66 REPORTS ON INVESTIGATIONS AND PROJECTS. 



Respiration in Cacti, by Prof. H. M. Richards. 



The first step to be taken in an investigation of respiration in cacti was, of 

 course, to obtain some definite knowledge of the acidity of the juices. It at 

 once developed that there was a striking diurnal periodicity in the acid con- 

 tent of the sap and a series of determinations was undertaken to determine 

 definitely the course and (in so far as possible at present) the causes of this 

 regular variation. Of all the forms of cacti experimented with, Opiintia 

 versicolor proved the most satisfactory and was chiefly employed, but the 

 same general phenomena were observed in Mamillaria grahami and others. 



In general the acidity curve in plants exposed to normal conditions of day 

 and night was nearly the reverse of the diurnal temperature variation. The 

 low point of acidity was reached at about 4 p. m., or slightly later than the 

 maximum temperature, while the maximum of acid content, averaging twice 

 and in some instances four times that of the minimum, occurred at about 

 5 a. m., when the temperature is nearly at its lowest. Other series of experi- 

 ments made (kept wholly in the dark and at different and constant tempera- 

 tures) showed an accumulation of acid at from 45° to 60° F., while at higher 

 temperatures, ranging to 100° F., the characteristic lessening of acid was 

 observed. On the other hand, plants exposed to full sunlight but kept cool, 

 at a temperature not above 75° F., maintained during the whole course of 

 the day the higher acidity of the early morning hours. Increased or dimin- 

 ished partial pressure of oxygen seemed to affect the results very little. 



At the present state of the investigation it would seem, then, that tempera- 

 ture was the controlling external factor in this manifestation. 



While the relation which this phenomenon bears to the respiratory func- 

 tions of the tissues is unquestionably the most direct, and must be definitely 

 determined by a careful comparison of an extended series of parallel obser- 

 vations, there should also be borne in mind the possible bearing of photo- 

 synthetic and anabolic processes on the question. 



The Analysis of the Condition of Parasitism, by Dr. D. T. MacDougal. 



The completion of the major experiments upon this subject and their de- 

 scription in Publication No. 141 left a number of parasitic arrangements in 

 an active condition and these have been made the subject of some further 

 analysis. In the earlier work the conclusion was reached that dependent 

 nutritive couples were possible only between a host with a low osmotic pres- 

 sure in the sap and an eligible parasite with a higher osmotic activity. Not 

 all such pairs of plants might come into nutritive contact, however, and no 

 explanation could be offered for the failure. Acidity was thought to be 

 without influence in the matter. The estimations of acidity were made of 

 material taken without regard to time of day and temperature. The recent 

 researches of Richards have shown, however, that cacti contain twice as 

 much acid in tlie morning as late in the afternoon. This changing acidity 

 may exert a very important effect upon the absorptive capacity of the muci- 



