Mas see . — The ‘Spot’ Disease of Orchids . 425 
leaving holes in the membrane, which, along with others pre- 
viously present, produce an irregular reticulation, the whole 
being of a brown colour (Fig. 7 a). The nucleus of the cell 
frequently remains unchanged throughout the entire cycle of 
disease, as shown in Fig. 7 a, x. 
In Viala and Sauvageau’s account of the vine disease pre- 
viously alluded to, vacuolated tannin-vesicles and the reticu- 
lated primordial utricle have been respectively interpreted as 
constituting the vegetative phase of their supposed Plctsmodio- 
phora vitis ; Figs. 2 and 4 illustrating their monograph repre- 
senting the former, and Fig. 1 the latter. 
The investigation of the disease under consideration was at 
first pursued along lines suggested by the preconceived idea that 
a fungus was the cause of the mischief, and it was only after 
numerous and varied experiments had failed to demonstrate 
the existence of the hypothetical fungus, that a search was 
made for bacteria, but with a like result. Finally, failing to 
induce the disease in healthy plants by inoculation with the 
expressed juice from diseased spots, even when introduced 
under the epidermis, thus proving the absence of an enzyme 
or organic ferment, which would have been due to the presence 
of fungi or bacteria, this was accepted as corroborative evidence 
of the absence of these organisms. 
At this stage Mr. W. Watson, Assistant Curator, Royal 
Gardens, Kew, whom I take this opportunity of thanking for 
numerous practical hints during this investigation, suggested 
a sudden chilling of the plants as a probable cause of the 
disease. Acting on this suggestion, the following somewhat 
drastic experiment was undertaken. 
A young healthy plant of Habenaria Susannae , R. Br., 
perfectly free from ‘ spot,’ and which up to the date of the 
experiment had been growing in a house having a tem- 
perature ranging between 75 and 8o° F., was selected for 
experiment. Minute particles of ice were placed at intervals 
on the uninjured epidermis of the upper surface of the leaves, 
the plant — along with the pot in which it grew — was then 
placed in a sink and covered with a bell-jar, and cold water 
