INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 253 



form of this region is brought about when these plants are grown in continuous 

 darkness. 



PhyUocactus, which produces flat, leaf-Uke stems and branches under usual 

 conditions, forms slender, cylindrical internodes in continuous darkness.* 



When darkness produces etiolation the anatomical structure of etiolated 

 plants is also different from that of 

 the same forms grown in light; the 

 dark-grown individuals are charac- 

 terized by exceptionally well-de- 

 veloped thin-walled parenchyma, by 

 exceptionally thin cuticle, by small 

 size and number of the vascular 

 bundles, and by a pronounced retard- 

 ation in the formation of mechanical 

 tissue. 



Experiments with colored light- 

 screens show that plants assume 

 their usual forms only when they 

 receive blue and violet light. When 

 grown in light of other colors [that 

 is, with the intensity of the blue and 

 violet rays very greatly diminished 

 as compared to their intensity in 

 sunlight], etiolation becomes mani- 

 fest.2 The curve XV, Fig. 132, 

 shows how greatly growth is re- 

 tarded by blue and violet light. 



The fact that photosynthesis can- 

 not occur in darkness was formerly 

 supposed to explain the phenomena 

 of etiolation, but the experiments 

 with colored lights just mentioned 

 show clearly that the photosynthetic 

 process has practically no direct in- 

 fluence upon plant form. In the 

 green- violet portion of the spectrum, 

 where photosynthesis is least pronounced, plants grow as usual, while in the 

 red-orange portion, where photosynthesis is most active, they become etiolated. 

 Furthermore, Godlewski^ obtained normal plants in the presence of light but in 



' Vochting, Hermann, Ueber die Bedeutung des Lichtes fur die Gestaltung blattformiger Cacteen. Zur 

 Theorie der Blattstellungen. Jahrb. wiss. Bot. 26 : 438-494. 1S94. [The same phenomenon is exhibited 

 by some of the platyopuntias of southern Arizona. — Ed.] 



^ Wiesner, J., Photometrische Untersuchungen auf pflanzenphysiologischem Gebiete. I Abt. Orien- 

 tirende Versuche iiber den Einfluss der sogenannten chemischen Lichtintensitat auf den Gestaltungsprocess 

 der Pflanzenorgane. Sitsungsber. (math.-naturw. Kl.) K. Akad. Wiss. Wien 102 : 291-350. 1893. 



" Godlewskj, Emil, Zur Kenntniss der Ursachen der Formanderung etiolirter Pflanzen. Bot. Zeitg. 

 37: 81-92, 97-107, 113-12S, 137-141. 1879. 



B A 



Fig. 134. — Potato sprouts grown in light {A ) 

 and in darkness (B). {After Pfeffer.) 



