ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 17 



Presumably some ferment is liberated by these cells which dis- 

 solves a portion of the membranes, so that the mere weight 

 of the leaf or a slight mechanical shock suffices to produce the 

 rupture. I do not know that the development of the layer of 

 separation as a result of fog-injury to the leaf differs in any 

 essential character from its development when a leaf dies from 

 natural causes. The rapidity with which it arises in the case of 

 fog is certainly remarkable. I have evidence showing that such 

 a layer can arise and the leaf disarticulate within forty-eight hours 

 of the commencement of a fog. Still, in a large number of cases, 

 possibly in a preponderating number — especially in " hard- 

 wooded " plants — the absciss layer is marked out from a very 

 early stage in the history of the leaf's development. In such 

 cases the solution or rupture of the membranes is alone required 

 to cause disarticulation. The production of these layers, when 

 they are not already present, and the changes in them which 

 lead to rupture of the tissues and fall of the leaves, are the result 

 of circumstances (in fog as in other cases) which render the leaf 

 no longer able to discharge its office. 



The Withdrawal of Starch from the Leaves. 



Whenever leaves fall from fog, whatever be their appearance, 

 whether green, yellow or blotched, there is one character in 

 which they almost invariably agree. This is in the withdrawal 

 of starch from their tissues. Even in the dull weather of 

 winter small quantities of starch are generally present in the 

 chlorophyll-corpuscles, at any rate of the palisade cells. The 

 leaves seem incapable of withdrawing a small residue, and in 

 this respect offer a point of divergence from their behaviour in 

 bright weather. In clear, sunny weather large quantities of 

 starch are produced, and readily and speedily converted into sugar 

 and withdrawn. In dull weather both these processes seem to 

 be interfered with. Not only is little starch formed, but little 

 is withdrawn from the leaf. I have been astonished to find 

 starch (in small quantities) so often present when the conditions 

 for its formation are unfavourable. I have been gradually led 

 to the view that in prolonged periods of dull weather the whole 

 health of the plant is affected, and that its tone is lowered. 

 Incidentally, I think this pathological condition of the plant 



c 



