ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 37 



ourselves to white flowers alone, we find bulbous plants, like 

 Tulips, Allium, Narcissics, and Hyacinths very hardy in this 

 respect. The behaviour of these flowers is in accord with that 

 of their foliage towards fog. Crinum likewise may be included. 

 Then amongst Orchids I would instance the genus Angr cecum. 

 Though A. sesquipedale (as we have seen) suffers severely, both 

 as regards its buds and flowers, A. hyaloides and citratum have 

 always passed through fog uninjured. Other white flowers 

 winch escape are Primula, Cyclamen, and Deutzia. These 

 inconsistencies are remarked on below. 



Further Comments, with Notes on the Action of certain 

 Reagents. 



It is not surprising that the fugitive and little protected floral 

 members should suffer so readily as they do in time of fog. 

 Their surfaces are so little furnished with cuticle, that poisonous 

 substances in the air easily penetrate the outer cell-walls. Neverthe- 

 less, in the few instances I have worked out in detail, the presence of 

 stomata, other things being equal, is not without noticeable result. 

 Take as an example the expanded flowers of Cattleya Triance. 

 The sepals suffer first ; then the two upper petals ; lastly the 

 labellum, which is the least sensitive of the organs cited. The 

 distribution of stomates is as follows. Upon the sepals stomates 

 occur in small numbers, but with fairly regular distribution. 

 Upon the two posterior petals they are about half as frequent as 

 upon the sepals. No stomates were found on the labellum. 



In Phalainopsis Schilleriana similar relations exist, though 

 the disparity in frequency of stomates upon the sepals and petals 

 is more considerable. 



The chief histological character of slightly injured petals (of 

 an Orchid, e.g. Ccelogyne cristata) is a plasmolysis of the cells 

 throughout. With plasmolysis, such very delicate and lacunate 

 tissues as those of the petals of Orchids exhibit collapse, more 

 or less complete. This, combined with the out-passage of water 

 into the lacunae, causes the translucency of the parts. Where 

 a yellowing of the petals &c. results, this is found to be caused 

 by the production in the cells of droplets of a yellow oil and to a 

 slight yellow colour developing itself in the previously colourless 

 protoplasm ; where browning occurs a fine dark precipitate is 

 found in the protoplasm. But any detailed statement at present 



