PLANT GROWTH IN RELATION TO WAVE-LENGTH 



BALANCE 



By earl S. JOHNSTON 

 Assistant Director, Division of Radiation and Organisms, Smithsonian Institution 



(With Four Plates) 



INTRODUCTION 



There can be little doubt that wave-length distribution exerts an 

 enormous influence on the growth of plants. Numerous experiments 

 show that stem elongation is greatly retarded under blue light, whereas 

 an acceleration takes place in the red and near infrared regions. 

 Chlorophyll production takes place better toward the red than toward 

 the blue end of the visible spectrum. Phototropic sensitivity is great- 

 est in the blue and zero in the red. For equal amounts of energy 

 falling on the leaf, two maximal regions of CO2 absorption have been 

 found — one in the red, the other in the blue. It thus appears that a 

 wave-length region best suited to a given plant process may be entirely 

 without effect upon another. 



In plant nutrition studies, experiments have shown that there is a 

 general balance in the proportionate amounts of mineral elements of 

 a nutrient solution that brings about a favorable growth response in 

 plants. Although there may be considerable latitude in the ratio of 

 amounts of elements in such a solution, it may be said that a balanced 

 condition exists. 



In a somewhat analogous manner, it is possible to think of the light 

 requirements of plants as a balanced condition of intensities of dif- 

 ferent wave lengths which bring about good plant growth. Both light 

 intensity and wave-length distribution vary to a considerable extent 

 over the earth's surface. Likewise the character of the vegetation 

 varies. Since plants have been growing on the earth for countless 

 ages, it is reasonable to assume that their physiology is adjusted best 

 to sunlight. Although there is experimental evidence to show that 

 different processes go on better in some wave-length regions of the 

 spectrum than in others, yet the best growth, when all the processes 

 are considered simultaneously, apparently takes place in the natural 



Smithsonian Miscellaneous Collections, Vol. 97, No. 2 



