Light 313 



QUALITY OF LIGHT 



Much work has been done on the effects of different wave lengths 

 (colors) of light. Many of the early results here are of doubtful value 

 since it was often difficult to change the wave length without at the same 

 time modifying intensity. There are many well-established facts, how- 

 ever, from which conclusions can be drawn. 



It seems clear, for example, that the longer wave lengths, notably 

 those in the red, promote a marked elongation of cells and thus of tissues, 

 whereas the blue rays (and white light) check this effect and tend to 

 prevent elongation. Teodoresco (1929) describes many examples of this, 

 especially from less highly organized forms such as young plants of liver- 

 worts or fern prothallia growing from spores. In these cases where white 

 or blue light is used, a rather compact group of cells develops from the 

 spore, but in red light a much elongated, spindly cell. The same effects 

 are evident in fungi. An important fact is that the plane of cell division 

 is usually controlled by the light. Mohr ( 1956 ) finds that in young fern 

 prothallia in red light cell division tends to be at right angles to the 

 polar axis of the structure so that filaments of elongate cells are formed, 

 whereas in blue light division is in various planes, so that a plate of cells 

 develops. In a normal and growing fern prothallium transferred to red 

 light, many of the cells grow out to form filaments. 



The same effects are to be seen in the more complex higher plants. Thus 

 Teodoresco finds that blue light checks petiole elongation but increases 

 blade area, and Vince ( 1956 ) that in many plants, when grown under 

 lights of equal energy levels, total stem length, internode length, and 

 leaf length increase with increase in the wave length. Not all plants 

 react alike, however. Whether the mechanism by which the red rays 

 promote elongation is like that by which low light intensity does so is 

 not clear, but presumably the same processes are affected by both factors. 

 Wassink and Stolwijk ( 1952 ) used equipment by which it was possible 

 to grow plants in various wave lengths of monochromatic light, and 

 under these conditions there was strong elongation of the stem and curl- 

 ing of leaves and petioles in green, yellow, and red light but essentially 

 normal growth in blue. Fortanier (1954), however, observed that only at 

 high light intensities is stem elongation greatest in red, yellow, and 

 green. At low ones it is greatest in blue. Leaf number was not affected by 

 wave length. 



Quality of light also affects flowering. Curry and Wassink ( 1956), work- 

 ing with annual Hyoscyamus niger, found that flowers were produced in 

 blue and infrared-plus-red radiation but that neither stem elongation nor 

 flower-bud formation occurred in green or red light. 



