344 VARIOUS CHEMICAL AND PHYSICAL AGENTS CHAP. 13 



wounds healed, stimulation gained over inhibition, and remained notice- 

 able for several weeks. If the perimeter of the wounds was too large, 

 the stimulating effect was pronounced even in the first two of three 

 days; but after this the photosynthesis sank rapidly below the normal 

 level. The wave of photosynthesis after injury ran parallel with a wave 

 of respiration. Lubimenko considered this as a demonstration of the 

 dependence of both processes on "protoplasmic stimulation," and thus 

 as an argument in favor of Kostychev's theory of protoplasmic control 

 of the photosynthetic apparatus (c/. Vol. II, Chapter 26). 



G. Physical Stimulants and Inhibitors * 



1. Ultraviolet Light 



The absorption bands of chlorophyll extend into the ultraviolet as 

 far as the absorption has been investigated — i. e., down to 220 m/i (c/. 

 Vol. II, Chapter 21). However, many other components of the cells 

 also absorb strongly in the ultraviolet— particularly below 300 m/x; and 

 this absorption often is injurious to the organism as a whole, and also 

 destroys its capacity for photosynthesis. What one would like to know 

 is whether the ultraviolet light absorbed by chlorophyll (or by the 

 carotenoids) also has this destructive effect, or whether it can be utilized 

 for photosynthesis in the same way as blue and violet light (i. e., in all 

 probability, by an immediate conversion of the excessively large quanta 

 into the smaller quanta of red light, and dissipation of the residual energy 

 as heat, cf. the discussion in Vol. II, Chapter 21). This question cannot 

 be answered without a quantitative analysis of the cell absorption in the 

 ultraviolet, and apportionment of the absorbed energy between the sev- 

 eral absorbing agents— a method whose application to visible light will 

 be described in chapter 22 (Vol. II). At present, we possess only scat- 

 tered data concerning oxygen liberation and starch formation in ultra- 

 violet light. More systematic information is available concerning the 

 lethal action of ultraviolet light on plants — but without an identification 

 of the compounds whose absorption is responsible for the injury. 



Photosynthesis, i. e., oxygen liberation and consumption of carbon 

 dioxide, undoubtedly still proceeds briskly in the near ultraviolet {cf., for 

 example, Gessner 1938). Starch formation was observed down to 300 m/x 

 by Ursprung and Blum (1917), and below 300 m/x by Richter (1932, 

 1935). In the latter case, however, the formation of starch appeared to 

 be a delayed consequence of light stimulation, rather than a direct result 

 of increased photosynthesis. (A short exposure to very intense ultra- 

 violet light caused an increased production of starch in the dark for hours 

 afterwards.) 



* Bibliography, page 350. 



