562 



.VA JURE 



[October i, 1908 



Rise of temperature affects nearly all physical and chem- 

 ical properties, but none of these is so greatlv affected bv 

 temperature as is the velocity of chemical reaction. For 

 a rise of 10° C. the rate of a reaction is generally in- 

 creased two or three fold, and this has been generalised 

 into a rule by van't Hoff. As this increase is repeated 

 for each successive rise of 10° C. either by the same factor 

 or a somewhat smaller one, the acceleration of reaction- 

 velocity by temperature is logarithmic in nature, and the 

 curve representing it rises ever more and more steeply. 

 Thus keeping within the vital range of temperature a 

 reaction with a temperature factor of X2 per 10° C. will 

 go si.xteen times as fast at 40° C. as at 0° C, while one 

 with a factor of X3 will go eighty-one times as fast. 



This general law of the acceleration of reactions bv 

 temperature holds equally for reactions which are being 

 accelerated by the presence of catalysts. As we regard 

 the catalyst as merely providing- for the particular reaction 

 it catalyses, a quick way round to the final stage by pass- 

 ing through the intermediate stag-e of forming a temporarv 

 addition-compound with the catalyst itself, so we should 

 expect rise of temperature to accelerate similarly these 

 substituted chemical reactions. 



If this acceleration is a fundamental principle of chemical 

 mechanics it is quite impossible to see how vital chemislrv 

 can fail to exhibit it also. 



ACCELER.ITION OF VlTAL PROCESSES nV Te.MTERATLRE. 



At, present we have but a small number of available 

 data among plants to consider critically from this point 

 of view. But all the serious data with w'hich 1 am 

 acquainted, which deal with vital processes that are to be 

 considered as part of the protoplasmic catalvtic cong-eries, 

 do exhibit this acceleration of reaction-velocitv by tempera- 

 ture as a primary effect.' 



Let us briefly consider these data. On the katabolic 

 sMe of metabolism we have the respiratory production of 

 CO,, and opposed to it on the anabolic side the intake of 

 carbon in assimilation. 



As a measure of the rate of the metabolic processes 

 constituting growth we have data upon the division of 

 flagellates ; and finally there is the obscure process of 

 circulation of protoplasm. 



The intensity of CO, production is often held to be a 

 measure of the general' intensity of metabolism, but any 

 relation between growth-rate and respiration has vet to be 

 clearly established. Our science is not vet in the stage 

 when quantitative work in relation to conditions is at all 

 abundant ; we are but just emerging from the stage that 

 chemistry was in before the dawn of physical chemistry. 



Taken by itself the CO,-production of an. ordinary green 

 plant shows a very close relation with temperature. In 

 the case of the cherry -laurel worked out by Miss .Matthjci 

 and myself the respiration of cut leaves rises bv a factor 

 of 2-1 for every 10° C. (See : Fig. ^s, ;Resp.) ' This has 

 been investigated over the range of temperatures from 

 16° C. to 45° C. .^t this 'higher temperature the leaves 

 can only survive ten hours in the dark, and their' respira- 

 tion is affected in quite a short time, but in the initial 

 phases the CO, output has the value of 0-0210 gr. per hour 

 and unit weight of leaf, while at •i6°-2 C. the amount is 

 only 0-0025 gr- CO,. Thus the respiration increases over 

 a range of ten-fold with ' perfect regularity solely by in- 

 crease of temperature. \o ■ reaction in a test-tube could 

 show less autonomy. ■\\. temperatures :above 45° C. the 

 temperature still sooner proves fatal unless the leaf is 

 illuminated so as to carry out a certain amount of photo- 

 synthesis and compensate for the loss of carbon in respira- 

 tion. Thus, with rising temperature, there is at no time 

 any sign of an optimum or of, a, decrease of the intensity 

 of the initial stage of respiration. 



Here, then, on the katabolic side of metabolism we have 

 no grounds for assuming that " temperature-stimuli " are 

 at work regulating the intensity of protoplasmic respira- 

 tion, but we find what I can only regard as a purelv 

 physical-chemical effect. The numbers obtained by 

 Clausen ^ for the respiration of seedlings and buds at 



^ A coHect'nn nf twenty cases, mostly from animal phvsi^logy, by Kanitz 

 {Zcits. ffir Ekktrochctnic, 1907, p. 707), e.\hibits coefficients ranging from 

 '■'to 3-3- 



2 Landwirtschn/tlkhc J ahrliichcr, Bd. .\i.\., 1890. 



KG. 2031, \"OL. 7S] 



different temperatures indicate a temperature coefficient of 

 about 2-5 for a rise of 10° C. 



To this final process of katabolism there could be no 

 greater, contrast than the first step of anabolism, the 

 assimilation of carbon by the protoplasm as a result of 

 photosynthesis. We must therefore next inquire what i-~ 

 the relation of this process to temperature. 



This question is not so simple, as leaves cannot satis- 

 factorily maintain the high rate of assimilation that high 

 temperatures allow. The facts of the case were clearly 

 worked out by Miss Matthtei,' the rate of assimilation by 

 cherry-laurel leaves being measured from —6° C; to 

 -(-42° :C. Up to 37° C. the curve rose - at first ■ gently 

 and then more and more steeply, but on calculating out 

 the values it is found that the acceleration for successive 

 rises of 10° C. becomes less and less. Between 9° C.-and 

 19° C. the increase is 2-1 times, the highest coefficient 

 measured, and exactly the same coefficient as for respira- 

 tion in this plant, which in itself is a striking point, seeing 

 how different the processes are. (See- Fig. 5, Assim.) 

 The decrease of the coefficient with successive rises is a j 

 state of things which is quite general among non-vital 

 reactions. \ critical consideration of the matter leads 

 one to the conclusion, however, that this failure to keep 

 up the temperature acceleration is really due to secondare 



10° 20" 5iS 40" fo'C^K^. 



causes, as is also the appearance of an optimum at about 

 38° C. Some of these causes have been discussed by me 

 elsewhere,- and I hope to bring a new aspectof the matter 

 before the Section in a separate communication. The con- 

 clusion formerly come to w^as that probably in 'its initial 

 stages assimilation at these very high' temperatures started 

 at the full value indicated by a theoretically constant 

 coefficient, but that the protoplasm was unable to keep 

 up the velocity, and the rate declined. "It must be borne 

 in mind here that quite probably no chloroplast since the 

 first appearance of green cells upon the earth had ever 

 been called upon for anything like such a gastronomic 

 effort as these cherry-laurel leaves in question. It is not 

 to be wondered that their capacities speedily declined at 

 such a banquet, and that the velocity-reaction of anabolic 

 synthesis traces a falling curve in spite of the keeping up 

 of all the factors concerned, to wit, temperature, illumina- 

 tion, and supply of CO,. This decline is not permanent, 

 but after a period of darkening the power of assimilation 

 returns. Physical-chemical parallels can easily be found 

 among cases where the accumulation of the products of a 



1 Phil. Trans Roy. Soc, Ser. B, vol. cxcvii., igoi. 



2 "Optima and Limiting Factors," Annais of Botany, wo\. xix., April, 

 1905. 



