ULTRA-VIOLET AND SEED PLANTS 881 



Further information concerning the effect of ultra-violet radiation 

 on anthocyanin development is given in another paper (Arthur, Paper 

 XXXV). 



CONCLUDING REMARKS 



In spite of the many publications on the subject, exact knowledge 

 regarding the infiuence of ultra-violet radiation upon seed plants other 

 than its destructive action is still to be ascertained. The interest in 

 ultra-violet in recent years has been so widespread as to justify the 

 accusation of some that the subject is a fad. It is to be hoped that the 

 "fad" will not run its course before accurate information has been 

 obtained. Needless to say, such information will not be forthcoming 

 from experiments of short duration, carried out with a few plants under 

 poorly controlled conditions such as have predominated in the work of 

 the past. 



Much of the present uncertainty of our knowledge of the effects of 

 radiation upon plants rests upon the complexity of the problem itself. 

 No other environmental factor is so variable or so difficult to control. 

 The fact that plants require visible radiation for normal growth neces- 

 sitates supplying them with this radiation. Sources of visible radiation 

 usually contain also infra-red radiation. Consequently these factors 

 must be considered and equalized in test cultures and controls when the 

 effects of ultra-violet are to be studied. Total-radiation measurements, 

 transmissions of screens, spectrograms of the radiation used, and the 

 like, are in themselves insufficient to give a complete picture of the 

 nature of the radiation reaching plants, although in many papers, even 

 these variables are not given. Few, if any, authors have measured the 

 total energy or the distribution of the energy in the ultra-violet to which 

 results were attributed, to say nothing of the failure to equalize the 

 radiant energy of other wave-lengths reaching the test plants as com- 

 pared with the controls. While it is conceded that these measurements 

 are difficult to make, it must also be admitted that so long as they 

 remain unknown in an experiment the results cannot be attributed to 

 ultra-violet any more than to any other operating variable. 



In addition to the necessity of having a complete description of the 

 radiation reaching plants it is no less important to know whether the 

 plants or plant parts studied absorb selectively different portions of 

 the spectrum and to what extent. Possible photochemical reactions in 

 the plant may be greatly accelerated in a relatively narrow region of the 

 spectrum that is strongly absorbed, whereas comparatively high intensi- 

 ties in a region that is feebly or not at all absorbed might be without 

 effect. 



REFERENCES 



1. Anonymous. Kulturversuche untcr Glassorten, die ultra-violetten Strahlen 

 durchlassen, Brephosglas und Ultravitglas. Veroffentl. Landwirtschaftsk. 

 Rheinprov. n. s. 18: 49-50. 1930. 



