2G JOURNAL OF THE EOYAL HORTICULTURAL SOCIETY. 



for relatively strong sulphurous acid. The point which I want 

 to emphasise is this. The action of small amounts of acid 

 differs from that of more considerable amounts, but neither 

 effect can be regarded as a good imitation of a fog-effect. With 

 large amounts death is rapid ; with traces only, death is much 

 prolonged. It is under the latter circumstances that, in a few 

 plants (of which Poinsettia is the best example), a normal death 

 of the leaf is brought about. The killing is so slow»that many 

 substances of nutritive value can be removed from the cells. I 

 found that when the leaves are killed in this way they often 

 (though not always) ultimately disarticulate. 



The reason why the above failed as an imitation of a fog- 

 effect is because due regard was not had to the conditions pre- 

 vailing in foggy weather. I had selected healthy plants for 

 experiment, and no precautions were taken to darken the chamber 

 in which they were exposed. When, however, plants are used 

 which have been previously rendered unhealthy by being placed 

 in a dark spot, or when the experiment is conducted in semi- 

 darkness, then a rapid disarticulation of the foliage ensues, 

 resembling that occurring in actual fog. This matter is further 

 discussed on p. 48. 



Action of Small Quantities of Sulphurous Acid upon 

 Transpiration. 



"When traces of sulphurous acid are introduced into a 

 chamber containing a transpiring shoot a slight diminution of the 

 rate of evolution of aqueous vapour takes place. I will, however, 

 describe here only one experiment, which may be regarded as 

 typical of the rest. In this experiment, however, rather more 

 acid was introduced (*G milligramme per cubic foot was the 

 strength employed in this instance) than is ever found in fog. 

 Its effect was marked and decisive. 



My procedure was as follows : • A vigorous shoot of Acer 



* In point of fact I used in these experiments the simple, but very 

 useful, piece of apparatus known as the " potometer." (Cf. Darwin and 

 1'hillips, "On the Transpiration-stream in Cut Branches," in Proceedings of 

 Cambridge 1'Jiilosophical Society, vol. v.) This method is admirable for 

 testing the effect upon the rate of transpiration of sudden changes in the 

 environment. In my account above I have simplified the apparatus to save 

 descriptive details. The assumption is, of course, made that the variations 

 in the rate of absorption which are actually recorded by the potometer— 

 Ftand in direct relation to the variations in the rate of transpiration. 



