EFFECTS OF REGIONS OF VISIBLE SPECTRUM lib 



the reverse was true. After adjustment had been made following a 

 transfer from one light condition to another the difference in rate in the 

 blue as compared with the red end was never less than 20 per cent and 

 sometimes as great as 50 to 60 per cent in potted plants of Cyperus. If 

 leaves were used which had just been killed by immersion in boiling water 

 for 5 min., change from one region of the spectrum to another did not 

 influence the rate of water loss. 



From these results, obtained under better controlled conditions, 

 Iwanoff and Thielmann conclude with Wiesner and his supporters that 

 the blue-violet end of the spectrum causes a greater rate of transpiration 

 than does the red end. However, they do not believe that the explana- 

 tion of this is the greater absorption of this region of the spectrum by 

 chlorophyll, nor that the transformation of this into heat energy is the 

 explanation of the increased rate of transpiration. From their experi- 

 ments with recently killed green leaves, the rate of water loss from which 

 was not altered by a change in light quality, they conclude that increased 

 transpiration in the blue-violet region of the spectrum must be a function 

 of the living protoplasm and not a result of increased evaporation due to 

 the availability of more heat energy. In other words, they believe that 

 light quality does affect the rate of transpiration, and this because it has a 

 physiological rather than a physical effect on the rate of water loss. 

 It is suggested that the effect of the blue-violet region on the degree of 

 stomatal opening or upon permeability of protoplasm might explain or 

 help to explain its effect. The meager evidence to date on either of 

 these points is inconclusive. Sierp's (45) work, considered in the section 

 on stomata, supports their first assumption. 



On the other hand, the general agreement among different investi- 

 gators that green leaves have a higher percentage absorption of radiation 

 in the blue-violet end of the spectrum w^ould tend to support a physical 

 interpretation. Furthermore, such absorption may be conditioned by 

 the chlorophyll content of the leaf. It is not surprising that the rate of 

 water loss from a living leaf is different from that of a dead leaf, yet this 

 fact by itself does not necessarily prove that the effect of radiation on 

 transpiration is primarily a physiological one. 



EFFECT OF QUALITY OF LIGHT ON STOMATA 



A number of investigators have attempted to determine the effect 

 of different parts of the spectrum on the movement of stomata. Among 

 these may be mentioned Kohl (21), Darwin (8), Lloyd (23), Sayre (35), 

 and Sierp (45). 



Kohl (21) found that stomata were widest open in the red end of the 

 spectrum between lines B and C and that a second maximum, less pro- 

 nounced, occurred between line F and the lower limit of the visible region. 

 Yellow, green, infra-red, and ultra-violet exerted essentially no influence 



