746 MINNESOTA BOTANICAL STUDIES. 



Experiment 5. A plant of Taraxacum in a pot was placed in 

 its normal upright position, where it could receive light from 

 one side only. The leaves of the rosette on the side farthest 

 from the source of light were curved upward at the end of the 

 third day in such a manner as to place themselves perpendicu- 

 lar to the rays of light. Those leaves on the side of the rosette 

 nearest the light remained in nearly their normal position. 



Experiment 6. — A normal specimen of Taraxacum was placed 

 in an inverted position in a dark chamber with strong sunlight 

 thrown in the chamber from below, by means of a large mirror. 

 The plant was kept in this position for a week, being well wa- 

 tered and cared for during the interval. At the end of seven 

 days a few of the leaves exhibited slight torsions in response 

 to the peculiar conditions of the light received, but the re- 

 mainder held their relative positions as before. 



The members of the rosette of Taraxacum are therefore dia- 

 heliotropic as shown by experiments 5 and 6, negatively geo- 

 tropic as shown by experiments 3 and 4, and hyponastic as 

 shown by experiments 1 and 2. The ordinary position of the 

 leaves is therefore due to their diaheliotropism. The diahelio- 

 tropic irritability of the plant so much overbalances the other 

 forms that their reactions are suppressed. When the helio- 

 tropic stimulus is removed the apogeotropic reaction is exhib- 

 ited, and the hyponasty of the leaf is suppressed. If both the 

 heliotropic and geotropic stimuli are removed the leaf assumes 

 a hyponastic position. The result of this set of experiments 

 supports the conclusion reached by Frank (IV), that the rad- 

 ical leaves of Taraxacum are apogeotropic. I am wholly una- 

 ble to account for the discordance between the experiment of 

 Vines (V, p. 426,) and experiments 3 and 4 of my own series, 

 which were many times repeated. 



Nicotiana. 



Experiment 7. — A healthy normal specimen of a rosette of 

 Nicotiana growing in a pot was placed in an inverted position 

 in a dark chamber, with the radical leaves horizontal. Forty- 

 eight hours later the leaves were pointing directly upward. 



Experiment 8. — A specimen of a rosette of Nicotiana similar 

 to that used in Exp. 7 was placed in an inverted position in a 

 dark chamber, with the radical leaves horizontal and the dorsal 

 surfaces downward. Sixty hours later the leaves had assumed 



