evolution of carbon dioxide at 31 

 to 33 C. was equal to that of Ricinus at 20 C. and not so very much less 

 than that of Triticum at 13 C. Since from 31 to 36 C. may be taken as 

 the normal temperature for these cacti in the summertime in their native 

 habitat, it would appear that under their natural conditions and in their 

 normal state of activity cacti are by no means as torpid in regard to the 

 evolution of carbon dioxide as is commonly supposed. 



TEMPERATURE EFFECT. 



For the study of the effect of temperature alone upon the rate and especially 

 the ratio of gas interchange, it is most instructive to take a group of data in 

 which the acidity conditions are similar. Such a one is the series under 

 discussion. In these experiments which were carried on in New York under 

 the equable conditions of the laboratory, not only was the acidity of the samples 



a Pfeffer, W. Physiology of plants. (Translated by Ewart.) Vol. I, p. 525, 1904. The 

 citation in the original German edition is correct. See also Czapek, F., Biochemie der Pflanzen, 

 vol. ii, p. 382, 1905. 



