ETIOLATION 361 
output of watery vapour. ‘The increased turgidity of the 
tissues resulting from this factor may very probably upset 
the normal course of metabolism. 
It is significant in this connection that the parts which 
show the excessive growth are in all cases those in which 
water accumulates as transpiration becomes checked. 
If, however, the effects are admitted to be due to the dis- 
turbance of transpiration, this is no satisfactory or final 
explanation of the phenomenon, for, as we have seen, the 
actual evaporation of the water of transpiration into the 
intercellular spaces is under the regulating influence of the 
protoplasm, and the effect must therefore be traced back to 
some interference with the latter, caused by the absence of 
illumination. With the lowering of the tone which follows 
the absence of the light we have a failure of the proto- 
plasm to exhibit its normal degree of permeability, which 
is maintained by some slight effort, and we find it retain in 
the cell more than the usual quantity of water. 
We cannot easily explain the effects which we have 
seen are produced upon the structural elements of the 
plant. We do not know why the usual development of the 
woody and sclerenchymatous cells of the stem should be 
interfered with, nor can we explain the effect of light upon 
the degree of differentiation of the mesophyll of leaves. 
We find that palisade tissue is developed more readily 
under the influence of bright light; a phenomenon which 
may be easily ascertained by comparing the structure of 
two leaves of the same tree, one taken from a brilliantly 
lighted and the other from a deeply shaded part. Indeed, 
the differentiation of the mesophyll into palisade and 
spongy parenchyma may be traced to the difference of 
illumination which the two faces of a leaf receive, for when 
both are well lighted, palisade parenchyma appears upon 
both sides, while etiolated leaves, as we have seen, do not 
develop this tissue at all. 
It is possible that this difference of structure on the 
two sides may be connected with the possibility of damage 
