568 PRINCIPLES OF GENERAL PHYSIOLOGY 



The phenomena seem to be related to the colours assumed by Wiener's (1895) phvlo<-hlori<l< * 

 under the action of light of various colours ; but the case with which we are concerned shows 

 the opposite effect, not the production of a similar colour. The production of a substance 

 of a colour similar to that of the light acting is well shown by Stobbe's (1908) fufyide* ; or;i!i;:r 

 light produces an orange dye, blue light, a blue one. Thus, a substance is formed which does 

 not absorb the light acting, so that no further change takes place ; although, if the product 

 is exposed to light of a different wave length from that under which it was produced, further 

 change is effected since the light is absorbed. 



FACTORS AFFECTING PHOTO-SYNTHESIS 



Temperature. A pure photo-chemical process has a low temperature coefficient, 

 like that of physical phenomena, as we saw above (page 42). The photo-synthesis of 

 carbon dioxide, not being a simple photo-chemical process, has a high one, especially 

 at low temperatures; it is 6 for the 10 between -5 and -I- 5, but only 1'76 

 between 20 and 30. Certain "limiting factors," to be referred to below, com- 

 plicate the measurements, so that, under ordinary conditions, the rate of the 

 reaction is the same between 3 and 30. 



The coagulation of the chloroplast by heat takes place at a lower temperature 

 than that of protoplasm in general. 



A high temperature coefficient at low temperatures, although only a more pronounced 

 effect of a general phenomenon (see page 42), is of frequent occurrence in complex physiological 

 processes. For example, that of the geotropic reaction is 6 "5 for the interval from - 10 to 0. 



Anaesthetics. The complex nature of the process is shown by the fact that 

 chloroform, even in traces, stops it ; 0'002 c.c. per litre of air suffices. 



Limiting Factors. The importance of these factors as regards the velocity of 

 the reaction has been emphasised by Frost Blackman (1905). They may be 

 temperature, light, or access of carbon dioxide. It will be clear that, if the 

 amount of carbon dioxide present is less than the system can deal with under a 

 certain intensity of illumination, no increase in rate will be obtained by increasing 

 the light, but will be obtained if the carbon dioxide is increased. This is, in fact, 

 the usual state of affairs. Under ordinary conditions of good illumination, the 

 leaf can deal with considerably more carbon dioxide than is able to diffuse to 

 the chloroplasts through the stomata and intercellular passages (see Brown and 

 Escombe, 1905). 



The effect of accumulation of sugar is to cause the stomata to close and cut off 

 the supply of carbon dioxide. 



THE EFFICIENCY OF THE CHLOROPHYLL SYSTEM 



Brown and Escombe (1905) made determinations of this value. 



First of all we require to know the amount of energy necessary to convert 

 1 c.c. of carbon dioxide to hexose. This can be calculated from the heat of 

 combustion of hexose, and amounts to 5 -02 calories. To obtain the maximal 

 efficiency, it is necessary to take account of the fact that the amount of light can 

 be diminished nearly twelve times without affecting the rate of synthesis with the 

 usual carbon dioxide tension of the atmosphere (Brown and Escombe, 1905, p. 86). 

 Of the total radiation falling on the leaf, 65 to 78 per cent, is retained by it ; the 

 rest is transmitted or radiated to the surroundings. The greater part of that 

 retained is used for purposes such as transpiration, that is, for evaporation of 

 water. To find out how much is actually used for photo-synthesis, Brown and 

 Escombe compared the amount absorbed by the white and green parts of a 

 variegated leaf. In a particular case the white part absorbed 7 4 '5 per cent, and 

 the green, 78-7 per cent., so that 4-2 per cent, was absorbed by the chlorophyll. 

 Now, it was found that, in Tropceolum majus, a total amount of incident light 

 equal to - 041 calorie per sq. cm. per minute caused the decomposition of 

 0-03034 c.c. of carbon dioxide per sq. cm. per minute. Since the energy stored in 

 the conversion of 1 c.c. of carbon dioxide to sugar is 5-02 calories, that stored in 

 0-00034 c.c. is 0-00034 + 5-02 = 0-001 7 calorie per sq. cm. per minute. This is 

 equal to 4'1 per cent, of the total incident radiation. We have just seen that 

 4'2 per cent, of the total incident radiation is absorbed by the chloroplasts, so that, 



