92 CARNEGIE INSTITUTION OF WASHINGTON. 



almost directly proportional to the relative dry weight of the tissue 

 tract, while in the late afternoon the acidity throughout the plant dimin- 

 ished, the decrease being considerably greater in the outer layers than 

 in the inner, as would be expected from the higher temperature and 

 light intensity acting upon the former. The water-content of both 

 the sahuaros and the bisnagas averaged about 95 per cent in the 

 central tissues and 90 per cent in the outer regions. The acidity of 

 the sahuaro is somewhat higher than that of the bisnaga, while the sap 

 of both is much lower in acid-content than that of the opuntias. 



A method is being developed to obviate certain difficulties encoun- 

 tered in the usual method of determining acidity in the expressed juice, 

 chief of which are the slimy product obtained by the latter method 

 and the rapid action of the oxidases on injury to the tissue, both of 

 which render titration difficult, and also to make possible the deter- 

 mination of the acid present in small quantities of material, as 1 to 5 

 grams of moist tissue. The essential feature is alcoholic extraction, 

 the subsequent titration being made with phenolphthalein, as usual. 



Growth and the Hydratation of Colloids in Cacti, by E. R. Long. 



In view of the suggestion of a number of investigators, especially 

 Borowikov and M. Fischer, that conditions which effect an increased 

 hydratation of plant colloids, notably the presence of acids, cause 

 increased growth of the plant, attempts have been made to correlate 

 the growth of succulents with their varying acidity, which has been 

 described in the reports of this department for 1911, 1912, and 1913, by 

 Dr. Richards and Dr. Spoehr. It was thought at first that the com- 

 mon observation of a higher rate of plant growth at night might be 

 accounted for bj' the increased acid-content of the plant-sap during 

 this period. However, early in the course of these experiments Dr. 

 MacDougal observed that the highest growth rate of Opuntia hlakeana 

 takes place during the day, i. e., in the period of decreasing acidity, 

 and experiments were made to determine whether the phenomenon 

 noted here might be an effect of the varying acidity of the plant-sap. 



It was immediately found, by placing pieces of 0. hlakeana in various 

 solutions, that the swelling induced in dilute acids was decidedly less 

 than that occurring in distilled water. It seems highly improbable 

 that this was an osmotic-pressure effect, in view of the fact that, in a 

 solution of a neutral salt, e. g., potassium nitrate, of the same molecular 

 strength as the acid, the swelling was much greater than in the acid 

 solution and fully as great as that taking place in distilled water. 



Experiments on the growth-rate of cacti paralleled the experiments 

 on swelling. The growth of (1) developing flowers and (2) of new 

 joints of 0. hlakeana and 0. discata was distinctly less when the parent 

 joint was absorbing N/100 and N/50 hydrochloric and malic acids 



