ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 47 



light was generally required. The fog was, however, emphati- 

 cally an overhead one, and was rarely on the ground. A fog of 

 such opacity and persistence would, had it rested on the ground, 

 have produced an enormous destruction of hothouse vegetation. 

 But, as it happened, neither at Kew nor at Chelsea was anything 

 but the most insignificant damage to be noted. When I reflect 

 on this, and on the results of my own experiments, I feel that 

 I can confidently assert that the reduction of light incident to 

 foggy weather cannot by itself be held as occasioning the whole- 

 sale dropping of foliage which occurs in severe fogs. I shall 

 revert to this matter again directly. 



The third symptom which plants exhibit in dull weather is 

 the relative immobility of the starch in their chlorophyll-corpus- 

 cles. Correlated with the much diminished or suspended activity 

 of carbon-assimilation is a marked sluggishness on the part of 

 the plant to withdraw residues of starch from the corpuscles. In 

 many cases (it is not a universal phenomenon) these residues 

 remain in situ through long spells of foggy weather. But, as I 

 have stated on p. 17, if the leaf actually falls, it first yields up its 

 starch. When the death of the leaf is inevitable, an effort is 

 made to save something of nutritive value. Thus the starch is 

 converted into sugar, and travels down into the stem. Nor is 

 this surprising, seeing how readily diffusible is sugar. Other 

 constituents, especially proteids, travel more slowly, and they 

 cannot be saved in the sudden destruction which is meted out by 

 the poison-bearing fog ; or, to state it more accurately, the slow 

 contraction or plasmolysis of the cells establishes a barrier, as it 

 were, around each cell, detaining all but the most diffusible 

 substances, which escaped prior to the contraction. 



That plants may suffer in the way described was recognised 

 by Sachs* nearly ten years ago. After remarking the rapid 

 formation and disappearance of starch, he continues : " But, on 

 the other hand, there exists a condition in which plants, though 

 apparently healthy, are in a state of rigidity or foliar inactivity. 

 In this condition little or no growth takes place, and the starch 

 in the leaves exhibits no variation sometimes for weeks." He 

 took a Tobacco-plant which exhibited these symptoms, "and 

 placed it in a dark room, where it remained for a week at a 



* Sachs, " Em Beitrag zur Kenntniss der Ernahrungsthiitigkeit der 

 Blatter," Arb. des bctan. Instituts in Wiirzburg, Bd. iii. 1884. 



