ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 45 



acids. The few experiments I have made towards satisfying 

 myself that this was the case do not add anything previously 

 unknown. The presence of increased quantities of organic acids 

 in the cells tends to make them increasingly turgid, whilst the 

 presence of so much water renders an abnormal wateriness of 

 the leaves inevitable. Under these circumstances, the whole 

 mechanism of the leaf is out of gear. The normal balance is 

 upset, and it is difficult to suppose that the protoplasm can offer 

 effectual resistance (as it might under normal conditions) to the 

 action of poisonous impurities in the air. 



Symptoms of Disease. 



That plants are in a pathological condition when grown in 

 dull light or semi-darkness is evidenced in some cases by events 

 which are quite apparent even to the passer-by. 



One of these manifestations is the appearance to which 

 Sorauer* has given the name of yellow-spottiness (" Gelbfleckig- 

 keit"). Many stove-plants are liable to this during dull weather. 

 Its occurrence is an expression of an enhanced wateriness of the 

 plants exhibiting it. It is caused by the stretching — generally 

 at right angles to the surface of the leaf — of groups of mesophyll 

 cells scattered over the leaf area. This gives rise to little 

 pustules or wartlike excrescences (" intumescences ") upon the 

 surface. The chlorophyll in the cells concerned turns yellow, 

 giving an appearance of yellow-spottiness. During my work I 

 have met with this symptom of disease, though I have not made 

 any extended investigation, except into one case. This is the 

 curious cupping of the leaves of Bouvardia, which is very 

 prevalent sometimes in dull weather, or when the conditions of 

 cultivation leave something to be desired. The surface of the 

 leaf shows irregular depressions, varying in size and form, with 

 wrinkles between the pits. These irregularities are due to the 

 stretching of the epidermis of the lower surface without there 

 being any equivalent extension of the other tissues. The 

 epidermal cells in question, as a result, cover a considerably 

 greater area than in sound plants. This throws the leaf into a 



* Sorauer, in Wollny's Forschungen auf dem Gebiete der Agricultur- 

 physik, Bd. xiii. p. 90. 



An instance of this phenomenon came under the notice of the Scientific 

 Committee some time ago (cf. Joum. Boy. Hort. Soc. vol. xi. p. lvi. under 

 heading " Warts on Vine Leaves "). 



