130 TIE PLANT 
have little influence, since both the direction and the angle of the incident 
rays change continually throughout the day, and the position of the leaf 
itself is more or less constantly changed by the wind. The influence of 
duration upon the character of light stimuli is difficult to determine. There 
can be no question that the time during which a stimulus acts has a pro- 
found bearing upon the response that is made to it. In nature the problem 
is complicated by the fact, that light stimuli are both continuous and 
periodic. The duration of sunlight is determined by the periodic return of 
night as well as by the irregular occurrence of clouds. Since one is a 
regular, and the other at least a normal happening, it is necessary to con- 
sider duration only with respect to the time of actual sunlight on sunny 
days, except in the case of formations belonging to regions widely different 
in the amount of normal sunshine, i. e., the number of cloudy days. In 
consequence, duration is really a question of the intensities which succeed 
each other during the day. The differences between these have already been 
shown to fall within the efficient difference for light, and for this reason 
the ratio between the light intensity of a meadow and of a forest is essen- 
tially the ratio between the sums of light intensity for the two habitats, i. e., 
the duration. The latter is of importance only where there is a daily alterna- 
tion between sunshine and shadow, as at the edge of forest and thicket, in 
open woodland, etc. In such places duration determines the actual stimulus 
by virtue of the sum of preponderant intensities. The periodicity of day- 
light is a stimulus to the guard cells of stomata, but its relation to intensity 
in this connection is not clear. 
The amount of change in light intensity necessary to constitute an 
efficient stimulus seems to depend upon the existing intensity as well as 
upon the plant concerned. Apparently, a certain relative decrease is more 
efficient for sun plants than for shade plants. At least, many species sooner 
or later reach a point where a difference larger than that which has been 
efficient no longer produces a structural response. This has been observed 
by E. S. Clements (2. c.) in a number of shade ecads. For example, a 
form of Galium boreale, which grew with difficulty in a light value of .o02, 
showed essentially the leaf structure of the form growing in light of .03, 
while the form in full sunlight showed a striking difference in the leaf struc- 
ture. In considering the light stimuli of habitats, it is unnecessary ' to 
discuss the stimulus of total darkness upon chlorophyllous plants, although 
this is of great importance in experimental evolution and in control experi- 
ment. The normal extremes of light intensity, i. e., those within which 
chlorenchym can function, are full sunshine represented by 1, and a diffuse- 
ness of .002, though small flowering plants have once or twice been found 
in an intensity of .oo1. The maximum light value, even on high mountains, 
never exceeds 1 by more than an inconsiderable amount, except for the 
