GAS INTERCHANGE." 



GENERAL CONSIDERATIONS. 



The problem presented in the study of the interchange of carbon dioxide and 



CT) 

 oxygen and the -Q- Z quotient, particularly in relation to the acidity conditions, 



is a complex one and some preliminary considerations are necessary before any 

 detailed discussion is attempted. 



Attention has been called to the fact that acidity and the rate of respiration 

 are very greatly influenced by the age and condition of the tissues. The 

 former is highest and the latter most rapid in the young joints, which are, 

 of course, highly turgid. Other things being equal, the acidity is lower and the 

 respiration somewhat less in amount in the mature joints that are turgid, 

 which is the condition ordinarily presented in the winter and summer rainy 

 seasons. In the mature joints which are flaccid, by reason of the loss of water 

 in the dry seasons, both the acidity and evolution of carbon dioxide are at 

 their minimum. The distinguishing of these three conditions of the plant, 

 the young, the mature-turgid, and the mature-flaccid, becomes of especial 

 importance. It is, of course, obvious that there can be no sharp line between 

 these stages so far as the life of any one individual is concerned; but in the 

 actual conditions presented by the plants at the time of year when most of 

 these experiments were carried on, the three classes of tissue were sharply 

 enough marked. A study of the transitional stages in the physiological 

 activity would, no doubt, be of considerable interest, although in this work no 

 definite attempt was made in this direction ; but, as will be shown later, the 

 examination of the data revealed certain instances of intermediate conditions. 

 For example, in many of the experiments on the mature joints which were 

 carried on during the summer of 1913, except those which were made from 

 especially irrigated plants, it was found, when the dry weights of the specimens 

 were determined, that they were essentially in the flaccid class, despite the 

 fact that some of them were putting forth new growth. It was also found that 

 this new growth, though in many respects in the typical young condition, was 

 by no means as high in water-content as the joints obtained from plants more 

 liberally supplied with water. 



The basis of separation for purposes of discussion and classification is made, 

 then, on three conditions of the tissue, determined in the main by the percent- 

 age of dry weight, although other characters occasionally have influenced the 

 decision into which group the material should be assigned. The young joints, 

 with the exception of those referred to above, averaged in these experiments 

 between 15 and 17 per cent of dry substance; the mature-turgid joints from 

 18 to 20 per cent, while those which were flaccid were all over 25 per cent, 

 running up in some cases to as much as 35 per cent of dry weight. 



The young joints are undoubtedly the most active in respiration, and prob- 

 ably also the most active in photosynthesis, but they are not necessarily the 

 most important part of the plant as far as nutrition is concerned. To begin 

 with, the actual amount of young growth in proportion to the mature is 

 relatively small, even in a plant which is growing rapidly, and it is not probable 

 that it keeps up its increased activity beyond the beginning of the dry after- 



"This portion of the work was done in collaboration with Miss M. E. Latham. 



63 



