1G JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



leaf, there is a marked diminution of the amount present as in- 

 jury due to fog becomes apparent. The cells in which destructive 

 changes have taken place are on the whole poorer in protoplasm 

 than in the healthy leaf. 



Type 3. The leaves undergo a more or less uniform change 

 in colour (to brown or yellow) before they disarticulate. 

 Examples : Poinsettia (and many other Euphorbiaceae), 

 Hoy a refiexa, Dendrobium nobile. 



The destructive changes may be studied by the examination 

 of a series of sections of leaves in various stages. 



Poinsettia. — In the uninjured leaf we may note the presence, 

 here and there, of oil-globules. With gradual change in colour the 

 margins of the chlorophyll-corpuscles become interrupted, the 

 contents of the corpuscles granular and yellowish. The cells are 

 plasmolysed, and the oil-globules are both larger and more 

 numerous. In more advanced stages there is an enormous in- 

 crease in the amount of oil — a number of yellow globules being 

 present in every cell, particularly in the spongy parenchyma. 

 The amount of protoplasm present is less, and the chlorophyll- 

 corpuscles show a marked disintegration and yellowing. It is at 

 about this stage that the leaf disarticulates. 



Hoya reflcxa. — Here the leaf is more stable, and remains at- 

 tached till a much larger proportion of its contents have travelled 

 away. In the healthy leaf a small amount of oil is present, but 

 as the chlorophyll-corpuscles turn yellow and disintegrate, the 

 amount of oil is first enormously increased. Later, however, the 

 oil entirely disappears. Up to this point no marked plasmolysis 

 is apparent. At the stage when the leaf falls hardly any proto- 

 plasm remains, whilst the chlorophyll-corpuscles are represented 

 by little aggregations of minute yellow granules. 



This last instance no doubt exhibits very nearly the same 

 changes as occur in the normal death of the leaf as it might 

 arise from causes quite independent of fog. 



Except in the few instances mentioned (Ccntropogon, &c), 

 the affected leaf falls. The disarticulation is actually brought 

 about by the development, in a transverse zone at the insertion 

 of the leaf, of a definite " absciss layer," or layer of separation. 

 In this zone there is an active production of cells by division, 

 and it is across this zone that the subsequent rupture occurs. 



