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CARNEGIE INSTITUTION OF WASHINGTON. 



the proportion of 21 or 22 to 16 or 17 per cent. Comparative tests 

 were finally based on disks taken from apical regions. The capacity 

 for absorbing water was seen to increase up to maturity (about 1 

 year old) then to decrease, as illustrated by the following set of tests 

 with Opuntia hlakeana, made May 17 to 29, 1916: 



Swelling (distilled water) 



Dry weight 



Total sugar 



Fiber 



Young. 



p. ct. 

 24.3 

 16.03 

 44.91 

 4.65 



Mature. 



p. ct. 

 50 

 24 



38.04 

 9.13 



Old. 



p. ct. 

 41.3 

 25.95 

 35.27 

 11.51 



The amount of imbibition is seen to be not a continuous function 

 of any one substance or group of substances. This would harmonize 

 with the results of swelling mixtures of gelatine and agar described in 

 the next paragraph. 



The phenomena of proportionate swelhng of gelatine in water, acids, 

 alkalies, and salt solutions have been mistakenly used hitherto in 

 attempts at explanation of the mechanism of growth. It has been 

 demonstrated by repeated tests that the tracts of the growing cells 

 studied as well as maturing or mature tissues do not swell more in acid 

 than in distilled water or alkali, as will be illustrated by the following 

 results : 



Swelling of disks of Opunlia. 



It is conclusively established that both young and old tissues take 

 up more water when neutral or alkaline. Acidity, therefore, in addi- 

 tion to retarding enzymatic action and respiration, indubitably oper- 

 ates to retard growth by its effects on imbibition by plant tissues. 



The Swelling of Colloidal Mixture in Water, Acids, and Alkalies, 

 hy D. T. MacDougal. 



It being demonstrated that growing masses of embryonic cells in 

 plants and tracts of mature tissue do not show their greatest capacity 

 for the imbibition of water in acidified but in alkaline solutions, it 

 was sought to find what substance or mixture of substances would 

 behave in a similar manner. The first inquiry was made with agar, 



