EXPERIMENTAL METHODS. 



21 



material from which the gas-interchange ratio was obtained. It may be added 

 that the consistency of the results among themselves was satisfactory evidence 

 of the sufficiency of this method. 



For purposes of experiment three classes of joints were distinguished, as 

 follows: young, mature-turgid, and mature-flaccid. Young joints were those 

 just formed or forming, very turgid and possessed of leaves. Mature-turgid 

 joints were those just back of the young joints or the terminal ones before the 

 young joints were formed. They were presumably of the previous year's 

 growth, highly turgid, and during the rainy season often showed the very 

 characteristic longitudinal fissures, lightly healed with periderm-like tissue. 

 Mature-flaccid joints were those of age similar to the last, but from plants that 

 had not recently had any water-supply, that is, in the condition that obtains 

 before the summer rains. Joints older than these were not used, partly 

 because they commonly become too large and partly because the amount of 

 woody tissue increases. For the sake of brevity, these two kinds of mature 

 joints hereafter will be designated simply turgid and flaccid, it being under- 

 stood that the young ones are necessarily turgid. 



In choosing material for experimentation, it was necessary always to exam- 

 ine it for the grubs of insects that frequently burrow into the tissues. They 

 can usually be detected with ease from the outside, but as an additional pre- 

 caution the joints were inspected when they were split preparatory to juice 

 extraction, after they had served for gas-interchange or respiration observa- 

 tions. Needless to say, if any contaminations of this kind were found the 

 whole experiment was discarded, but as a matter of fact only once or twice was 

 it necessary to do this. 



TABLE 2. Acidities of tissues of different ages. 



The young joints were generally more turgid and showed higher acidity and 

 greater oscillation thereof than did the old ones. As has been said, the mature 

 joints were presumably of the previous season's growth, but it is not always 

 easy to determine the age of a joint from its external appearance. Commonly 

 a terminal joint is but one season old, but if the plant put forth no new 

 growth during the previous growing period the distal one may be at least two 

 years old. Since this is the case, tests were made to determine how greatly 

 age affects acidity. The results show that the difference in this regard is 

 very small as between the 1 year and 2-year-old shoots, but begins to show 

 in the third year. Even this applies only to total acidity, for the juice differs 

 in strength very little. It is no doubt owing largely to the increase in the 

 heavy mechanical and vascular tissue that the total acidity decreases with 

 age, for the actual amount of succulent parenchyma must be relatively smaller 

 in a given weight of old joints than in younger ones. The averages given in 

 table 2, from the experiments set forth in table 22, show that no serious error 

 could have resulted if occasionally 2-year-old joints were used instead of those 

 but a year in age. It is safe to say that no material older than this was used. 



