PHOTO-SYNTHESIS I N TROPICAL SUNLIGHT. PART IV Synthesis of Sugars and Complex Nitrogenous Substances BY ATMA RAM RAJVANSHI AND N . R . DHAR
In a previous paper we have recorded our results on the photosynthesis of formaldehyde. I n this paper we will deal with ( I ) polymerisation of formaldehyde to reducing sugars, (2) synthesis of complex nitrogenous compounds and (3) discussion of results recorded in both the papers. Polymerisation of Formaldehyde to Reducing Sugars
It was Butlerow‘ who for the first time showed that formaldehyde is polymerised to reducing sugars when heated with dilute alkalies. Nef2 has shown that this mixture of reducing sugars, so obtained, contains nearly 2 4 members of the carbohydrate family. The condensation of formaldehyde to reducing sugars by means of sunlight has been recently studied by Baly and co-workers3 who have reported the formation of sugars from dilute solutions of formaldehyde in ultraviolet light. Dhar and SanyalJ4 Gopala Rao and Dhar have reported the formation of sugars from formaldehyde solutions exposed to sunlight in presence of ferric chloride, chlorophyll and zinc oxide. Aqueous solutions of formaldehyde (Merck’s formalin 40%) usually 3% were prepared using pure water, and exposed in pyrex glass beakers and sealed bulbs for a period extending over 60 to IOO hours and some times even 1 2 5 hours. These solutions were also exposed in big sealed bulbs having a capacity of zoo to 2 5 0 cc. using different photocatalysts. Similar solutions were kept in bulbs and beakers in the dark. The solutions exposed and unexposed were examined simultaneously and always Benedict’s solution was used to test sugars as Fehling’s solution is also reduced by formaldehyde. The procedure of removing the sugars was the same as described already. I n Table I are the results obtained with these experiments. Temperature Coefficient of Sugar Formation A study of the temperature coefficient of sugar formation from HCHO was undertaken with a view to the elucidation of the process of polymerisation of HCHO to sugars. The experimental procedure adopted was as follows: Two big bulbs of ordinary glass whose capacity was one litre were well cleaned with hot chromic acid and subsequently with hot caustic soda solution in order to remove any impurity present. Two litres of 3c0formaldehyde ‘Ann., 111, a42 (18j9);Compt. rend., 53, r4j (1861). 2Ann., 403,355 (1914). a J. Chem. SOC.,121, 1078 (1922);123, 165 (1923);Proc. Roy. SOC., 116A, 197,2 1 2 219 (1927);122, 393 (1929). J. Phys. Chem., 29,926 (1925).
576
ATMA RAM RAJVANSHI AND N. R. DHAR
TABLE I Experiments
Results
Time of exposure
2.
37,HCHO loo hrs. 3% HCHO with ferric chloride 40 hrs. in a beaker 3. 3% HCHO with chlorophyll 40 hrs.
no sugar detected. sugar detected.
4. 3% HCHO with ZnO 7 0 hrs. 5 . 3% HCHO with methyl orange 50 hrs. 6. 37c of HCHO with methylene 7 0 hrs.
sugar detected, but less than the previous case. very small quantity of sugar. very smaIf quantity of sugar. very little of sugar.
I.
blue 7 . 370 HCHO with nickel do. carbonate 8. 3% HCHO with FeSOr 90 hrs. 9. 393 HCHO with MgO IOO hrs. IO. 370 HCHO with BaCOa do. I I . 3% HCHO with CaC03 do. 12. 3% HCHO with SrC03 do. cuco3 do. do. 13. cuso4 do. do. 14. uranium nitrate 80 hrs. do. 15. safranine IOO hrs. do. 16. cartharamin do. do. 17. rhodamin do. do. 18. cqbalt carbonate 75 hrs. do. 19. 80 hrs. SnO do. 20. do. 21. chromium sulphate 90 hrs. nickel sulphate do. do. 22. 7 0 hrs. Ca(0H)z do. 23. erythfosin do. do. 24. phosphomolybdic do. do. 2.5. acid praseodymium do. 26. do. nitrate yttrium nitrate do. 27. do. ceric oxide do. do. 28. 50 hrs. ferric chloride in do. 29. a sealed tube 30. 3% HCHO with ferric chloride 50 hrs. and little ferrous sulphate 3 I. do. chlorophyll in do. a sealed tube 32. do. with ferric do. hydroxide sol
little reducing sugar. very little reducing sugar. no sugar. do. do. do. do. do.. do. do. do. do. do. do. do. do. do. do. do do. do. do. appreciable amounts of sugar. less than in the previous case. more sugar than in the beaker. no reducing sugar.
