i 72 Schunck and Brebner .—On the Action of Aniline 
with watery cells, which contain a large amount of cell-sap 
with little protoplasm, of which Echeveria leaves afford a good 
example, seldom or never react well with aniline. There can 
be no doubt too that the thickness of the cuticle, the greater 
or less waxiness of its surface, and other physical conditions, 
affect the rapidity of the discoloration. In cases where little 
or no discoloration takes place, the cause must be sought in 
the absence of one or both of its essential conditions, these 
conditions being, as before stated, the simultaneous presence 
of some form of active oxygen and an organic acid, other than 
oxalic. In the case of Rumex it is probably the presence of 
oxalic acid, or rather of superoxalates, that prevents the re- 
action taking place. 
When a leaf belonging to the first class is painted with 
aniline, the discoloration shows itself in a very short time, 
either in the shape of spots, as in the holly, or along the veins, 
as in the mint leaf, and then the whole leaf acquires a uni- 
form brown colour (Plate IX, Figs. 1-3). When the cells of 
such a leaf are examined under the microscope, the appear- 
ance is such as depicted in Fig. 4, which represents a few cells 
from the mesophyll of a holly leaf near the midrib. The 
chlorophyll-corpuscles, which practically occupy their normal 
position, are of a rich brown colour, and the cell-sap is hardly, 
if at all, tinted ; the outline between the cytoplasm and cell- 
sap is not well marked ; a slight plasmolysis has taken place, 
but such is not always the case. With such leaves as are 
discolored very slowly, several distinct stages in the process 
may be traced under the microscope. The first stage is re- 
presented in Fig. 5, which shows several cells of a Tropaeolum 
leaf after being painted with aniline. Here viscid green drops 
have exuded from the chlorophyll-corpuscles, and run together 
to form larger ones, which lie scattered about in the cells of 
the mesophyll, the corpuscles, which are now completely 
colourless, occupying their usual peripheral position. In the 
second stage, shown in Fig. 6, the green viscid drops have 
turned brown ; otherwise the appearances presented are much 
the same. In the third stage of the reaction, as shown in 
