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 
Ozxalis 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 toa different arrangement of the chloro- 
plasts in the two cases. Ina 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 
