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PLANT PHYSIOLOGY AND ECOLOGY 



rays, so that they screen each other from the full effect of the 

 light, and thus reduce the amount of water they lose. This arrange- 

 ment is not a necessary protection against over-illumination, for 

 the chloroplasts of the guard-cells receive stronger light without 

 serious harm, but it appears to be a device to prevent injurious 

 water loss from the chloroplasts while they are active. In the 



Fig. 24. Position of chloroplasts in sun leaf (1) of Allionia linearis, in a 

 leaf of the shade form (2), and in one found in very deep shade (;f). 



shade the danger of excessive water loss is slight, while the need 

 of obtaining all the light possible is imperative. Accordingly, the 

 plastids arrange themselves at right angles to the light ray, thus 

 increasing the surface for light absorption. This arrangement of 

 the plastids is foimd not only in sun and shade leaves respectively, 

 but it is also typical of horizontal leaves. In fact, the latter 



