476 PHYSIOLOGY 



is the red-yellow, the most effective light as a stimulus is that near the 

 violet end of the spectrum. Since this is the region of least energy, the 

 shortness and frequency of the waves are the important features of light 

 as a stimulus. In this respect the red end of the spectrum, though its 

 energy is far greater, behaves as darkness. 



Response. — In general the response of plants to light differs according 

 to the usual attitude of the organ and its mode of growth, for which 

 indeed light is largely determinative. Parallelotropic organs respond 

 by directing their tips toward or away from the source of light, while 

 plagiotropic organs place themselves more or less at right angles to the 

 direction of the rays. Primary stems, therefore, are mostly positively 

 phototropic, and some roots, particularly aerial roots, are negatively 

 phototropic; while leaves are mostly transversely phototropic or diapho- 

 totropic. 



These phenomena were first known as heliotropism, etc., and are often still so 

 called, because the sun in nature is the source of all light. It seems better, however, 

 to use the wider term, since plants respond in the same way to artificial light, which 

 is so largely used in experimental work. The general result of these reactions is 

 the same as of those to gravity, so far as the same organs are sensitive to both 

 stimuli, though the two act from opposite directions in nature. 



Intensity. — The intensity of the light may determine either a positive 

 or a negative curvature, and within certain limits between these two there 

 is a range of intensity which calls forth no visible reaction ; this is the 

 point of phototropic indifference. It is by no means the point of no 

 excitation. At high intensities that call forth negative curvature, injury 

 soon appears. Near the lower limit of intensity that can produce an 

 end reaction, plants show themselves very sensitive to light. Thus, 

 radish seedlings respond to the light of a single candle at a distance of 

 about 8 m., the broad bean (Vicia Faba) at 22 m., and a cress {Lepidium 

 sativum) at about 55 m. The differences that plants can distinguish are 

 within the limits of error for the unaided eye, and are not very easily 

 distinguishable even with the photometer. 



Time relations. — The presentation time, of course, depends upon the 

 intensity of light used, and is approximately inversely proportional to it. 

 The greatest range of presentation time recorded is that for etiolated 

 seedlings of oats, being 0.001 second with light intensity of 26,520 Hefner 

 candles, and 13 hours with light intensity of 0.000439 Hefner candle. 

 Intermediate light intensities give corresponding inverse proportional 

 intermediate presentation times. As a rule the younger an organ is, the 



