362 VEGETABLE PHYSIOLOGY 



to the chloroplasts if they are too brilliantly illuminated. 

 The arrangement of the palisade cells shields them to a 

 certain extent. 



If we pass to consider the effects of too intense an 

 illumination we find that it is attended with considerable 

 danger to the well-being of the protoplasm. When the 

 leaves of certain plants, among which may be mentioned 

 Oxalis acetosella, are kept exposed to very strong sunlight, 

 and prevented from shading themselves as they normally 

 do by changes in their position, they rapidly die, the dura- 

 tion of their life being reduced from two or three months 

 to as many days. Bright sunlight has in other cases been 

 found to check the growth in length of seedlings, the effect of 

 different degrees of illumination having been compared by 

 direct measurement. We find various arrangements in 

 different plants which appear to be directed towards pro- 

 tecting them from the effects of too brilliant an insolation. 

 Many which normally have their leaves so arranged as to 

 expose their upper surfaces to the incident rays are found 

 under bright sunlight to place them so that their edges 

 and not their surfaces receive the light. This phenomenon 

 has been called Paraheliotropism. It is exhibited normally 

 by the leaves of Oxalis which have just been alluded to. 



Another phenomenon, having for its purpose the protec- 

 tion of the chlorophyll, can be seen in many ordinary dorsi- 

 ventral leaves. When brightly illuminated they are of a 

 lighter green colour than when shaded, and this has been 

 found to be due to a different arrangement of the chloro- 

 plasts in the two cases. In a leaf exposed to diffused light 

 these are collected on the upper and lower walls of the cells 

 just under the epidermis, and they present their broader 

 surfaces to the incident rays. When the light is cut off 

 altogether for a considerable time, and other conditions 

 are unfavourable, they collect on the lateral and lower 

 walls. When the leaf is brilliantly illuminated they place 

 themselves upon the lateral walls only, and rotate on their 

 long axis so as to present their edges instead of their 



