Mas see , — The ‘ Spot 9 Disease of Orchids. 423 
(Fig. 2 b). Within three days from the first appearance of 
the spheres, the enclosed starch grains have become completely 
dissolved and their substance added to the common mass. If 
at this stage a section through a diseased spot be placed in 
water and examined under the microscope, the spheres will 
be seen to undergo vacuolation, gradually changing from the 
previous solid condition into hollow vesicles, the walls of 
which become irregularly perforated or reticulated, the con- 
figuration of the network slowly and constantly changing 
after the fashion of the movements presented by the vegetative 
phase of Plasmodiophora Brassicae , but differing in the move- 
ments being of a purely physical nature, the perfectly homo- 
geneous membrane becoming thinner in proportion as the 
vesicle increases in size, and in constantly retaining a rounded, 
even contour. Increase of size usually continues until the 
vesicle fills the cell in which it is contained (Figs. 2 c, and 4). 
Vacuolation takes place exactly as stated above when sections 
are placed in a one per cent, solution of osmic acid instead of 
water, which, taken alone, is sufficient proof that the spheres 
in question are not of an amoeboid nature. In some instances, 
instead of one, several spheres are formed in a cell, each under- 
going vacuolation, but remaining comparatively small in size. 
The composition of the spheres is proved by the action of 
reagents to be complex, and although I have sometimes spoken 
of them as tannin-vesicles, it must be clearly understood that 
I do not intend to convey the idea that they consist entirely 
of tannin, although agreeing in many respects with the 
structures called tannin-vesicles by Klercker 1 . 
That tannin is present is shown by the following reactions. 
Potassium bichromate produces a bright brown precipitate, 
insoluble in water ; an aqueous solution of cupric acetate 
causes a dingy brown colour, which changes to green when 
subsequently treated with an aqueous solution of ferrous sul- 
phate; a 1 per cent, solution of osmic acid blackens the spheres, 
but as previously stated, does not prevent vacuolation. Finally, 
1 Studien Uber die Gerbstoffvakuolen. Tiibinger Inaugur. -Dissert. 1888. 
