ARRANGEMENT OF CHLOROPLASTS 



273 



ment Frank calls apostrophe. Here we are once more dealing with 

 the " surface " and " profile " portions of chloroplasts, which in this 

 case, however, are not immediately determined by the direction of the 

 incident light. Very intense illumination, such as that produced by 

 bright sunshine, causes an aggregation of the chlorophyll corpuscles, 

 which has been termed systrophe by Schimper. In this condition the 

 ehloroplasts leave the cell-walls altogether, and become massed together 

 into a more or less compact clump in each cell ; they consequently 



Fio. 109. 



T.S.S. through fronds of Lemna trisulca. A. Chloroplasts in surface-position (epi- 

 strophe). B. Chloroplasts in profile-position (apostrophe). After Stahl. 



shade one another, and thus obtain as much protection as possible 

 against the injurious effects of very intense illumination. 



In the palisade-tissues of Higher Plants, it is generally the radial 

 walls that enjoy the conditions of illumination which are most con- 

 genial to the chloroplasts ; the latter are in any case forced to occupy 

 these walls to a large extent, since only a small number can. be 

 accommodated on the tangential walls. Stahl's description of this 

 arrangement as a " profile " position of the chloroplasts with reference 

 to the incident light is however inadmissible. The error is obvious in 

 the case of leaves which maintain a fixed position, since here the light 

 meets the leaf at various angles at different times of the day, as the 

 sun changes its position. The author has further observed that in 

 curved palisade-cells the chloroplasts are generally massed upon the 

 radial walls ; if, however, the ends of the cells project freely into air- 

 spaces, chlorophyll-corpuscles also occur upon the tangential walls, 

 although those which do so must necessarily assume the " surface " 

 position. 



