486 MANUAL OF BOTANY 



That these changes are to be attributed to the absence of 

 the light can be seen by comparing two similar plants, the first 

 cultivated in darlmess and the second under ordinary conditions 

 of illumination, the other conditions being kept the same for both. 



The explanation of the changes is somewhat difficult. The 

 absence of light is clearly the cause of the different colour, for we 

 have seen that, under such conditions, the pigment chlorophyll 

 is not formed, but is replaced by the yellow etiolin. "When an 

 etiolated plant is exposed to light, the etiolin is shortly replaced 

 by chlorophyll and the plant becomes green. Etiolin appears, 

 indeed, to be the antecedent of chlorophyll. The question of 

 the non-development of the woody elements and the generally 

 increased succulence is more difficult to explain, and many 

 hypotheses have been advanced to account for it. There is a 

 change in the metabolism, evidently, as shown by the greater 

 production of organic acids, to the osmotic properties of which 

 the increased succulence is partly due. The reason for this 

 change of metabolism is, however, not explained. Diminished 

 transpiration may, perhaps, account for a good deal,' for, as we 

 have seen, in the absence of light there is but little output of 

 watery vapour. 



It is significant in this connection that the parts which show 

 the excessive growth are in all cases those in which the water 

 accumulates as transpiration becomes checked. The effect 

 must, however, ultimately depend upon the protoplasm, which 

 we have seen regulates all these functions, and which is pro- 

 foundly susceptible to the influence of the altered conditions. 



If we now examine the effects of too brilliant an illumination, 

 we find other changes taking place. Many plants 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 ParaheUotropism. The effect 

 is to preserve the chlorophyll from the destruction which follows 

 upon too bright an illumination. 



Another phenomenon, having for its purpose the protection 

 of the chlorophyll, can be seen in many ordinary dorsiventral 

 leaves. When brilliantly illuminated they are of a lighter 

 green colour than when shaded, and this is found to be due to a 

 different arrangement of the ohloroplasts in the two cases. In 

 the leaf exposed to diffused light these are collected on the 

 upper and lower walls of the superficial cells, and they present 

 their broader surfaces to the incident rays. When the light is cut 



