July 21, 1923] 



NA TURE 



95 



Letters to the Editor. 



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Effect of Infinitesimal Traces of Chemical 

 Substances on Photosynthesis. 



The possibility of ultra-measurable traces of certain 

 chemical substances affecting assimilation is a matter 

 of much importance in physiology. The carbon- 

 assimilation of water-plants affords an extremely 

 sensitive process for the investigation of the subject. 

 The usual method of counting the number of bubbles 

 of oxygen given out by the plant under light is, how- 

 ever, most untrustworthy for quantitative determina- 

 tions, since the size and frequency of the bubbles 

 undergo spontaneous variation. This difficulty has 

 been completely removed by a new device which I 

 have been able to perfect, by which the evolution of 

 equal volumes of oxygen is automatically recorded on a 

 revolving drum by an electromagnetic writer ; records 

 thus obtained enable us to determine the normal rate 

 of photosynthesis and its induced variations. I have 

 also found that there is a definite relation between the 

 evolution of oxygen and the formation of carbo- 

 hydrate in the leaf. The automatic apparatus 

 referred to can be so adjusted that the successive dots 

 in the record represent the photosynthetic production 

 of amounts of carbohydrate as small as a millionth 

 of a gram. It is impossible in this short communica- 

 tion to give a detailed account of the apparatus, which 

 will be found fully described in my forthcoming work, 

 " The Physiology of Photosynthesis," to be published 

 by Messrs. Longmans. 



My attention was directed to the possible effect of 

 traces of chemical substances on carbon-assimilation 

 by the extraordinary increase in the photosynthetic 

 activity after the thunderstorm and rain which 

 lasted from February lo to February 13 of this year. 

 The coefficient of photosynthetic activity of the 

 aquatic plant, Hydrilla verticillata, growing in the 

 pond of the Institute, had been carefully determined 

 for January and for the first week of February, 

 and found to be practically the same in different 

 specimens. The coefficient for light, i.e. the ratio of 

 increment of activity to the increment of light, was 

 found to be 13 "2 per 100 lux immediately before 

 the thunderstorm (February 9) ; whereas after the 

 thunderstorm it was found to be 26-9, the activity 

 having been thus increased 100 per cent ; later the 

 value decreased by stages to 22-9, as if the beneficial 

 effect of the thunderstorm were subsiding to a cer- 

 tain extent. There was no variation of temperature, 

 which remained constant at 22° C. 



The rain could not have produced any variation of 

 turgor in the plant, which was submerged in water. 

 A plausible explanation of the enhanced activity is 

 that the electrical discharges during the thunder- 

 storm produced oxides of nitrogen which, washed 

 down by the rain, added traces of nitric acid to the 

 water of the pond in which the plants were growing. 

 The quantity thus added would, however, be incon- 

 ceivably minute. The correctness of the above 

 hypothesis may for the present be left an open 

 question. There can, however, be no doubt that 

 minute traces of nitric acid exert a potent influ- 

 ence on photosynthetic activity, as is shown by the 

 results of the following experiments carried out 

 under constant light and temperature. At first I 

 applied a dilution of one part in ten thousand 



NO. 2803, VOL. I 12] 



which caused a depression of activity. I therefore 

 went to the other extreme and prepared different 

 dilutions of i, 10, and 100 parts in 100,000,000,000. 

 It is difficult to form any clear conception of ultra- 

 measurable quantities from a row of zeros, and I will 

 therefore, following the French system of measure- 

 ment, designate a thousand millions as a billion. 

 Application of a solution of one part of nitric acid in 

 100 billions induced no change in photosynthesis, but 

 one part in 10 billions produced a marked increase in 

 activity of about 100 per cent ; i part in 2 billions 

 caused a further increase of nearly 200 per cent. This 

 was the climax. The enhanced activity underwent 

 a slight decline at dilutions of one to ten parts in a 

 billion, the activity being still greater than the 

 normal by 100 per cent. There was an abrupt 

 depression of activity at lower dilutions than 1000 

 parts in a billion (Fig. i). The above figures may be 

 taken to be typical of the effect of traces of nitric acid ; 

 for a dozen different specimens taken at random gave 

 very similar results. In subtonic specimens, with 



-Of 



10' 



iO" 



1 1 10 



nHuiiM . 

 Fig. I.— Curves showing effects of traces of nitric acid, of extracts of 

 thyroid gland, and of formaldehyde, on the activity of carbon .issimila- 

 tion by the aquatic plant Hydrilla verticillata. The ordinals 

 represent induced change in photosynthetic activity, the normal beirig 

 taken as loo ; the abscissas represent dilution from o-oi to lo* parts in 

 a billion. 



photosynthetic activity at standstill, an addition of 

 two parts nitric acid in ten billions caused vigorous 

 photosynthetic evolution of oxygen, the renewed 

 activity persisting for a very considerable length of 

 time. 



I obtained similar increase in carbon-assimilation 

 with traces of certain other substances of which I will 

 give only two examples. The dotted curve in the 

 middle of the figure exhibits the effect of extract of 

 thyroid gland. Here the maximum activity was pro- 

 duced at a dilution of ten parts in a billion. The 

 noticeable fact is that there was no reversal for a 

 considerable range : the increased activity of about 

 80 per cent persisted up to the lower dilution of one 

 part in a million. 



The effect of traces of formaldehyde, which is a 

 highly poisonous agent, is of much theoretical 

 interest. A dilution of one part in a billion caused an 

 increase of photosynthetic activity by 85 per cent. 

 At higher concentrations, formaldehyde produced its 

 normal poisonous effect. The action of traces of 

 formaldehyde has special significance in regard to the 

 " first product " of assimilation. According to 

 Baeyer's theory, formaldehyde is one of the first pro- 

 ducts from which carbohydrates are formed by poly- 

 merisation. This theory labours under the difficulty 

 that formaldehyde is extremely poisonous to plants. 



