524 



ECOLOGY 



velopment ; in any event, green plants may become variegated by grafting. The 

 boundaries of the white spots usually are veins, which perhaps act as barriers to the 

 virus. White spots exposed for a time to darkness and then to light become 

 green, as though the virus were destroyed by darkness. In some plants (fig. 757), 

 but not in all, the albescent leaves or parts of leaves are smaller than those that are 

 green, indicating defective food migration from other parts. 



The movements of chloroplasts. In diffuse light, plastids commonly 

 are close to the outer cell walls, and so arranged as to expose a maxi- 

 mum surface to the light 

 (epistrophe, fig. 758). When 

 exposed for a time to in- 

 tense light, the plastids for 

 the most part move to the 

 side or rear walls, to which 



they are closely appressed, 



FIG. 757. An albescent leaf 

 of Abutilon; the shaded portions 

 represent the parts .containing 

 chlorophyll, the other portions 

 being colorless; note the greater 

 development of those parts where 

 the chlorophyll is more abundant. 



758 



FIGS. 758, 759. Variations in the form and 

 position of chloroplasts (c) in Isoetes: 758, a 

 surface view of epidermal cells that have been 

 exposed to diffuse light; the position taken 

 is that of epistrophe; 759, similar cells that have 

 been exposed to direct and rather intense sun- 

 light, illustrating apostrophe; note that the 

 chloroplasts near the walls differ in shape from 

 those found elsewhere; highly magnified. 



and their chief axis is parallel rather than perpendicular to the incident 

 light (apostrophe, fig. 759). If exposed to intense or long-continued 

 light, the plastids tend to become grouped in the center of the cell 

 (systrophe). These movements commonly are believed to be advan- 

 tageous. The advantage of epistrophe is obvious, since light is so 

 important for the development and role of chlorophyll. The advantage 

 of apostrophe is less obvious, though there are reasons for believing 

 that chlorophyll is injured or destroyed in intense light. The alga 