PHOTO-SYNTHESIS I N TROPICAL SUNLIGHT
577
solution were prepared and an excess of ferric chloride was added, because some of the ferric chloride is reduced to ferrous chloride by the action of formaldehyde. 800 cc. of this solution were filled in each bulb and the bulb sealed at the end. The bulbs were put in big glass jars used as a thermostat and exposed to the bright sunlight. The temperature of the two jars was kept constant a t 30' and 30' C respectively. After an exposure of q j hours, the two bulbs were removed and their ends broken. 750 cc. of the contents were taken from one bulb and the ferrous salt produced during the exposure (the ferrous salt also reduces Benedict's solution) and some ferric chloride left were first removed by precipitating them with ammonia as hydroxides. After the whole of ferrous and ferric chloride were removed, the remaining solution was evaporated on a water bath to dryness. The dried mass was extracted with pure methyl alcohol. The extract was then evaporated on a water bath and dissolved in water. A similar process was adopted with the contents of the other bulb. A small quantity of the solution, so obtained, was tested for HCHO and in both cases negative results were obtained. 40 cc. of a standard Fehling's solution were heated on a water bath for half an hour and the first solution added to it. h marked reduction of the Fehling's solution was noticed. After a complete reaction the cuprous oxide was well washed, dried, ignited, and weighed as cupric oxide. Similarly the solution obtainedfrom the other bulb was treated with Fehling's solution and the cupric oxide weighed. The following are the results obtained:I. at 30' C. Wt. of CuO = 0.061 gm. at 40' C. Rt. of CuO = 0.077 gm. 2. Temperature coefficient (0.077/0.061) = 1.2 nearly I. the wt. of sugar calculated as glucose a t 30' C. = 0.13jj grm. the wt. of sugar calculated as glucose a t 40' C. = 0.16 grm. 2. K e have discussed later on these preliminary results, which are being confirmed by further experiments.
The Photo-synthesis of Nitrogenous Compounds and Nitrification The photosynthetic utilisation of nitrogen by plants in the formation of proteins is in a large manner complementary to the production of carbohydrates from carbon dioxide and water. These substances equally with carbohydrates claim attention on account of their great importance as food materials. According to Baly and Barker, the photosynthesised formaldehyde is capable of reacting with ammonia or potassium nitrite to give complex nitrogenous compounds, i.e. amines, pyridine, quinoline bases, and alkaloids. Snow and Stone' consider the evidence adduced by Baly as insufficient. Dhar and Sanyal have also been able to synthesise methylamine and an alkaloid from formaldehyde and ammonia exposed to tropical sunlight. It is believed that the proteins synthesised in the plants are produced in a similar manner. Now as regards the source of plant nitrogen, it is generally believed that nitrites and nitrates, present in the soil, constitute the chief source of its J. Chem. Soc., 123,
1.59
(1923).
ATMA RAM RAJVANSHI AND N. R. DHAR
578
supply. Moore and Websterl and Dhar and Sanyal believe that sunlight causes a slight union of nitrogen and oxygen in the formation of oxides of nitrogen, which are absorbed by the plants. The following experiments were carried on regarding the production of nitrogenous compounds in the plants and the nitrification process in the soil.
Experimental A. Photosynthesis of nitrogen compounds. As is generally believed and has further been carefully pointed out here that formaldehyde is first synthesised by the plants from carbon dioxide and water. Experiments were therefore made with 2% formaldehyde and ammonia using suitable photocatalysts. Time of exposure
Experiments
HCHO with ammonia
24 hrs.
2y0HCHO with ammonia
3 0 hrs.
I . 2%
2.
and‘ Ti02
3 . 2YoHCHO with ammonia
30 hrs.
and copper carbonate
4.
2%
HCHO with KKOs
30 hrs.
do. HCHO with ammonia and nickel carbonate 6 . Air free from nitrite fumes by 3 hrs. passing through FeSOc solution was passed in conductivity water in a Jena glass beaker covered by a watch glass 7. The experiment (6) repeated 2 hrs. by passing air free from nitrites in a quartz beaker covered with a quartz cover 16 hrs. 8. 2y0HCHO with ammonia and copper carbonate 5.
