30 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



BouvardiaJ* It originates in patches on the corolla ; but if the 

 exposure is continued long enough, these spread until the whole 

 of the corolla is involved. The presence of much tannin is 

 readily demonstrable in these buds. The scarlet bracts of Poin- 

 scttia are transformed to a yellowish-brown colour after pro- 

 longed exposure to pyridine. The red colour is located in the 

 epidermis, and the long exposure necessary is explained by the 

 fact that these red bracts are quite destitute of stomata. The 

 pyridine produces a copious brown precipitate in these epidermal 

 cells. This again may be on account of the tannin. The 

 scarlet bracts of this plant are of interest because, in my 

 experience, they are never affected by fog. The green leaves of 

 this plant, on the other hand, are very susceptible to it. The 

 cuticularised and slightly waxy epidermis, which is destitute of 

 stomata, probably excludes the toxic constituents of fog, though, 

 of course, it succumbs when any of them are applied in greater 

 concentration — as in the present instance. 



To determine the mode of entry of pyridine into the tissues, 

 experiments were made in much the same manner as when in- 

 vestigating this point for sulphurous acid (cf. p. 23). The 

 same apparatus was employed. When the under surface is 

 exposed to pyridine a rapid effect is noted. In an hour the 

 mesophyll exhibits plasmolysis. The tissues are affected in the 

 following order : Spongy parenchyma, palisade parenchyma, 

 epidermis. When the upper surface is exposed (to the same 

 vapour and at the same time) for an hour no plasmolysis or 

 other action is observable. After the expiration of three and a 

 half hours' exposure a Coiiocliniam leaf showed here and there 

 in the upper epidermis slight plasmolysis. After twelve hours 

 this was more complete, and the subjacent palisade layer was 

 affected. These and similar experiments indicate that the 

 cuticle exerts a considerable resistance to pyridine, and to its 

 allies. The entry of pyridine is, I believe, assisted by the 

 stomata to a marked degree. What I have said in reference to 

 sulphurous acid is true also of this substance. 



The other substances of the pyridine series with which I 

 experimented are lutidine, picoline, piperidine, chinoline, thio- 

 phene, and nicotine. All were applied as vapours, and in 

 precisely the same manner as pyridine — i.e. one or two drops on 

 * For full discussion vide Section III. 



