14 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Further Notes on Class of Injuries B. 



The changes in the leaf which lead to its speedy disarticula- 

 tion, with or without the accompaniment of colour change (as 

 described under B, p. 10), are due in great part to an attack 

 upon its delicate unprotected internal tissues. The fog actually 

 effects an entry into the lacunae or system of intercellular spaces 

 of the leaf through the stomata ; and, circulating here, the 

 poisonous substances are brought into immediate contact with 

 the moist, delicate, and uncuticularised cell-membranes of the living 

 cells. These cellulose membranes can offer but little resistance, 

 and the protoplasm is directly attacked. The capacity of any 

 plant to resist such an attack must depend essentially on the 

 constitution of the protoplasm. Many apparently fragile plants 

 exhibit no injury, whilst others, with thick leathery leaves, 

 readily drop their leaves. A thick external cuticle may arrest 

 corrosion from an acid deposit ; few and small stomata may 

 render the entry of noxious vapours less rapid than where they 

 are more numerous or larger ; but it is upon the constitution of 

 the protoplasm that the production or non-production of injuries 

 will in the main depend. This constitution of the protoplasm 

 is a quality which is inherent in it. 



One cannot, either by means of high powers of the microscope 

 or by a chemical investigation of the cell-contents, tell beforehand 

 which plants will suffer when brought in contact with London 

 fog. But one can, by taking into consideration all the circum- 

 stances of the environment, understand how in some cases the 

 constitution of the protoplasm may be impaired and the whole 

 plant lose tone, as it were, so that the protoplasm is readily de- 

 stroyed by the noxious substances present in the air. Nor does it 

 follow that the agent which actually attacks the protoplasm is 

 the same as the one which enfeebles it and reduces its tone. The 

 latter may be distinct, and merely prepare the way for the former. 

 It will be necessary, in pursuing the discussion, to bear in mind 

 all that a fog implies, and not merely to search for some toxic 

 constituent. 



I stated on p. 10 that leaves may disarticulate still green, 

 or that they may gradually change to brown or yellow, and then 

 fall, or that scattered discolorations may be developed before 

 this happens. I will now describe the details in a few plants 



