DEPARTMENT OF BOTANICAL RESEARCH. 55 



of the essential features of the earUer stages of the metabolic activity 

 of cacti is still very incomplete. In studying the various vital pro- 

 cesses of these plants, it is possible to deal with the visible products, 

 and these products afford certain proof of and insight into the metabolic 

 activity, although the exact nature and causes of these changes may be 

 still undetermined. 



The carbohydrates predominate in the general food economy of the 

 cacti. It was necessary to develop accurate methods to determine 

 various groups of sugars of different physiological significance. Opun- 

 tia blakeana and 0. discata have been most generally used, and analyses, 

 together with determinations of the rate of respiration, have been 

 carried out under a variety of experimental and natural conditions. 

 A growing young joint seems to affect the food-supply of the mother 

 joint but very slightly, apparently becoming autonomous in this 

 respect very early in its development. The rate of respiration, as 

 well as of photosynthesis, is naturally much higher in the growing 

 than in the mature joint. There is no evidence of the existence of 

 special formative and respiratory substances, and in regard to food- 

 content young and mature joints exhibit the same or parallel reactions 

 to changes in environic conditions, e. g., temperature or water-supply. 

 The nature of food consumption is shown in a series of starvation experi- 

 ments; these, when compared with the results from normal material, 

 indicate that although ordinarily the metabolic functions make sub- 

 maximal demands on the food-supply, during times of intense heat 

 and drought the plant is in a state of privation in regard to certain 

 sugars. It remains to be determined whether this labile equilibrium 

 of starch "^z^i monosaccharide is purely a temperature effect or is here 

 related also to the water-content of the plant. 



The function of the slimes, which are of pentosan nature, is still 

 quite obscure. It is certain, however, that the pentoses are not to be 

 regarded as inert components, but are drawn into the complex of 

 metabolic activity quite as actively as the hexoses. In this work very 

 helpful assistance has been rendered by Mr. J. M. McGee. 



Teviperature and Growth, by D. T. MacDougal. 



The lower limit at which growth may begin, the degree at which it 

 proceeds most rapidly and continuously with regard to the stage of 

 development of the organ m^easured, the point at which a higher rate 

 may be maintained for a short period, and the upper limit of tempera- 

 ture have been the subject of many observations. Some attention 

 has also been paid to the acceleration ensuing from definite rise in 

 temperature with reference to the van't Hof law of chemical action. 



Growth includes a number of separate processes, each of which goes 

 on at a rate modified by the temperature by the amount of material 



