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 etiolm. 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, 
