ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 15 



which may serve as types of the three main varieties. But at the 

 same time I would emphasise the fact that the distinction made 

 is of an arbitrary nature ; transitions from one to the other exist ; 

 and the same species under different circumstances (not yet 

 understood) does not exhibit identical characteristics. 



Type 1. The leaf falls green and apparently uninjured. 

 Examples : Malpighia urens, Platonia antioquensis, 

 Rheedia sp., Phyllanthus montanus and P. angastifolius 

 (cladodes), Daphnopsis tinifolia, Bignonia sp., &c. 



A microscopic investigation of vertical sections of these leaves 

 shows that the cells, especially in the regions rich in lacunae {i.e. 

 in the spongy parenchyma), are slightly plasmolysed. The pro- 

 toplasm is not discoloured. The chlorophyll-corpuscles appear 

 normal, and in most cases the green colour is perfectly pre- 

 served. 



Type 2. There is local discoloration, especially at the 

 margins or towards the apex, or in irregular blotches. This 

 change in colour does not, however, involve the whole area 

 of the leaf before it falls. Included here are the few forms 

 in which the leaf does not disarticulate. Examples are most 

 numerous : Clerodendronmacrosiphon, Beaumontia grandi- 

 flora, Aristolochia tricaudata, Bouvardia (leaf not falling), 

 Centropogon Lucianus (leaf not falling), Buellia, Justicia 

 carnea, Conoclinium ianthinum, Begonia. 



Microscopic examination of such material shows that the cells 

 throughout are slightly plasmolysed, and this both in the dis- 

 coloured regions as in those parts apparently unaffected. The 

 green portions show no further change. The cells of the dis- 

 coloured regions, in addition to plasmolysis, often exhibit a 

 browning of the protoplasm, which may be slight or may be con- 

 siderable. This is an especially marked character in the epider- 

 mis. The chlorophyll-corpuscles present various appearances. 

 They may be green and, as far as can be judged, sound ; or their 

 outline may be broken, indicating an incipient stage of dispersion. 

 They may contain brown-coloured granules or they may be broken 

 up into fragments. Oil-globules, either numerous and minute or 

 large and less frequent, are in many cases found. Sometimes, as 

 in Beaumontia grandiflora, where considerable quantities of oil 

 appear to exist in the chlorophyll- containing cells of the healthy 



