STIMULATING INFLUENCE OF LIGHT 



127 



194. Stimulating Influence of Light. The absorption and use 

 of the energy of light depends upon the angle with which the 

 rays strike the surfaces, and the intensity of the impinging rays. 

 A few species of green plants have become adapted to living in 

 faint diffuse light, but the greater majority find their optimum in 

 direct sunlight. In order to be able to attain the most advan- 

 tageous positions it has been necessary for the plant to acquire 

 irrito-motility to light, and to be able to place its body at proper 

 angles to the impinging rays. Species which have a habit of 

 clinging closely to a horizontal or vertical substratum or support, 

 tend to move their bodies away from the source of light as is 

 also the case with typical roots constituting aphototropism, while 

 shoots generally tend to move toward the center of the radiations 

 because of their prophototropism. The organization of the shoot 



FIG. 64. Diagrams of shields for allowing phototropic stimuli to fall on restricted 

 regions of a seedling. A, paper shield adjusted to seedling of Avena exposing the 

 basal portion to action of light the direction of which is indicated by the arrows. 

 Cross section of tip at //, and basal portion at ///. J5, a band of tinfoil is wound 

 tightly around a hypocotyl of Brassica at v, and a free part is twisted into a cap which 

 fits tightly over the cotyledons at/. C, black paper shield for covering basal portions 

 of hypocotyl of dicotyledonous plants. D, cylinder of tinfoil r, to surround basal 

 portion of seedling of Avena with cap J, through which the apical portion projects. 

 a, a, level of the soil. After Rothert. 



is such that this generally carries the leaves into a zone of 

 stronger illumination. Leaves exhibit still a different form of re- 

 action by which they place their surfaces at right angles to the 

 direction of the rays in response to their diaphototropism. This 

 form of irritability is also exhibited by many zygomorphic flowers. 

 Phototropism is also exhibited by some chlorophylless forms. The 

 reactions in such organisms are generally of advantage in the dis- 



