GAS INTERCHANGE. 65 



abundance of leaves. Their acidity, it is true, was usually lower than that 

 of young material from more turgid plants, with the consequential effect 

 on their -Q-* ratio. In a measure they constituted a condition intermediate 

 between the young and the old stems. 



Thus it will be seen that there must be expected a very considerable variation 

 in the condition and consequent behavior of cacti from season to season and, 

 indeed, as experience has shown between different plants in the same season. 

 It becomes imperative, therefore, to have a large number of experiments 

 in order to establish any kind of a reliable average, a fact that is additionally 

 true in the case of such physiological processes as those involved in gas inter- 

 change, which are also so greatly influenced by relatively small changes in 

 the external conditions. The attempt has been made to accumulate a con- 

 siderable body of data, which, while relatively large, would no doubt be more 

 satisfactory if doubled. 



Not every one of the experiments carried out has been incorporated in the 

 account given here. As has been said in the chapter on experimental methods, 

 all experiments were thrown out in which the analysis showed that more 

 than 5 per cent carbon dioxide was accumulated, with the exception of a few 

 selected ones, retained for special reasons and so indicated. For various 

 causes a number of other experiments were also neglected, especially in the 

 beginning, but none has been abandoned except for well-defined and cogent 

 reasons which rendered their results untrustworthy. 



In connection with the conditions of dryness and turgidity, it is interesting 

 to note a possible relation of the color of the plant. In almost all cases record 

 was made of the color of the exterior of the joint, which varies from the typical 

 somewhat bluish green to a reddish tint that shades to a yellow brown in the 

 grooves and on the under side of the joint. When these notes were com- 

 pared with the results of the acidity determinations and the gas interchange it 

 did not develop that there was any very definite or marked relation to be found 

 between them. In a general way, the drier the plants are the more likely 

 is their color to be red, but except in so far as the dry condition affects both their 

 acidity and rate of gas interchange, which is lower in each case, there is no 

 close correlation. For instance, in those cases where the mature and also 

 turgid joints were red which while not common is also not infrequent there 

 appeared no regularity as to a greater or lesser quantity of acid or to an in- 



f^O 



creased or decreased -Q 1 quotient. The young joints also showed a reddish 

 tinge at times, notably those mentioned as having been used in the summer of 

 1913, but except for their somewhat lower activity there was no effect to be 

 correlated with color. 



However, it is probable that redness of color is generally associated with 

 loss of water by the tissues. It would be interesting to investigate this point, 

 as the formation of anthocyan in chlorophyll-bearing tissue is usually held to 

 be correlated with low temperatures. Of course, one effect of cold is to deprive 

 the plant of water and the question may be asked whether desiccation due to 

 whatever circumstance is not a predisposing cause for the formation of antho- 

 cyan compounds. From observations made, which were necessarily incom- 

 plete, it seems improbable that the reddening of the Opuntia versicolor joints 

 is connected with the lower winter temperatures, for in March and April, 



