MORPHOLOGY OF ANGIOSPERMS. 



107 



time in the morning, and diffused daylight during the rest of the 

 day, the position of the upper surface is at right angles to the 

 incident rays of daylight, and not to that of the rays of the morn- 

 ing sun. This phenomenon may be studied in the house geranium 

 and other window plants. In endeavoring to explain this behavior 

 of the leaves, Frank assuriies it to be due to a kind of heliotropic 

 irritability peculiar to dorsiventral organs, and terms it trans- 

 verse HELIOTROPISM. 



Fig, 70. A, leaf of violet (Viola tricolor) showing broad lamina, long petiole, and one 

 of the palmately-lobed stipules at the base of the petiole. B, C, stages in the develop- 

 ment of the leaf. The lobes of the stipules (s) develop before the lamina (1). 



The stem, as well as the petiole or stalk of the leaf, is also 

 influenced by the light, and is said to manifest' positive helio- 

 tropism. Those parts of plants that turn away from the light, as 

 the aerial roots of the ivy, are said to possess negative helio- 

 tropism. 



Depending upon their relation to external agents, several forms 

 of leaves are distinguished. In those which assume a more or. 

 less horizontal position the two surfaces of the lamina are quite 

 different, and the leaves are said to be dorsiventral, or bifacial. 



