82 ACIDITY AND GAS INTERCHANGE IN CACTI. 



at individually or in the mass, it would appear, then, that there can be no 



c*o 

 question as to the relation of the course of acidity and the -Q ratio. Of 



course individual experiments occasionally depart from the general rule; for 

 internal causes, concerning which we can have no information, may, no doubt, 

 influence either the exhalation of carbon dioxide or the absorption of oxygen or 

 both, and so affect the ratio in its relation to the acidity. However, the 

 results agree in the main and show unequivocally that the above generali- 

 zation is sound. 



The very high ratio and exceedingly rapid gas interchange shown in 43a and 

 44a (table 60) have no great significance. In both cases, despite the relatively 

 short period of the experiment, a very large amount of carbon dioxide accumu- 

 lated in the flask, amounting to more than 12 per cent. As has been shown, 

 the carbon dioxide-oxygen relation is disturbed when the former gas collects 

 so largely. The experiments are included simply to show the effect of a long- 

 continued application of a relatively high temperature. 



EVOLUTION OF CARBON DIOXIDE IN DIFFUSE AND DIRECT SUNLIGHT. 



Because of the highly interesting results obtained by the Pettenkofer method 

 to determine the formation of carbon dioxide in diffuse light and in direct 

 sunlight, a considerable number of experiments were made under these con- 

 ditions to ascertain the complete gas-interchange relations. 



In the table giving the results of the experiments with diffuse light (table 62) 

 it will be seen that the amount of carbon dioxide given off is not very different 

 from that evolved in the dark. There is a tendency for the gas ratio, as it 

 shows in the analysis of the atmosphere surrounding the plant, to rise some- 

 what. This is probably due to the use of oxygen formed in the tissues by the 

 photosynthetic processes, for the higher ratio appears to be due rather to a 

 lesser absorption of external oxygen than to an increase in the formation of 

 carbon dioxide. In the two experiments 45a and 46a (table 62) there was 

 almost no oxygen absorbed, in consequence of which the ratio rose in one case 

 to 7.00 and in the other to 6.33. 



It would be a point of the highest interest to determine whether this phe- 

 nomenon can be observed at all seasons of the year, or is, perhaps, only char- 

 acteristic of the period of the summer rains, when the temperature is high and 

 the fully turgid plants are rich in acid. It would also be important to ascer- 

 tain the effect of lower temperatures and whether under such conditions the 

 photosynthetic activity was less influenced than the processes which result in 

 the liberation of carbon dioxide. It is not at all impossible that under low 

 temperatures the amount of this gas evolved would be so reduced that it 

 would all be consumed by the active chlorophyll-bearing tissue. The low 

 temperature series (48 to 52, inclusive, table 62) was carried on with material 

 that was really in a flaccid condition, although it appeared nearly turgid to 

 the eye. The percentage of dry weight was in the neighborhood of 30. Also, 

 they were not carried on at Tucson, but at Carmel, California, from material 

 which had been shipped from Arizona some weeks previously. The light 

 conditions were not comparable with those in Tucson, but of much less inten- 

 sity, although described as the " bright diffuse light" of that seacoast region. 

 Consequently, these experiments are not directly parallel with the others 



