MISCELLANEOUS CHEMICAL STIMULANTS 343 



A large variety of stimulation effects caused by almost infinitesimal 

 quantities of different chemical agents was described by Bose (1923, 1924). 

 His investigation originated in a casual observation: The rate of photo- 

 synthesis of certain water plants was observed to increase sharply during 

 a thunderstorm. Bose attributed this phenomenon to the oxides of 

 nitrogen produced by electric discharges in the atmosphere; this conclu- 

 sion induced him to investigate the effects on photosynthesis of various 

 stimulants. He found that the photosynthesis of Hydrilla verticillata 

 was trebled by approximately 5 X 10"*% of nitric acid and doubled by 

 10-^% of a thyroid gland extract. Iodine (10"^%) caused a rate increase 

 by 60%, and formaldehyde (10-^%), by 80%. These curious results 

 certainly are in need of confirmation and elaboration. 



Kholodny and Gorbovsky (1939, 1941) observed that the rate of 

 photosynthesis of Hydrangea and hemp is temporarily doubled by 0.1% 

 of /3-indoleacetic acid (this compound — the so-called " heteroauxin " — is 

 an "artificial growth hormone"). 



Bukatsch (1939) described a similar stimulation of photosynthesis by 

 ascorbic acid. (It was mentioned on page 273 that ascorbic acid may 

 play a part in photosynthesis; Bukatsch's experiments were intended to 

 test this hypothesis.) 



Treboux (1903) observed a strong stimulation of photosynthesis of 

 aquatic plants by dilute (e. g., 10" '^ normal) acids. However, Wilmott 

 (1921), while confirming the experimental results of Treboux, found the 

 interpretation to be erroneous. The increase in photosynthesis was 

 caused, not by stimulation, but by the dissolution of incrustations of 

 solid carbonate which often occur in plants grown in hard water, and by 

 the consequent increase in the supply of carbon dioxide. No "stimu- 

 lation" could be observed if plants were grown in soft water; even in 

 hard water the effect disappeared if the supply of external carbon dioxide 

 was made ample. 



The effect of mechanical injury on photosynthesis also must be 

 mentioned in this chapter, because it is probably due to the internal 

 production of stimulating chemical agents or "wound hormones." The 

 effect of wounding was first investigated by Kostychev (1921), who 

 was unable to find any stimulation; to the contrary, leaves of Betula 

 puhescens, shredded by a needle, showed a somewhat reduced rate of 

 photosynthesis. Lubimenko and Shcheglova (1933) attributed Kosty- 

 chev's result to a loss of active leaf surface, and repeated his experiments 

 (with leaves of wheat and barley) by punching holes of known area and 

 comparing photosynthesis per unit surface of wounded and intact leaves. 

 They found a marked stimulation which, however, became apparent 

 only two or three days after the injury. The authors attributed this 

 "induction period" to the water loss caused by wounding. After the 



