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 o.ooi 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 
