368 VEGETABLE PHYSIOLOGY 



pigment which they contain, known as etiolin, being a pale 

 yellow. In the leaves the differentiation of the mesophyll 

 into palisade and spongy parenchyma does not take place. 

 The parenchymatous cells of the ground tissue of the 

 elongated organs, whether they are stems or leaves, become 

 altered in shape, their longitudinal diameter being con- 

 siderably increased. Plants thus affected by darkness are 

 said to be etiolated. 



That these differences are to be attributed to the absence 

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

 the first cultivated in darkness and the second under 

 ordinary conditions of illumination, the other conditions 

 being kept the same for both. 



The explanation of these changes is somewhat difficult. 

 The absence of light is clearly the cause of the different 

 colour, for, as we have seen in a preceding chapter, under 

 such conditions the pigment chlorophyll is not formed, but 

 is replaced by the yellowish- white etiolin. When an 

 etiolated plant is exposed to light, the etiolin is soon re- 

 placed by chlorophyll, and the plant becomes green. 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 disturbance of the normal course 

 of the metabolism evidently, as shown by the greater 

 production or accumulation of organic acids, to the osmotic 

 properties of which the increased succulence is partly due. 

 It is known that in plants possessing considerable succu- 

 lence the free organic acids which are produced during the 

 night undergo oxidation when light finds access to them. 

 The reason for the disturbance in question is, however, 

 not explained. Diminished transpiration may perhaps 

 account for a good deal, for, as we have seen, in the absence 

 of light the stomata remain shut and there is but little 

 output of watery vapour. The increased turgidity of the 

 tissues resulting from this factor may very probably upset 

 the normal course of metabolism. 



