INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



255 



marked influence upon form and structure even in light. It therefore appears 

 that Palladin' is justified in supposing that etiolation in darkness is at least 

 largely caused by diminished transpiration. The anatomical characters of 

 etiolated plants are quite like those of plants grown in light but with water- 

 saturated air, and all of the formal responses of etiolation appear to be explain- 

 able as resulting partly from alterations in the conditions controlling the rate 

 of water loss and partly from the consequent alterations in the internal influences 

 of the different organs upon one another. Thus, Bellis perennis, which forms a 

 stem with spirally arranged leaves when grown in darkness, also shows the same 

 response when grown in Ught with a water-saturated atmosphere. 



Plants in which the leaves are slow to develop, such as the hop, form almost 

 as long internodes in light as in darkness. In such cases stem growth is not 

 influenced by leaf formation either in light or in darkness, so that the inter- 

 nodes can elongate freely. 



Finally, Weber's^ studies show that etiolated plants are poorer in ash, espe- 

 cially in calcium, than are green plants. Some results of Weber's analyses of 

 etiolated and green pea leaves are given in the following table, which shows 

 the total ash content and that of seven constituent elements (the latter reckoned 

 as oxides), in percentage, on the basis of total dry weight in each case. In the 

 same table are given similar results for bean leaves as obtained by Palladin. 



Similar results were obtained by Schlosing from plants that had been grown 

 with light but in a chamber with very moist air. 



Among the conditions causing the structural peculiarities of etiolated plants 

 are therefore to be considered: reduced rates of transpiration, and the conse- 

 quent modification in the distribution of water and dissolved mineral substances 

 in the plant body, the non-occurrence of the photosynthetic process, and, to 

 some extent, light as such. 



Some of the chemical reactions that are necessary for normal growth occur 

 only in the presence of the blue-violet light rays. In the general influence of 

 light upon plant growth and structure, many different kinds of reactions have 



> Palladin, W., Transpiration als Ursache der Pormanderung etiolirter Pflansen. Ber. Deutsch Bot. 

 Ges. 8: 364-371. 1890. Idem, Ergriinen und Wachsthum der etiolirten Blatter. Ibid. 9: 229-232. 

 1891. Idem, Eiw-eissgehalt der Griinen und etiolirten Blatter. Ibid. 9 : 194-198. 1891. Idem, Aschen- 

 gehalt der etiolirten Blatter. J6W. 10: 179-183. 1892. Idem, 1893- [See note i, p. 208.) 



' Weber, Rudolph, Ueber den Einfluss farbigen Lichtes auf die Assimilation und die damit zusammen- 

 hSngende Vermehrung der Aschenbestandtheile in Erbsen-Keimlingen. Landw. Versuchsst. 18: 18-48. 

 i87S. 



