402 RELATION TO ENVIRONMENT. 



flower head follows the sun. The young head will follow the 

 sun all day even if all the leaves are cut off, and the growing 

 stem will also if all the leaves as well as the flower head are cut 

 away. Young seedlings will also turn even if the cotyledons and 

 plumule are cut off. 



This phenomenon of the rotation of leaves with the sun is 

 much more general than one would infer, as may be seen from 

 a little careful observation of rapidly growing plants on bright 

 sunny days. In Alabama I have observed beautiful rosettes of 

 Cassia marilandica rotate with the sun all day. The peculiarity 

 |s very striking in the cotton plant, especially when the rows 

 extend north and south. In the forenoon or afternoon it is 

 most striking as the entire row shows the leaves tilted up facing 

 the sun. There are many of our weeds and common flowers 

 of field and garden which show this rotation of the leaves. Some 

 of these form rotating rosettes; while in others the leaves rotate 

 independently as in the sweet clover. 



771. Fixed position of old leaves. In many of the cases cited 

 in the preceding paragraph, the rotation of the leaf only occurs 

 on sunny days. During cloudy days the leaves of the sunflower, 

 for example, are in a nearly horizontal position, or the lower 

 ones may be somewhat oblique, since the stronger illumination on 

 such a plant would be the oblique rays rather than the zenith 

 rays. As the leaves reach maturity also the epinasitic growth is 

 equalized by hyponastic growth so that the growth movements 

 bring the leaf to stand in a nearly horizontal position, or that 

 position in which it receives the best illumination. In age, then, 

 many leaves have a fixed position and this corresponds with the 

 position assumed on cloudy days. 



772. Position on horizontal stems. On horizontal stems the 

 leaves have a horizontal position, and if such a stem is stood in 

 an erect position the appearance is very odd. If the leaf arises 

 directly from the horizontal stem, its petiole will be twisted part 

 way around in order to bring the face of the leaf uppermost. 

 It is interesting to observe the different relation of stem, petiole 

 and blade and the amount of twisting as the horizontal stem or 



