NYCTITROPIC MOVEMENTS 



141 



and to the walls at right angles to these in strong illumination as 

 in Fig. 72, although some investigators deny the economy of 

 such movement. These movements may be seen in leaves of 

 Oxalis placed in various intensities of light and there sectioned. 

 214. Paraphototropism. A large number of species have the 

 power of changing the positions of the laminae in such man- 

 ner that the angle at which rays strike the surfaces are varied 



with the intensity. By reason 

 of this adaptation many forms 

 exhibit movements during 

 the intense illumination of 

 midday, which are termed 

 paraphototropic movements. 

 Generally such movements 

 consist in reactions resul'ting 

 in directing the apices of the 

 leaves or leaflets toward or 

 away from the source of 

 illumination. If the leaves 

 of almost any leguminous 

 plant are examined at noon 

 on a hot summer day, or in 

 a tropical greenhouse these 

 positions may be observed. 

 Trifolium, Mimosa, Cassia, 

 Oxalis, Phaseolus, and others 

 are good objects for these 

 observations. 

 Note the positions of the leaf- 



FlG. 72. Positions assumed by chloro- 

 plast in Lemna trisulca ; A, in diffuse light, 

 B, strong diffuse light and C, in direct sun- 

 light. After Stahl. 



215. Myctitropic Movements. 



lets of any of the leguminous species mentioned in the last ex- 

 periment after 5 P. M. and early in the morning. The positions 

 assumed are much more marked than those of the paraphoto- 

 tropic reactions and may consist in moving the laminae down- 

 ward or upward. The nyctitropic movements seem to be very 

 clearly due to differences in illumination, although the reaction is 



