Atkins — The H/jdroyen Ion Concentration of Plant Cells. 423 



D. rotundifolia was obtained growing on Dartmoor, and on washing the 

 glandular leaf-hairs with a drop of water they were found to be quite strongly 

 acid, pH 4'8, or slightly more acid using methyl red. The crushed leaf was 

 carefully compared with standards, and agreed with pH 3'0, using brom phenol 

 blue. This was confirmed with thymol blue. The significance of this in 

 connexion with enzyme action will be mentioned later. 



Relation of the natural acidity of plant tissues to the activity of their 



enzymes. 

 ■ Determinations made on the fruit of Carica papaya at Pusa showed that 

 the latex of the skin of the unripe fruit was as acid as pH 5"4, an identical 

 value being given by the hard central tissue. On looking up r]'ankers(1917) 

 work on purified commercial papain, it was seen that this enzyme works most 

 rapidly at pH 5 when the temperature is 37° C. Taking this rate as 100, 

 the rates from pH 3 to pH 7 are respectively 27, 91, 100, 63, 47. 



The natural reaction of the fruit is, therefore, close to the optimal value 

 for papain action. According to Kilmer (quoted from Mendel and Blood, 

 1910), the ripe fruit gives no latex when the skin is cut, and the pulp shows 

 very little proteolytic activity. Unfortunately no ripe fruits were available 

 for the examination of pH values. The marked action of hydrogen cyanide 

 in activating papain, especially as regards the progress of the action up to the 

 formation of amino-acids, as described by Mendel and Blood, may, perhaps, be 

 of significance in the physiology of plants having cyanophoric glucosides. 



Again, the recent work of Talk and M'Guire (1921) has shown that the 

 action of soluble sucrase from bananas is most active at pH 3'5 to 4'5. 

 Taking the activity atpH 4'0 as 100, the series at intervals of pH 0'5, from 

 pH 3-0-6-5, is as follows :— 37, 98, 100, 98, 94, 78, 27, 11, the rates being 

 measured at 35° C. In this case also the reaction of the fruit lies in the 

 neighbourhood of the optimum for the enzyme, a freshly picked small unripe 

 fruit being of acidity pH = 4-6-4-8, that of the skin being 4-6 approximately. 

 For the leaf-stalk a value pH 4-9 was found. The pulp and skin of a fully 

 ripe banana, examined in England, were found to lie between pH 5'6 and 

 pH 6'0 in different cells. In considering the optimum pH value for any 

 enzyme, account must be taken of the temperature, for Compton (1921) has 

 shown that the optimum value varies with the latter. Thus, for the maltase 

 of Aspergillus oryzae, he found that for pH 3'0 the maximum hydrolysis 

 occurred at 35-5° C, whereas at pH 7'2 it was not reached till 47° C. In 

 the natural conditions of growth of the papaw and banana at Pusa, an air 

 temperature in the shade of 35°-38° is quite normal, and in full sunlight 

 the fruits, in spite of surface evaporation, must often attain considerably 

 greater temperatures. When one considers that the action of maltase at 



