SUN RAYS AND PLANT LIFE— JOHNSTON 365 



The leaf stems or petioles turn toward the light source. Ordinary 

 house plants such as the geranium show this same response as the}' 

 grow by a well-lighted window. Unless such plants are turned 

 occasionally, the stems will grow out toward the light, giving them a 

 lopsided appearance. 



From superficial observations it would appear that light hinders 

 or retards elongation of plant cells. It is frequently noted that the 

 stems of many plants grow more rapidly at night than during the 

 day. Potatoes send forth greatly elongated shoots in a darkened 

 cellar; if these same potatoes were permitted to remain in strong 

 light, the sprouts would be very much shorter and the internodes 

 greatly reduced. 



In the case of plants illuminated on one side it is noted that the 

 shaded sides of the stems have stretched more than those receiving 

 direct illumination. The uneven rate of growth on the opposite sides 

 results in curved stems and a general appearance of the plant turning 

 toward the light. This characteristic bending is very well illustrated 

 with the oat sprout shown in plate 4, figure 1. Because of its con- 

 venience in handling and its ready response to light the oat seedling 

 or coleoptile, as it is technically called, has been used very extensively 

 in phototropic studies. 



Although superficial observations clearly indicate that the sensitiv- 

 ity of the plant toward radiant energy is such that it reacts differently 

 to light and darkness, the question as to its sensitivity to different 

 colors or wave lengths of light is not so readily answered. To obtain 

 an answer a plant might be placed half-way between two equally 

 intense lights, for example blue and green, and the direction of bend- 

 ing noted. The plant's sensitivity to different colors could thus be 

 determined in a general way. Such experiments have been conducted 

 by the Smithsonian Institution to determine growth sensitivity to 

 wave lengths of light (Jolinston, 1934, 1935 a). 



The general procedure used in studying the wave-length effects in 

 phototropism, as this type of response is termed, is to place an oat 

 seedling between two lights of different color. After a time interval 

 the seedling is examined for a one-sided growth. If, for example, the 

 seedling being exposed to blue light on one side and to green on the 

 other, a distinct bending was noted toward the blue light, it was then 

 known that the blue light exerted a greater retarding action, since 

 the side of the seedling toward the green light grew more, thus bend- 

 ing the seedling toward the blue light. The lights were then so 

 adjusted as to increase the green, or decrease the blue intensity. An- 

 other seedling was used and the process repeated until a balance 

 point was obtained where the effect of one light neutralized the effect 

 of the other in such a manner that the seedling grew vertically. 



