190 



LIGHT AND PROTOPLASM 



[Ch. VII 



whilst in the light green (illuminated) part of the leaf the 

 chlorophyll lies upon the walls parallel to the rays. When 



the grains are upon the 

 superficial face of the 

 cells they are said to be 

 in epistrophe ; when they 

 have turned away from 

 this face they are in apos- 

 trophe. This apostrophic 

 position is found under 

 two opposite conditions 

 of illumination : under 

 intense light, as we have 

 just seen (positive apos- 

 trophe, Moobe), and upon 

 prolonged standing in the 

 dark (negative apostro- 

 phe). (Fig. 55.) 



It appears, then, that 

 epistrophe occurs only 

 within certain limits of 

 light intensity. The in- 

 tensities included between 

 these limits constitute 

 what MoOEE calls the 

 epistrophic interval. The 

 epistrophic interval varies 

 in position and in extent 

 in different species.* It has been found that in the case of 

 plants which normally live in the bright sun the epistrophic 



Fig. 55. — Cross-section through the leaf of 

 Lemna trisulca. A. Position of the chlo- 

 rophyll grains in diffuse daylight — epis- 

 trophe. £. Position of the chlorophyll 

 grains in intense light — positive apos- 

 trophe. G. Position of the chlorophyll 

 grains in darkness — negative apostrophe. 

 (After Stahl.) 



* The limits were determined by Moore, in a roughly quantitative way, by 

 means of his photrum, constructed as follows. A room with a single window 

 illuminated by the sun was chosen and 12 feet spaced off from the window back 

 into the darkness. The intensity of the light diminished of course as one re- 

 treated from the window. Plants of various species were allowed to stand, 

 simultaneously, at varying distances from the window, and the distance back at 

 which epistrophe began to appear, and, finally, at which negative apostrophe 

 came in, were noted. Then a diagram ^j- the actual scale was made (Fig. 56), 

 showing the position of the points of beginning and ending of epistrophe (so- 

 called positive and negative critical points) . 



