I30 THE 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 dift'erence 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 (/. c.) in a number of shade ecads. For example, a 

 form of Galium horcale, which grew with difficulty in a light value of .002, 

 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 darlcness 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 i, and a diffuse- 

 ness of .002, though small flowering plants have once or twice been found 

 in an intensity of .001. The maximum light value, even on high mountains, 

 never exceeds i by more than an inconsiderable amount, except for the 



