MOVEMENT 



369 



are diaphototropic and set themselves at an angle to the incident 

 rays, where they are in a position for maximum photosynthesis. 

 On the edge of the crown of a tree, it is interesting to observe how 

 the leaves are arranged to get the maximum of light with the 

 minimum of shading. On the sides of buildings and walls the 

 leaves overlap so little that they form what are called leaf mosa- 

 ics. If the light is too intense, the position may be shifted, and 

 in plants with the leaves in a vertical plane, they are so arranged 

 that they get the moderate light of the forenoon and afternoon 

 without being exposed to the intense light of midday. The edges 



A B C D E 



Fig. 28. — The rate of growth and phototropic sensitiveness of plants in various 

 parts of the spectrum. The letters at the bottom indicate the Frauenhofer 

 lines. The solid line is the curve of growth of sunflower seedlings. The dotted 

 lines represent the phototropic sensitiveness of water cress seedlings. In what 

 part of the spectrum is growth the fastest? In what part does phototropism 

 occur most quickly? (After Palladin.) 



of the leaves are thus directed to the north and south, and such 

 plants as the wild lettuce (Lactuca scariola) and Silphium laci- 

 niatum have been given the name of "compass plants." That this 

 adjustment is actually dependent upon the light intensity is 

 shown by the fact that when the lettuce is grown in the shade, 

 the north and south position is not assumed. 



Some flower heads are also positively phototropic. Species of 

 Helianthus, Hieracium, and Tragopogon stand erect at night, but 

 before sunrise the stems bend over towards the east. As the sun 

 rises the heads open and then change their position during the 

 day as the sun passes across the sky, so that in the evening they 

 face the west. At sunset the stems become erect again, remain- 

 ing in this position until morning. From this behavior the com- 

 mon name of " sunflowers" has been given to such forms. 



Even some fungi show phototropism. The most conspicuous 



