478 PHYSIOLOGY 
It has been shown that the impact of the ether waves of full sunlight produces 
a pressure equal to about half a milligram per square meter. In a seedling of oats 
at this rate the plant would have to be sensitive to a difference of five millionths of a 
milligram and probably to one tenth of this infinitesimal amount. This is simply 
inconceivable! 
It seems most likely that it is the difference in the lighting that is per- 
ceived, for the intensity of the stimulus has an important bearing on 
the form of the reaction, and plants are able to respond to differences 
of illumination coming from different sides that are too small for the eye 
to distinguish. 
Plagiotropic organs. The behavior of plagiotropic organs toward 
light is especially interesting, because it seems to be usually of the very 
greatest importance for the welfare of the plant in food making by leaves, 
thalli, etc. The fact that the leaves of most common plants, set before 
a window, place themselves at right angles to the incident ligiit, attracts 
attention at once. If the pots be turned around, the position of the leaf 
blades will soon be changed, and they face the window again. Thus the 
leaves obviously come into a position most advantageous for receiving the 
maximum of energy for photosynthesis. The corresponding orientation 
in the open shows that it is not the direct sunlight alone to which the 
leaves respond, but rather what may be distinguished as sky light; that is, 
the brightest diffused or reflected light. Indeed in some cases the direct 
sunlight is evidently too intense, and the plane of the blades is set at an 
angle to the direct light, the edge in some plants being directed upward. 
Compass plants. When the position of leaves is uniform or nearly so, and cor- 
responds approximately with the plane of the principal meridian, the plants are 
known as compass plants. The wild lettuce, Lactuca Scariola, is the most widely 
distributed of these, and on the prairies and along railways, the compass plant, 
Silphium laciniatum, which illustrates the habit far better, is common. Other 
plants in this and other countries have the same habit. That this is a response 
to intense light is seen easily in the lettuce, for when this plant srows in the shade, 
its meridional position is not assumed. 
Fixed light position. The reaction of a leaf to light can occur only 
while it (especially the petiole, which is the seat of most curvatures) is 
still growing or capable of growing. During this period the habitual 
responses lead finally to a position known as the fixed light position, a 
sort of resultant, which on the whole gives the blade the most advan- 
tageous illumination. One result of this is the arrangement of blades 
in such a way as to avoid shading one another. This produces the 
so-called leaf mosaics (see Part III, p. 543.) The movements of the leaf 
