46 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



number of folds and depressions, the appearance being especially 



marked in the lower epidermis, which tends to split away from 



the subjacent tissues locally. I have little doubt this is only 



a special case of " gelbfleckigkeit," though, there being no 



chlorophyll in the epidermis, there is consequently no yellow 



coloration. At any rate it is of interest as indicating an undue 



wateriness and turgidity of the tissues. 



Another symptom exhibited by plants which are insufficiently 



illuminated is a tendency to drop some of their leaves. This is 



very widespread, and is for the horticulturist a question of the 



first magnitude. In ordinary dull weather the actual falling of 



leaves is not as a rule very marked ; but if any additional factor, 



inimical to the plant, be introduced the result may be a profound 



one. When a healthy plant is covered with an opaque shade, so 



that it is in darkness, we find that leaf-dropping begins in due 



time. I have experimented with selected examples of " hard- 



w r ooded" and of soft herbaceous plants. On the average a leaf 



will drop about the fifth day, then another a few days later, and 



so on. The oldest leaves drop first, and the others follow in the 



sequence in which they were developed. In this way more than 



a month elapses before the plant is stripped. When a plant is 



grown under a translucent screen, so that the light is reduced * 



but not extinguished, as in foggy weather, the rate at which the 



leaves fall is slow compared with what obtains in total darkness. 



The first leaf will not fall till perhaps the eighth or ninth day, 



and the succeeding ones at proportionately remote intervals. 



The whole process of leaf-dropping is prolonged to double the 



time occupied in darkness. It will be noticed that the rate at 



which leaves drop in actual fog is out of all proportion to the 



length of exposure to reduced light, or even darkness, which the 



plants have incurred. The thing was illustrated on a grand scale 



this winter for the first time since I have been occupied with the 



question. For the five days ending December 23, the west of 



London (including Kew) was covered more or less continuously 



by a dense mantlo of fog. This was so opaque that artificial 



* I took some pains in making these experiments to obtain, as nearly as 

 possible, an average reduction of light similar to that experienced in foggy 

 weather. I was able to do this by employing the method of photometry 

 elaborated by Dr. G. H. Bailey, of Manchester, for the registration of sun- 

 light intensity. In reviving and developing an idea of the late Angus Smith's, 

 J)r. Bailey has provided the plant physiologist with a much-needed imple- 

 ment of research. 



