62 CARNEGIE INSTITUTION OF WASHINGTON. 



As was previously established, low water-content of the plants results 

 in a condition of general reversion of the carbohydrates to polysac- 

 charides, while low temperatures result in a general inversion to 

 monosaccharides. The continued dry weather kept the monosac- 

 charide content unusually low for that season, until the cold nights 

 of the late winter months effected a change, in that in spite of the 

 low water-content the monosaccharides rose, and finally the late 

 winter rains brought conditions of the carbohydrate equilibrium about 

 to normal. It would seem that the water requirements of the cacti 

 are easily satisfied, as in spite of the fact that the winter rainfall was 

 very light, the water-content of the plants thereafter was about 

 the same as during the same season of normal precipitation. 



Special experiments on the effect of water-content and temperature 

 on the carbohydrate equilibrium were repeated under improved con- 

 ditions. The results obtained have substantiated and somewhat elabo- 

 rated the former findings, which were given in the report of last year. 



An effort has been made to coordinate the results obtained on the 

 rate of respiration with the carbohydrate economy and the conditions 

 influencing it. No simple or constant relation exists between the 

 rate of carbon-dioxide elimination and the supply of carbohydrates; 

 at least no method has been found of calculating the values obtained 

 which permits of any such interpretation. Nor can the rate of respi- 

 ration be constantly or definitely associated with any one group of 

 carbohydrates. It has been commonly assumed that the mono- 

 saccharides or disaccharides play this role. However, it has been 

 found that the content of the plant of the sugars may reach a point 

 where a further increase is without effect upon the rate of respiration, 

 while also a decrease in the content of these sugars below a certain 

 point does not result in diminished carbon-dioxide emission. The 

 limits of the range of content of these sugars which directly influence 

 respiration are not fixed, but undoubtedly represent a combination of 

 conditions intimately associated with a complex in which the protein- 

 content and certain enzymes are controlling factors. 



These considerations are of particular interest in the carbohydrate 

 economy of the cacti in that under natural conditions the monosac- 

 charides and disaccharides are present at times only in very small 

 amounts. In spite of this condition, the rate of respiration does not 

 show any marked diminution. It would seem, therefore, that under 

 stress the plant possesses the power of utilizing the polysaccharides 

 and aplastic material. An indication of this was already obtained by 

 MacDougal, Long, and Brown in the study of cacti which had been 

 starved for a long time. 



The formation of pentosans is intimately associated with this con- 

 dition. As has been previously reported, the simpler sugars, or 

 monosaccharides, decrease in amount in the plants as the water- 



