496 BOTANICAL GAZETTE [JUNE 
was due, it is evident that substances of much different physical 
nature from such colloids as gelatin and fibrin, which show a 
marked increase in swelling in acid solutions, are concerned. It 
should be noted that common cornstarch (boiled and dried) swells 
no more in N/s50 and N/25 HCl than in distilled water, although 
the effect of NaOH of the same molecular strength in this direction 
is marked. 
One important point left to be considered is the effect of the 
varying acidity of the plant sap upon the water-absorbing power 
of the plant. Dr. MacDoucat, to whom I am indebted for many 
suggestions in this work, has observed that the greatest growth 
rate of Opuntia Blakeana takes place in the day. In the daytime 
the acidity of the plant sap is on the decrease, the acid products of 
carbohydrate metabolism, after accumulating during the night, 
being destroyed by light and increased temperature during the 
day. Can it be that this decrease has an effect upon the water- 
absorbing power and in consequence upon growth? The following 
experiment was carried out to test this. A joint of Opuntia 
Blakeana was cut at 5:00 A.M., 0.75 hr. before sunrise, the period 
of maximum acidity. Four disks were cut from it, and the holes 
plugged to prevent abnormal evaporation. The swelling of these 
in one hour in distilled water at 25° C. in diffuse light was determined 
immediately. The joint was exposed to the bright sunlight until 
TABLE III 
Time | Weight in grams | Increase in grams | Percentage increase 
A.M. | 
I GE SESE NL eae an 23:02 
Oreh yee ae 25.25 1.63 6.9 
P.M. 
ice AGES Eee Ne Se De rau ial, 22.25 
BiSO ee A ee. 25.00 | 2.75 ii’ 
4:00 P.M., the period of minimum acidity, and then four disks 
were cut and their swelling determined in diffuse light for one hour, 
also at 25°C. The results are shown in table III. 
3 Spoenr, H. A., Biochem. Zeitschr. 57:95-111. 1913. 
