ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. H 



time, until sufficient acid is collected to cause local lesions. The 

 drops of water, further, doubtless themselves directly absorb 

 sulphurous acid from the air. This will be speedily oxidised into 

 sulphuric acid. 



The deposits on the leaves of stove plants contain an appre- 

 ciable amount of sulphuric acid. The general nature of the 

 deposits is similar to those collected from the glass lights, the 

 analyses of which are given on p. 4. Of the substances 

 there enumerated, the sulphuric acid is the chief one soluble in 

 water ; thus it is to this that I attribute these corrosions. If the 

 attached drops of water from a number of plants be shaken off 

 into a vessel and collected in some quantity, distinct traces of 

 sulphuric acid can be demonstrated. The condensed vapour 

 which drips from the roof, and incidentally falls upon the plants, 

 also contains sulphuric acid. 



I quoted Dalechampia Roezliana and Pavonia Wiotii as 

 marked instances, with a special purpose in view. The leaves 

 of these two plants depend from their stems at a considerable 

 angle, perhaps 45° on the average. Consequently moisture very 

 readily accumulates at the apex. It is instructive to note the pro- 

 gress of the discoloration from the tip backwards. The epidermis 

 at the apex is first attacked. The corrosion works on into the 

 deeper-lying tissues until the whole of the substance of the leaf is 

 destroyed in this region. In many cases the tip actually falls 

 away, or is detached by the shaking which the plant undergoes 

 incidental to horticultural operations. From the truncated apex 

 the corrosion travels upwards, and one may see leaves still attached 

 to the plant and performing their functions in which only the 

 lower, i.e. proximal, moiety is uninjured ; the injured and 

 uninjured portions being separated by a sharp transverse line. 



When sections are made of a portion of leaf-surface which has 

 been exposed to this corrosive action from above, and examined 

 under the microscope, certain definite changes may be noted. 

 The upper epidermis is first attacked. The acid appears to 

 traverse the cuticle and to destroy the cells. The contents of these 

 cells at first exhibit a slight contraction from the walls — a plas- 

 molysis ; the protoplasm then turns brown, due to the appearance 

 in it of a very finely divided dark precipitate. Then, bit by bit, 

 the same action is continued in the subjacent tissues, till the acid 

 has worked its way right through the thickness of the leaf. 



