424 Scientific Proceedings^ Royal Dublin Society. 



pH 3 has fallen from its maximum rate at 35-5° C. to about two-thirds of 

 this rate at 39" C, and to roughly one-third at 42° C, the importance of 

 high temperatures combined with high acidity is made clear. With lower 

 acidity, however, the optimum has not been reached at 42° C. It is thus 

 very natural that the reaction found for the plant tissue should be rather 

 below the optimum acidity for 35-5° C, since the fruit is certainly exposed 

 during its development to higher temperatures at which a degree of acidity 

 optimal for 35'5° C. would have serious destructive action on the enzyme. 

 In tropical climates the black bulb thermometer may reach 72° C. ; and it 

 has been found that thermometers placed inside small wooden frames about 

 a foot square and two inches deep, covered with linen, aeroplane dope, 

 and varnishes of various colours, quite commonly reach 50°-60° in sun- 

 light. These measurements were not made to investigate fruit tempera- 

 tures, but they serve to show how high are the temperatures that may be 

 found under such atmospheric conditions. It accordingly seems reasonable 

 to suppose that temperatures of 45°-50° may be met with in ripening fruit 

 in the tropics. In England during full sunshine in July it was found that 

 with an air temperature 25'1° C. an untreated mercury thermometer rose 

 to 33-7 when insolated ; and a similar thermometer inserted in a ripe banana 

 reached 33-4°. The fruit is thus over 8° above air temperature; and in 

 view of the fact that in Upper Egypt and the Sudan temperatures of 44° are 

 common, it is unlikely that 50° is at all too high an estimate for the tempeia- 

 ture to which fruits in tropical climates may be exposed. In this connexion 

 the decrease in acidity of fruits as they ripen is to be noted, and as the ripen- 

 ing usually takes place in the hot weather, it is possibly advantageous for 

 enzyme action that the initial higher degree of acidity should be reduced. 



As previously mentioned, immature castor-oil seeds were found to give 

 a reaction of pH 5'4, though the leaves were at pH 4*8. No details, quite 

 parallel with those on papain, of the effect of various pH values on lipase 

 action are to hand; but Armstrong and Gosney (1913, 1914) have shown that 

 after the enzyme has been liberated from its zymogen by dilute acetic acid, 

 the low acidity of oleic acid is most favourable to its action. A table given 

 by these workers shows the action of 0"5 gram of castor bean lipase upon a 

 solution of ethyl succinate according as increasing amounts of various acids 

 were present. The percentage hydrolysed in twenty hours was 56'5 with no 

 added acid, as against 60-5, 61-8, and 57-3 with N/40, N/20, and N/10 

 butyric acid respectively, and 8-1 per cent, with N/40 sulphuric acid. If 

 one neglects the Cjuite possibly considerable effect of the enzyme preparation 

 in diminishing the hydrogen ion concentration, though acid itself, and 

 determines the pH values corresponding to the above concentrations, it is 

 found that the butyric acid values are pH 2-67, 2-85, and 2-93, arrived at by 