2%
Results
A small quantity of hexamethylene tetramine. A large quantity of hexamethylene tetramine and a very small quantity of urethane but no urea, some nitrite. No urea, some nitrite, hexamethylene tetramine, and an oily substance which could not be identified being in very small quantity. some nitrite but no formhydroxamic acid. hexamethylene tetramine. no nitrite.
a very small quantity of nitrite detected. small quantity methylamine M.P. hydrochloride 95.9OC. and hexamethylene tetramine.
The tests employed to detect the compounds were those described by Plimmer in his “Practical Organic and Bio-Chemistry.” The test for the nitrite was carried in the following way: 1
J. Chem. Soc., 119, 1555
(1921).
PHOTO-SYNTHESIS IN TROPICAL SUNLIGHT
579
A 0.57~ solution of a-naphthylamine was prepared and another 1% solution of sulphanilic acid. I cc. of each solution was mixed and subsequently I O cc. or 2 0 ccs. of the solution to be tested for nitrite were added. A similar blank containing pure conductivity water used in the experiments was also put up to compare the colour as the amount of nitrite produced was very small. On keeping the two beakers, a pink colour was developed in case when nitrite was present. This test is extremely sensitive and depends upon the well-known diazo-reaction with the nitrite formed.
Discussion of Results Unequivocal evidence is brought forward for the formation of HCHO from C o r and water using numerous organic and specially inorganic catalysts. The chief inorganic catalysts used are MnC12, cobalt and nickel carbonates, copper sulphate, copper acetate and carbonate, chromium oxide, etc. All these are coloured substances having marked absorption in the visible spectrum. In the case of organic substances when used as a catalyst] the presence of formaldehyde has been attributed to the decomposition of the organic substances in sunlight. Experiments were conducted in which a solution of the organic photocatalyst was separately exposed to sunlight in which no carbon dioxide was passed. These solutions were distilled and the amount of formaldehyde detected. The presence of greater amounts of formaldehyde in the distillate in which carbon dioxide was passed clearly points out that the excess of formaldehyde is due to the photochemical reduction of carbon dioxide to formaldehyde. The amount of formaldehyde detected in presence of MnClz used as a photocatalyst is 50 times greater than the limit to which the Schryver’s test of formaldehyde used in this investigation is sensitive. One part of formaldehyde in 625,000 parts of water can be detected by this test. In the light of these experiments there is hardly any doubt that the presence of formaldehyde is due to its photoformation from CO, and HzO. The production of only small amounts of formaldehyde has to be ascribed to the fact that the formaldehyde so synthesised being in the reactive state is polymerised to the reducing sugar. We are of the opinion that there are two simultaneous reactions occurring with the active formaldehyde :I. Reversion to inactive or ordinary HCHO. 2 . Polymerisation into reducing sugars. Active formaldehyde
M
T-l
Inactive formaldehyde iS Reducing sugars. Baly and Barker differentiate this active formaldehyde from the ordinary variety by assigning to it a different structural formula. There is, according to the scheme, an equilibrium of the following type established in the process of photosynthesis: Active formaldehyde iS Reducing sugar
M
Inactive formaldehyde
M
Starch and other such products.
