362 VEGETABLE PHYSIOLOGY 



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 explanation 

 of the phenomenon, for, as we have seen, the actual evapora- 

 tion 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, 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 from 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 



