208 



MEMOIRS OF THE NEW YORK BOTANICAL GARDEN. 



diurnal adaptive movements of the leaflets according to this account 

 during the two-months-long day.^*^ 



Fig. 159. Helleborus niger. D, section of basal portion of petiole of normal leaf. 

 C, section of basal portion of petiole grown in continuous electrical illumination. E, 

 epidermis. CC, cortical tissue. L, bast fibers. B, wood cells, en, endodermis of 

 entire stele. E^, endodermis of separate bundles in etiolated petioles. C^, pericjle. 

 CC^^, parenchymatous cells in position of medulla. After Bonnier. 



The influence of illuminations of a duration and intensity dif- 

 ferent from the normal in the culture of economic plants has received 

 some attention from physicists and horticulturists. Siemens ^^^ pub- 

 lished a report of some experimental observations in 1880, in which 

 the comparative behavior of plants in continuous darkness, normal 

 daylight, normal daylight supplemented by nocturnal illumination 



i^^Wiesner, J. Untersuchungen ueber den Lichtgenuss der Pflanzen im Ark- 

 tischen Gebiete. A. d. Sitzungsber. d. kaiserl. Akad. d. Wiss. i. Wien, 109: Abth. i, 

 May, 1900. 



'"Siemens, C. W. On the influence of electric light upon vegetation and on 

 certain physical principles involved. Nature, 21 : 456. 1S80. See also Proc. Roy. 

 Soc. 30 : 210-230. 



