138 RELATIONS OF PLANTS TO LIGHT 



is generally known as the fixed light position. When the body 

 of a plant is moved in such manner as to disarrange the leaves 

 with respect to the light equilibrium, curvatures ensue, to restore 

 the chlorophyl -bearing portions of the organ to their original 

 positions. Here, as in diageotropic reactions, torsions may also 

 accompany the reactions of adjustment to light. The cause of 

 the horizontal position of dorsiventral organs, including stems, 

 thalli and other structures may be determined only by actual 

 analysis. The unequal growth of the two flanks (epinasty, 

 hyponasty) of such organs may also play a part irrespective of 

 external inductions. 1 



210. Paraphototropism of Leaves of Taraxacum. Secure a few 

 young plants of Taraxacum in which the rosette includes a num- 

 ber of vigorously growing leaves, which usually lie flat upon the 

 surface of the soil. Transfer them to pots filled to heaping with 

 soil, or enclose the roots in a compact mass of damp sphagnum. 

 Place in a position near a window where the illumination will be 

 strong and from one side only. Note the positions of the leaves 

 a day or two later. Place the plant in an inverted position in a 

 phototropic chamber where it may receive illumination from below 

 at right angles to the dorsal surfaces of the leaves. Note posi- 

 tion a day or two later. Close the chamber, shutting out light, 

 and observe position of the leaves after an equal period. Place 

 a plant in a dark room with the root in a horizontal position, and 

 the leaves vertical with respect to their planes. Note position 

 of leaves two days later. Fasten a plant to the clinostat in the 

 last named position and revolve it on its axis. An analysis of 

 the above results will show that the leaves of Taraxacum are 

 diaphototropic, apogeotropic, and epinastic. 2 



211. Diaphototropism of Leaves of Arisaema. Place an awak- 

 ening corm of Arisaema in a pot filled with soil and covered 

 with sphagnum and fasten to a clinostat in such manner that the 

 main axis of the plant is horizontal and perpendicular to a win- 



1 Ewart, A. J. Diaheliotropism of radial members. Annals of Botany. 10 : 294. 1896. 



2 Day, R. N. The forces determining the positions of dorsiventral leaves. Min- 

 nesota Botanical Studies, I : 743. 1894-98. 



