314 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1934 



which everyone is familiar is the bending toward the light of common 

 house plants that are grown on window ledges. The necessity for 

 turning them from day to day in order to have them grow symmetri- 

 call}'^ is common knoAvledge. All these examples illustrate the fact 

 that plants are sensitive to light and that they have a definite light- 

 response. 



The lop-sided growth of a potted window plant such as a geranium 

 is an indication that the external conditions conducive to growth 

 are not symmetrical. Although the temperature and humidity of 

 the air may be the same on all sides of the plant, the light conditions 

 are decidedly different. Those portions of the plant next to the 

 window receive more light than those toward the darker interior, 

 and the plant grows toward the more intense light. The phenomenon 

 of unsymmetrical growth due to unilateral light conditions has been 

 used by plant investigators to determine the sensitivity of plants 

 to light. This response is known as phototropism. 



From a superficial observation 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 appear- 

 ance of the plant turning toward the light. 



Although superficial observations clearly indicate that the sensitiv- 

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

 ently to light and darkness, the question as to its sensitivity to dif- 

 ferent 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, to ascertain to- 

 ward which one the plant bends. The plant's sensitivity to different 

 colors could thus be determined in a general way and compared to 

 the sensitivity of various animal reactions or even to human vision. 



There is abundant evidence that phototropism is a special case of 

 the more general light-growth phenomenon in plants, and that in- 

 tensity as well as wave length and duration of radiation must be 

 carefully considered. In phototropic experiments the specific char- 

 acteristics of the plant must be known. The regions sensitive to light 

 are frequently localized. The plant's response is sometimes positive, 

 that is, bending toward the light ; sometimes negative, bending away 

 from the light, and again it may even change from positive to nega- 



