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JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



vapour could be studied as it penetrated the upper and lower 

 surfaces respectively. The method is not a new one, but it has 

 only been used previously to show that the destruction of the 

 chlorophyll is much more rapid by the lower than by the upper 

 surface. 



I find in the cases investigated by me that, whilst the 

 mesophyll is very readily attacked from below, the action from 

 above is exceedingly slow ; whilst in the latter case the thickness 

 of the cuticle has a decided influence. With thickly cuticularised 

 leaves no injury will accrue when the upper surface is exposed 

 for a period quite long enough for the total destruction of the 

 leaf were the under surface the one exposed. With soft, un- 

 cuticularised, or very slightly cuticularised leaves, the result is, 

 of course, very different. But still, where stomata are absent 

 from the upper surface, the epidermis always serves as a barrier 

 to very rapid action. When the leaf used possesses stomata on 

 both faces, there is less disparity in the observed action, though, 

 in most cases, the action is more rapid through the lower surface. 



By the histological examination of a great number of leaves 

 belonging to various genera, I conclude that the sulphurous acid 

 reaches the interior of the leaf most readily by the stomata. By 

 examining leaves that had been exposed for periods varying from 

 a few minutes to six or eight hours, I was able to show that the 

 soft mesophyll cells in the neighbourhood of the stomata are the 

 first to be affected, the action progressing from these points to 

 the whole of the spongy parenchyma. The first change in these 

 cells, by which contact with the sulphurous acid is signalised, is 

 their plasmolysis. It is only later that their chlorophyll-corpuscles 

 exhibit any change of colour, and later still that the droplets of 

 chlorophyllan make their appearance. In a leaf such as that we 

 are studying -with stomata on the under surface only — the pali- 

 sade layer is the last part of the mesophyll to be attacked. Very 

 generally, though I cannot say universally, it is the lower ex- 

 tremities of the protoplasmic bodies of these cylindrical cells 

 which exhibit the greatest amount of contraction ; the sides are less 

 strongly contracted, whilst the upper extremities are usually found 

 in their original position* Where there is a considerable develop- 

 ment of lacuna) between the palisade cells, so that the acid vapour 



* The same phenomenon was frequently observed when experimenting 

 With the pyridines. 



