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 light, 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, Lacluca 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 grows 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 



