22 



ACIDITY AND GAS INTERCHANGE IN CACTI. 



The dry weights of the material show that the manner of selection resulted 

 in a pretty constant type of tissue, as they did not vary in any class over a 

 great range, at least not in any one season. The turgid joints averaged 

 about 20 per cent dry substance, varying from 16 to 22 per cent in a series 

 taken at random. The flaccid joints showed, as would be expected, a much 

 greater percentage of dry weight, the average of all the determinations made 

 being 29 per cent. For the young joints, which are by far the most succulent 

 of all, the amount of dry substance is lowest, a little over 16.5 per cent. 

 However, in discussing these general averages, which include material taken 

 during the summers of 1912 and 1913, attention should be directed to the fact 

 that the amount of solid material in proportion to the fresh weight was not the 

 same both seasons as the table below shows. The winter of 1911-12 was a 

 normal one, both as regards precipitation and temperature; and the summer 

 rains, which began in July, while somewhat below the average, were fairly 

 copious. Not so in the following season, when the winter temperatures were 

 at times abnormally low and the rains of both the winter and the following 

 summer were very light. As a result of these conditions the flaccid-mature 

 joints in 1913 showed an increase of nearly 6 per cent in dry weight over those 

 used during the previous year. The young joints also contained a greater pro- 

 portion of dry substance, but the gain was less, being only about 3 per cent. The 



TABLE 3. Average per cent of dry weight of material in all gas-interchange 



experiments in 1912-1913. 



'Excluding the one instance mentioned above, where it was 9.7 per cent. 

 2 These were from irrigated plants. 



old turgid joints were about the same, but they do not come into consideration 

 in this connection, as the turgidity in all cases in 1913 was attained by irri- 

 gation. In the one instance in 1913, where young joints were collected from 

 irrigated specimens, the percentage of dry substance was the lowest ever 

 observed, namely, 9.7 per cent. 



It was found that at the time of year when the flower-buds are forming, 

 the acidity of the joints on which they appear is higher than usual and fluc- 

 tuates irregularly, and consequently such joints were avoided. This applies 

 only to the experiments carried on in the spring of 1911, since all of the other 

 work was done in the summer long after the flowers had disappeared. 



In the plants of Opuntia versicolor there is a considerable range of color, 

 from a clear, somewhat bluish green throughout to a decidedly reddish tint 

 on the upper exposed side, with a lower side green or brownish yellow. Gener- 

 ally the red color is characteristic of flaccid plants, but not always by any 

 means. Casual observation shows that many plants which are distinctly red 

 during the dry period lose a great part of that color when the rains come, but 

 some of the reddest plants have been found among those which are appar- 



a Experiment 41, table 63. 



