IRRITABILITY 



217 



intensity and direction of the light. This is shown most 

 clearly by the movements of chromatophores. * The ac- 

 companying figures a, b, c, were drawn on a warm sunny 

 morning from leaves of a moss (Funaria) growing under a 

 bell-glass in the laboratory. Figure a is from a leaf taken 

 from a plant when the dish was on a table six feet from the 

 nearest window : the chlorophyll grains are against the 

 upper and lower walls of the cells, presenting their maximum 

 area to intercept the light. This is the typical daylight posi- 

 tion. Figure b is from a leaf taken from a plant after the 



Figure 17. Cells from leaves of Funaria Sp. 

 a — cell from leaf in ordinary diffuse daylight. 

 b — position of chlorophyll-grains in intense light, 

 c — position of chlorophyll-grains in darkness. 



dish had been standing 5 to 10 minutes in brilliant sun- 

 shine on the window-sill inside the laboratory : the chloro- 

 phyll grains are against the lateral walls of the cells, pre- 

 senting their minimum area to intercept the light. Figure 

 c is from a leaf which had been kept in the dark for 5 to 10 

 minutes ; the chlorophyll grains are against the lateral and 

 bottom walls of the cells. These figures are drawn at the 

 same magnification, but the cells and chromatophores differ 

 considerably in size. Changes in position of the chlorophyll 

 grains take place very rapidly, and the drawings must be 



* Frank, .1. B. tjber die Veranderung der Lage der Chlorophyllkorner 

 und des Protoplasmas in der Zelle und deren innere und aussere Ursachen. 

 Jahrb. f. wiss. Bot., VIII., 1872. Stahl, E. Uber den Einfluss von Rich- 

 tung und Starke der Beleuchtung auf einige Bewegungserscheinungen im 

 Pflanzenreiche. Bot. Zeitung, 1880. Oltmanns, F. tJber photometrisehe 

 Bewegungen der Pflanzen. Flora, Bd. 75, 1892. 



