1 8 PLANT RESPIRATION 



tigations of the influence of light on plant respiration have been 

 carried out in the absence of materials sensitive to light. It 

 would be most interesting to perform experiments with inten- 

 sive illumination, on the action of catalysts sensitive to light 

 on the respiratory changes. Likewise in the study of the effect 

 of light, the influence of the heating of the material by the rays 

 of light has not always been taken into consideration.^- A. 

 IMayer and Deleano^ noted a daily periodicity in the intensity 

 of respiration under natural conditions. Spoehr' has made the 

 interesting assumption that during the day the respiration of 

 plants is increased by reason of the fact that under the influence 

 of the sun's rays an ionization of the atmospheric oxygen takes 

 place and in consequence thereof autoxidation is instigated in 

 the protoplasm. 



The Influence of the Concentration of Oxygen on Respira- 

 tion.— Saussure^ has already pointed out that a reduction of 

 oxygen content in the surrounding atmosphere to half the 

 normal concentration has no eft'ect on plant respiration. Wil- 

 son'* subsequently proved that plants respire normally in an 

 artificial mixture of 0.2 parts atmospheric air and 0.8 parts 

 hydrogen, and indeed exhibited the same rate of respiration as 

 in ordinarv air. Even with an oxygen content of i % no check- 



. CO2 



ing of plant respiration could be noted. Likewise the ratio ^^ 



grew larger only with a very low content of oxygen (1-2%).^ 



' Mayer, A. und Deleano. Zeitschr. f. Bot. 3 : 657. iQir. 



sSpoehr, H. A. Bot. Gaz. 59: 366. igiS- iThis hypothesis has been tested recently 

 by students of Blackman who have increased the ionization of air by exposure to polo- 

 nium. Middleton (Ann. of Bot. 41 : 343-356. 1927) used barley seedlings and Whimster 

 (Ibid. 41 : 357-374- 1927) worked with leaves of Pelargonium zonale. Under carefuly con- 

 trolled conditions, both workers found that the rate of COj-production is appreciably 

 increased by the presence of a high concentration of ions in the air. the effect being 

 greater with the green leaves.) 



>de Saussure, Th. M6m. de la soc. phys. de Geneve 6: 552- i833- 



* Wilson. Untersuch. aus d. bot. Inst. Tubingen i : 685. 1885. 



'Johannsen, W. Untersuch. aus d. bot. Inst. Tubingen i: 716. 18S5; Stich, C. 

 Flora. 74: I- 1891. 



'' Neither has the strength of the iUumination been properly measured. As 

 illustrations of the results obtained by the use of the katharometer in studying 

 both respiration and photosynthesis. Waller (New Phyt. 25: 109-118. 1926) 

 gives a few measurements which do recognize the several factors that must be 

 measured. — Ed. 



