EPISTROPHE AND APOSTROPHE 



487 



off altogether for a considerable time and other conditions are 

 unfavourable, they collect on the lateral and lower walls. In 

 the illuminated leaf tliey are found to have partiality rotated so 

 as to present their edges instead of their surfaces to the light, 

 and to have collected upon the lateral walls. In the first 

 case the chloroplasts lie parallel to the surface of the organ ; in 

 the last they are at right angles to the surface. These two 

 conditions are known as epistrophe and apostrojyhe respectively. 

 ^Yhen the conditions of the incidence of the light are altered, 



Fig. 1192. 



Fig. 1192. Bfsmodiioyi gyrans. A. Stem with leaves during the day. B. A 

 similar stem with leaves in tlie nocturnal position, pointiug downwards. 

 (After Darwin.) 



the chloroplasts change then" positions accordingly. We see in 

 these phenomena a power of the protoplasm to respond to an 

 increase of illumination in such a way as to protect the plant 

 from mjurious consequences. 



The effect of light upon structure also can be noted in the 

 case of such leaves as are brightly illuminated ; the palisade 

 parenchyma being much more fully developed than in leaves 

 which have grown in the shade. Indeed, the differentiation of 

 the mesophyll into palisade and spongy parenchyma may be 

 traced to the difference of illumination which the two faces 



