ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 51 



universal rule by any means, though it applies in the vast majority 

 of cases when the leaves are dorsiventral. The radial or bilateral 

 leaves characteristic of many Monocotyledons become excessively 

 elongated in darkness, just as shoots do, but their breadth is 

 diminished. And even among dorsiventral leaves exceptions 

 occur. The leaves of the Beet, for example, attain a considerable 

 size in darkness.' From Sachs' observations it appears that leaves 

 which, when they unfold under normal conditions, become fully 

 exposed to light at a comparatively early stage in their develop- 

 ment, are those which are most affected in their growth by con- 

 tinuous darkness ; whereas those, such as sheathing leaves, 

 which are naturally protected more or less from exposure to 

 light by others investing them, attain a relatively more perfect 

 expansion." I shall not attempt to discuss this difference in 

 behaviour amongst leaves, but what is of interest for us to note 

 liere is the fact that those plants which present a contrast in 

 regard to their reaction towards diminished light should also ex- 

 hibit very different capacity in resisting fog. 



Action of the several Impurities. 



It is hardly possible to sharply distinguish between the 

 lesions caused by the various toxic ingredients of fog. Wherever 

 *' tannin " is present in tissues a dark coloration maybe assumed, 

 sooner or later, after the death of the protoplasm. 



With sulphurous acid this change is certainly slow in its 

 •development. Frequently no such change is observed. 



With the pyridines there is often a browning with tannin. 

 But where tannin cannot be shown to exist, or where its pre- 

 sence is doubtful, the colour-change does not proceed usually 

 "beyond a slight yellowing (as in many corollas). Still, I should 

 not like to make any definite generalisation as yet. 



With phenol, nitro-benzene, and several other coal-tar pro- 

 ducts a very general browning results ; and this in green and 

 colourless tissues alike. But the degree in which this colour is 

 formed varies in different plants. We find every stage between 

 no change in colour to the production of a deep chocolate-brown. 

 The distribution of tannin in the organs affords no clue here. 



But we must remember that our knowledge of post-mortem 

 oolour-changes in plant-tissues is limited and uncertain. We 



e2 



