THE PRINCIPLES OF OAK GALL FORMATION. 7 
' exterior, consisting of epidermis with unicellular hairs 
containing red pigment in their cell contents ; (2) 
colourless hypodermal cells, beneath which is (3) a 
layer of small meristematic cells; then (4) a thick 
layer of large cells rich in tannin, below which follow, 
(5) the cambian ring, (6) primary starch cells, (7) cells 
containing crystals, and (8) a thin layer of primary 
nee tissue composing the walls of the larval 
cell. 
re colours in many oak galls are very varied and 
rich. 
They range from white and cream through all 
tints of yellow, from very pale to deep orange, and 
from a very pale green to a rich dark hue, and through 
almost every shade of red, some of the tints of which 
are very attractive. Only one kind of gall yields a 
purple, and that colour is in the hairlets with which it 
is clothed (Spathegaster Taschenbergi). The browns 
vary from very pale, through various shades of reddish- 
brown, to chocolate. 
No English gall student appears to have published 
results of investigations into the origin of the colours. 
This is rather remarkable, because the colours are 
always so prominently before the collector, and in 
many instances it is the colour which gives the charm 
to the gall. 
Dr. Marion Newhbigin’s researches (‘Colour in 
Nature,’ 1898) have shed much light upon the 
physiology of pigments and colours in plants and 
animals. But the colours of vegetable galls do not 
appear to have received her attention. Since most 
galls are almost as complex in their structure as the 
plant on which they grow, and as it has already been 
shown, are so indissolubly associated with the ener- 
gising functions of the tree, and also built up of the 
same materials, may it not be inferred that the 
coloration of galls is produced in the same manner 
as in other parts of the plant ? 
Chlorophyll-green is, of course, the supremely im- 