ATMA RAM RAJVANSHI AND N . R . DHAR
5 80
According to Bohr’s conception of atomic structure an active molecule or a nascent molecule as popularly known, is characterised by possessing a higher energy i.e. being in a higher state of quantum orbit. Whether this nascent formaldehyde molecule is by itself capable of being polymerised with other active molecules to reducing sugar, without taking up any energy absorbed by the photocatalysts used, or this polymerisation of active formaldehyde into reducing sugars will also require a further supply of energy, is a question which is difficult to answer a t this stage. Our knowledge of energy relations between the various stages is insufficient to enlighten upon this point. To throw light on this question, however, a study of the temperature coefficient of sugar synthesis from formaldehyde was undertaken. I n order to follow the arguments developed in this paper it will be worth while to consider some of the current views on this problem. From an analogy with these experiments in “vitro” it can be said with a certain amount of confidence that in the green plants too, the primary product of photosynthesis is formaldehyde which is polymerised to sugars. The reaction CO2 HzO-+HCHO 0 2 takes place with an absorption of energy. The light energy absorbed by chlorophyll is in some way converted into chemical energy. On account of the readiness with which this photo-synthesised formaldehyde is polymerised into sugars, Baly and Barker have given it a different structural formula: HCHO (ordinary) HCOH (active). It is generally believed that chlorophyll plays the part of an optical sensitiser, but the mechanism of this sensitisation is not very clear. Spoehri has stated that several authors incline to the view that the fluorescent property of chlorophyll may be the guiding factor in its capability to act as a sensitiser. I n order to test this view, atternpts uere made by us to effect photosynthesis using different fluorescent organic dyes. A glance a t the experimental results will a t once make it clear that nearly all the fluorescent dyes used, failed to act as marked photosensitisers in the formation of formaldehyde. The efficiency of a fluorescent substance for photosensitisation will depend mainly upon three factors:(a) the amount of light absorbed by the fluorescent substance. (b) the probability of activation of the molecules of the reactants on collision with these activated niolecules of the fluorescent substances. (c) the probability of the activated fluorescent molecule to decompose into some other products. According to the newer conception of the quantised molecule, if a fluorescent molecule has absorbed less energy, it will not be in a sufficiently higher state of quantum orbit and hence the energy which this molecule can give to other reactant molecule in falling to a lower orbit will not be sufficient to activate a molecule like that of carbon dioxide which requires IZO,OOO calories for a gram mol to be activated for formaldehyde formation. Moreover, if the substance acting as photosensitiser has a great tendency to decompose into
+
1
+
“Photosynthesis” (1926).
PHOTO-SYNTHESIS IN TROPICAL SUNLIGHT
581
other constituents, as was found notably in the case of fluorescein and cartharamin and they were easily decolourised in light in three or four hours, there will not be sufficient chance of activation of a COZmolecule. In the light of these negative results obtained with fluorescent substances, it appears that the fluorescent property of chlorophyll is not of much importance in bringing about photosynthesis. It seems that this fluorescent property of chlorophyll helps all the more in its decolourisation when exposed to sunlight. Moreover the probability of activation of a COI molecule will depend upon the intake of energy by the fluorescent chlorophyll molecule and this energy will be given out as a mono-chromatic radiation on its reversion to its original state. As COZ shows an absorption in the ultraviolet region, it cannot be possible for this molecule to be activated by this monochromatic radiation, which certainly will not be in the ultraviolet region Thus it is quite clear that the marked photo-activity of chlorophyll has to be ascribed to properties other than fluorescence. Weigert’ advances the view that the absorption of light by chlorophyll results in an internal photo-electric effect, which results in the break-down of water into Hz02 and hydrogen, which react with carbon dioxide according to the following equation:Hz HzOz COz $ HCHO H20 0 2 Besides these theories where an essentially physical significance is assigned to chlorophyll, there are some which give it an active part in the chemical reactions. Of these mention may be made of the theory of Willstatter and StolP who are of the opinion that carbonic acid forms a complex with chlorophyll, which with absorption of energy passes into an isomeric peroxide type of compound. This breaks up thermally probably by the action of an enzyme with the formation of formaldehyde, oxygen, and regeneration of chlorophyll. A modification of this theory of Willstatter and Stoll has been put forward by Gopala Rao and Dhar. They state that apart from all these properties of chlorophyll, it also exercises reducing properties to a certain extent. They have been able to demonstrate the reduction of COz to CO in presence of chlorophyll when exposed to light. The CO was tested by the iodine pentoxide test. According to them photosynthesis consists of three stages:(a) I n the first stage chlorophyll reacts with carbonic acid under the influence of light with the formation of chlorophyll peroxide and carbon monoxide. (b) The nascent carbon monoxide reacts with water giving formaldehyde, and oxygen is evolved. Hz0 CO $ HCHO 0 the other half of the molecule of oxygen comes from the decomposition of the chlorophyll peroxide and is probably thermal in nature and due to the enzyme present in the chloroplast. (c) Polymerisation of this formaldehyde to sugars.
+
+
+
+
+
Z. physik. Chem., 106, 3 1 3 ( 1 9 2 3 ) .
* “Vnterauchungen uber Chlorophyll”
(1913).
