ACIDITY DETERMINATIONS. 31 



greatest at 30 C. and least at 40 C. (table 15). According to the figures 

 shown in the. table, mature joints with an average initial acidity of 0.24 per 

 cubic centimeter juice show accumulation of acid after 18^ hours to the 

 following amounts: at 20 C., 1.15; at 30 C., 1.54; at 40 C., 0.62. Similarly 

 the younger parts, which had an average initial acidity of 0.24 per cubic centi- 

 meter juice, showed during the same period the following gain: at 20 C., 1.44; 

 at 30 C., 1.74; at 40 C., 0.75. As with the acidity decrease, the younger 

 tissues are more active than the older ones, but the difference is not very great. 



The results of earlier determinations made in 1911 of the accumulation of 

 acid in mature joints, extending over much longer periods and at greater range 

 of temperature (table 16), are not directly comparable with those just discussed, 

 for in these cases only total acidity and not juice acidity is given. The 

 greatest accumulation of acid took place at 7 C. where, after 96 hours, the 

 acidity was 1.38 per gram fresh weight, or somewhat more than double the 

 initial acidity of the sample. The shorter the period of exposure the higher 

 the temperature at which the maximum is found. After 18 to 24 hours it was 

 at 20 C.; at 31 C. the acidity remained nearly at its initial point, but on the 

 whole it falls the longer the exposure. At 47 C. there is a decrease for every 

 period, and after 96 hours at that temperature the acidity had sunk to half of 

 its original amount. In other words, at low temperatures, while the acid is 

 formed slowly, it is also broken down even more slowly and hence accumulates 

 to a greater degree; while at high temperatures it is formed less rapidly and is 

 broken down faster and hence diminishes instead of increasing. These experi- 

 ments also bear out what has been reported by previous investigators regarding 

 the effect of long-continued darkness. The series at 20 C. is a good illustra- 

 tion. After 69 and still more after 96 hours of darkness at that temperature 

 a decrease of acidity from the point that was attained at 18 hours is shown. 



That the plants are affected by the normal meteorological changes was a 

 matter of common observation. After a cool night in summer, the acidity of 

 the tissues on the following morning was noticeably higher than when the 

 night had been warm. Similarly the effect of relatively low temperatures 

 during the day, not above 27 C., especially when accompanied with a dull, 

 overcast sky, was to maintain a higher degree of acidity than normal. On 

 July 17, 1912, when these conditions were realized, the average acidity of the 

 juice from six different plants at 1 p. m. was 1.23 per cubic centimeter; the 

 same plant a few days later, when the sky was clear and the temperature 

 much higher, gave an average acidity of only 0.65 at the same hour. Again 

 on July 24, when the day had been relatively cool and dull, three plants gave 

 an average acidity of 1.15 per cubic centimeter of juice at 4 o'clock in the 

 afternoon, the hour at which acidity is usually at itsminunum; subsequently, 

 on a bright, warm day the acidity was only 0.32 at the same time. From these 

 data it appears that the acid-content of the plants is sensitive to the immediate 

 fluctuations in the external conditions and that induced after-effects of the 

 normal periodic changes are not to be anticipated. None of the experiments 

 exposed for long periods in the dark at constant temperatures shows any indi- 

 cation that there is a persistence of periodicity in acidity beyond the immediate 

 effect of the factors which caused it. Special experiments to reveal any 

 derived periodicity did not in any case indicate diurnal oscillations other than 

 those caused by illumination and temperature. Material of low acidity, when 



