124 METABOLISM 



it cannot be replaced by any other compound of carbon, least of all by carbon- 

 monoxide. 



Along with carbon-dioxide we must study the oxygen. There are many facts 

 which prove to us that, at the commencement of assimilation, no traces of oxygen 

 are observable, and this is the more remarkable inasmuch as practically all the 

 vital processes in the green plant are dependent on the presence of this gas. 

 In the experiments on assimilation referred to above (p. 105) it was noted that 

 free oxygen was definitely absent in reduced haemoglobin, and yet that the 

 decomposition of carbon-dioxide could commence in such a medium ; when 

 decomposition does commence the experiment comes to an end, since oxygen 

 is at once produced. We learn from E WART'S (1897) investigations, however, 

 that certain plant -colouring matters are capable of combining loosely with 

 oxygen, and we are led to believe that this capacity is more widely distributed 

 than is generally assumed, and that, in experiments such as those quoted, it is 

 not free oxygen but oxygen in loose combination that is at the disposal of the 

 plant. At all events the plant's power of bringing about assimilation ceases in 

 the long run in absence of oxygen ; we have still, however, to decide whether 

 this takes place when the loosely attached oxygen is used up or when the 

 chloroplasts become inactive. 



Heat, which has such a fundamental influence on plant life in general, 

 affects assimilation also to a great extent. The determination of the dependence 

 of assimilation on temperature from a quantitative point of view is by no 

 means easy, because, in addition to the construction of assimilation products, 

 a destruction of these same bodies, owing to respiration, is always going on at 

 the same time, and because this latter process is dependent on temperature in 

 other ways than the former. The most accurate experiments in this relation are 

 those of KREUSSLER (1890). His researches show first of all that a gradual 

 increase in assimilation is concomitant with a rise of temperature ; the maximum 

 assimilative activity is obtained somewhere between 25 and 40 C., followed by 

 a decrease as higher temperatures are reached. Since assimilation soon ceases 

 at temperatures above 45 C., while respiration increases with a further rise of 

 temperature to over 50 C., there must be a certain degree of temperature at 

 which the two processes neutralize each other. The following table summarizes 

 KREUSSLER' s (1890) chief results on this subject : 



Ricinus. 

 Temperature. 25 40 45 50 60 



a. mg. of CO a formed per 



hour in the dark owing 



to respiration . . . 8'5 15-0 14-8 16-4 0-75 



b. mg. of CO ? decomposed 



per hour in light . . 35.9 29-0 16-1 29-9 

 c . a + b = mg. of CO 2 assi- 

 milated per hour in 



44-4 44-o 30-9 13-5 



Prunus laurocerasus. 

 25 40 45 50 60 



7.7 19.9 21-7 21.3 2-5 

 20-9 4 x -7 3- 1 337 



28-6 61-6 24.8 12-4 



Looking at line c it will be noticed that the energy of assimilation in Ricinus 

 is still as great at 40 as at 25 ; at 45 it has sensibly decreased, and must 

 soon after reach zero. In the case of Prunus it is greater at 40 than at 25, 

 but falls rapidly with increase of temperature, soon becoming nil above 45. At 

 all events, at 50 in light, in both cases, only carbon-dioxide is formed. 



Line b shows the net profit or loss of the gaseous exchange occurring in light ; 

 Ricinus, in spite of concomitant assimilation, shows a smaller gain in carbon 

 at 40 than at 25, while in Prunus the conditions are reversed ; at 45 the gain 

 in carbon in Prunus is reduced to a minimum, and, beyond that, naturally 

 entirely ceases, indeed there is a loss instead. 



KREUSSLER (1888) has also investigated the lower limits of temperature. 

 He found feeble but still recognizable assimilation taking place in bramble 