+
582
ATMA RAM RAJVANSHI AND N. R. DRAR
Thus oxygen evolution takes place in two stages, the one half being liberated in the action of carbon monoxide with water to form formaldehyde; the other half being liberated during the decomposition of chlorophyll peroxide. Gopala Rao and Dhar state that the photosynthetic activity is aided by the reducing action of chlorophyll, and that in the presence of a photocatalyst having reducing properties as well, formaldehyde formation is likely t o be quicker and much facilitated. Additional support to this theory has been given from the experiments described in this paper. It has been found that MnClZand FeS04 give a larger amount of HCHO than the other insoluble powders. Baly and co-workers have stated that insoluble powders are the best catalysts for photo-synthesis. In view of this work of Baly, nickel, cobalt and copper carbonate and other insoluble powders mentioned in the experimental section were tried. No doubt they are good catalysts but MnC12 was found to be better than cobalt carbonate. The amount of formaldehyde was fifty times greater than the limit of sensitivity of the test. It seems to me that MnClz and F&04 apart from being photosensitisers exert reducing action, thus reducing some COZto CO which easily forms formaldehyde. Why more formaldehyde is not detected with FeS04, is due to the fact that some of the FeSOd is oxidised, and the ferric salt so formed being an efficient converter of formaldehyde into sugars polymerises this formaldehyde a t once. The temperature coefficient of formaldehyde formation from COz and water vapour in presence of MnC12 and chlorophyll was found to be nearly 1.5. This value of the temperature coefficient is greater than unity. But it is a well-known fact that purely photochemical reactions have their temperature coefficients nearing unity. It appears, therefore, that there are two stages in formaldehyde synthesis. One is the photochemical one and the other is the thermal process and hence the total process has a temperature coefficient greater than unity. Moreover, Miss Mathaei and Blackmann, Warburg and Willstatter have found that the temperature coefficient of photosynthesis in plant is greater than unity. Miss Mathaei and Blackmann have found that the temperature coefficient is as high as 2. This cannot certainly be characteristic of a photochemical reaction, but of thermal and enzymic reaction. The temperature coefficient will depend upon the slowest reaction. If the photochemical reaction is slow, the temperature coefficient will tend to be unity and if the thermal reaction is a slow one, the temperature coefficient will be higher. This behaviour has been well observed by Willstatter in the case of green and yellow varieties of the same species. The experimental results of Warburg on the relation between light intensity and temperature coefficient also support the new scheme of photosynthesis as advanced by us. All conditions remaining the same, the temperature coefficient is unity when the light intensity is low and the temperature coefficient is greater than unity when the light intensity is higher. I n the first case, because the light intensity is low the photochemical reaction will be slow and will limit the whole process. When the intensity of light is high, the photochemical reaction will be sufficiently rapid and the thermal reaction will be slow and
PHOTO-SYNTHESIS IN TROPICAL SUNLIGHT
583
hence the thermal reaction will limit the whole process. Thus from considerations already explained it is clear that in the former case, the process being limited by the light reaction and in the latter by the thermal reaction, the temperature coefficient in the former case will be smaller than in the latter. Blackmann’s principle of limiting factor which has been utilised in the discussion of the temperature coefficient has also shown that an internal factor like the content of chlorophyll with which carbon dioxide reacts is limiting the whole process. It was pointed out in an earlier part of this thesis that increase of carbon dioxide flow gave smaller amounts of formaldehyde, under similar conditions. This feature of the problem is also evident from Warburg’s researches on the effect of carbon dioxide concentration, on the rate of photosynthesis. With increasing carbon dioxide concentration above a certain limit Warburg found that the increase in the rate of photosynthesis rapidly falls off, until a certain concentration of COz, further increase in the latter produces no effect. According to this hypothesis COZ reacts with chlorophyll as follows: Chlorophyll carbon dioxide water. = chlorophyll peroxide CO.
+
+
+
At low concentrations of COS when chlorophyll is in comparatively greater quantities, it is clear that the photosynthesis will increase with increasing concentration of COz. But this will not increase ad infinitum. The content of chlorophyll being limited, a certain stage will be reached where the content of chlorophyll is no longer in excess but is just sufficient to interact with the carbon dioxide present a t any instant, if the concentration of carbon dioxide is further increased there will be no further increase in the reaction velocity, as the chlorophyll with which carbon dioxide interacts is in limited quantities. The extra carbon dioxide finds no chlorophyll to react with, hence no photosynthetic increase with increase of carbon dioxide concentration is observed.
