XEW RESEARCHES OX THE EFFECT OF LIGHT WAVES 

 ON THE GROWTH OF PLANTS 



By F. S. BuAcKKTr and Eakl S. Johnston 



[With 3 plates] 



Ono of the most commonly observed relationships between plants 

 and light is that of phototropism, or the directional growth of plant 

 organs as inliiienced by light. A potted plant, such as tiie geranium, 

 [daced on the window ledge where light will fall on it from one side 

 will, in a comparatively short time, turn its leaves in a position to 

 receive maximum light from outside the room. The beautiful sym- 

 metry of plants is veiy easily lost when they are exposed to unilat- 

 eral illumination. 



Various parts or organs of plants respond differently to light. 

 Young shoots and leaf petioles usually grow toward the source of 

 light, while the leaves frequently set themselves at right angles to its 

 beams. Occasionally there are organs which grow away from the 

 source of light. Tliese three types of growth are known respectively 

 as positive, transverse, and negative phototropism. One of the few 

 plants which shows all three types of phototropism is the mustard 

 seedling illustrated in Plate 1. A 200-watt electric lamp served as 

 the light source in this particular case. The direction of the light is 

 indicated by the arrow. The shoot shows positive phototropic bend- 

 ing, the roots negative, and the small leaves distinctly show a 

 tendency to expand at right angles to the beam of light. 



Light has a threefold eflFect on plants. The plant's response to a 

 bright light is different from that to a dim light. Chlorophyll, the 

 green pigment, is formed in the presence of light, but if the light is 

 very intense, this coloring matter is destroyed. The number of hours 

 per day that the plant is exposed to light governs to a large extent 

 the nature of its growth. The very interesting and extensive experi- 

 ments of Garner and AUard of the United States Department of 

 Agriculture have conclusively shown that plants may be made to 

 grow vegetatively, or to set flowers and fruit, merel}' by changing the 

 length of time they are exposed to light. The color, or wave length 

 of light, also plays an important j^art in the plant's behavior. 

 Chlorophyll formation will be more raj)id in red than in blue or 

 violet light. On the other hand, the short wave lengths at the violet 



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