Polymerisation of Formaldehyde to Sugars The results obtained with such experiments are interesting and lead to important conclusions regarding the mechanism of the polymerisation of formaldehyde to sugars and ultimately to the mechanism of photosynthesis. Solution of formaldehyde exposed to light is not polymerised to sugars when it is exposed to sunlight but if a formaldehyde solution is exposed with catalysts like FeC13 and chlorophyll, a sure and distinct test for reducing sugars is obtained. Thus it is quite clear that in presence of chlorophyll, formaldehyde can be polymerised to reducing sugars by using the energy absorbed by chlorophyll. The yield of sugars, however, is not so great as to account for the large photosynthetic activity going on in the plants. But it should not be concluded from this that formaldehyde cannot form the first stage in the process of photosynthesis. There is an essential difference in this formaldehyde used in the laboratory and the formaldehyde synthesised in the plants. The formaldehyde synthesised is in a nascent state when it is
5 84
ATMA RAM RAJVANSHI Ah’D N. R. DHAR
just formed and thus possesses a great store of energy. This stored energy in nascent formaldehyde makes a formaldehyde molecule capable of being polymerised into sugars. From the fact that a large amount of energy is needed for the polymerisation of ordinary formaldehyde to reducing sugars, it is quite apparent that a nascent molecule of formaldehyde must be in possession of a large amount of stored energy. The results with ferric chloride and chlorophyll leave no doubt as to the validity of the statement that formaldehyde even in the ordinary condition is capable of being polymerised into reducing sugars. The experiments on the temperature coefficient of formaldehyde formation are interesting and point out clearly that the polymerisation of formaldehyde into reducing sugars is essentially a photochemical process because the temperature coefficient between 30 to 4ooC is nearly unity and this is a characteristic of the truly photochemical reactions. Baly,l however, has stated that the photosynthetic activity falls in “vitro” at about 31Oc and in plants above 36’C. These results of Baly in “vitro” are not corroborated by our observations. Further work will explain the difference between our results and those of Baly. It is difficult to state definitely whether nascent formaldehyde itself polymerises to sugars or requires an extra amount of energy to be polymerised. It is very likely that the polymerisation of formaldehyde into reducing sugars might be taking place in two stages. First the formaldehyde molecule simply polymerises into a substance having a formula (HCHO), and then this is transformed by an intra molecular change into glucose or other sugars, as the case may be. Our experimental results show definitely that ferric salts and chlorophyll are efficient photocatalysts in the polymerisation of formaldehyde to reducing sugars.
Oxidation of Ammoha and its Compounds to Nitrate Dhar and Sanyal have shown that ammonia solution can be slightly oxidised to nitrate by passing air when exposed to light. Gopala Rao and Dhar have shown that the velocity of oxidation can be greatly increased by adding some photo-sensitisers like titanium dioxide, zinc oxide, cadmium oxide, etc. They have further shown that ammonium sulphate, chloride, carbonate and phosphate can also be oxidised. Similarly on the basis of these experiments they have given out the view that the nitrification in soils is a t least partly photochemical taking place at the surface of various photocatalysts present in the soil in the presence of sunlight. They have shown that this theory is capable of explaining many facts inconsistent with the bacterial theory of nitrification. The present authors have now shown that ammonium oxalate, tartrate, and formate can similarly be oxidised though much less than the inorganic salts. They have shown that the rate of oxidation is greater in the March sun than in the January sun. Nature, October 2 5 (1930).
PHOTO-SYNTHESIS IN TROPICAL SUNLIGHT
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summary The polymerisation of formaldehyde to reducing sugars has been effected by light in the presence of ferric chloride, chlorophyll, nickel carbonate, methyl orange and zinc oxide. Of these ferric chloride is the best photochemical polymerising agent. 2. The temperature coefficient of the polymerisation of formaldehyde to reducing sugars has been found to be 1.1 for a rise of I O between 30 and 40. 3 . A few cases of the photosynthesis of nitrogenous compounds has also been reported. 4. A discussion of the results of Part 111 and I V is presented I.
Chemieal Laboratory, University of Allahabad, Allahabad, India. June
4,1931.
