20 THE ORCHID REVIEW. 
most susceptible. One other cause of irregularity in the appearance of the 
spots proved to be the relative amount of moisture in the plants, those 
saturated with water being much more readily affected than those compara- 
tively dry. These results were corroborated in the garden, both with the 
Habenaria in question, and with some species of Satyrium, which became 
affected with ‘spot’ when a spell of cold weather succeeded the excessively 
hot weather of the early summer. 
The disease first appears in the form of minute pale spots on the upper 
surface of the leaf, sometimes few and scattered, at others more numerous. 
At first they are not conspicuous, and likely to escape observation unless 
specially looked for, but they soon assume a pale brown colour, and gradually 
increase in size, sometimes becoming confluent. The tissues are completely 
disorganised, and owing to the collapse of diseased cells beneath the 
epidermis the surface of the spot becomes somewhat depressed. 
Mr. Massee’s summary is practically as follows :—The Orchid disease 
known as ‘spot’ is of non-parasitic origin, the initial cause being the 
presence of minute drops of water on the surface of the leaves at a time 
when the temp is ptionally low and the roots copiously supplied 
with water. The effect of the chill produced by the drops of water under the 
above conditions is to cause disorganisation of the cells of the leaf under- 
lying the drops, and the di Pp of the chlorophyll, which is followed 
by the precipitation of tannin and other substances, forming a globular sphere 
in each cell, and eventually the complete disintegration of the cells. It is 
mainly caused by the three following conditions :—(1) too high a tempera- 
ture, (2) too much water and not sufficient air in contact with the roots, 
and (3) watering or spraying with a falling instead of a rising temperature. 
The plate shows the appearance of the diseased tissues, and the spheres 
within the cells which were at first mistaken for the vegetative phase of a 
species of Plasmodiophora. 
Thus it would appear that the malady is of a totally different nature to 
the Vanilla disease (Supra., III., page 51), which is a true parasitic fungus. 
On the diseased tissue known as “ spot,’ a saprophytic fungus is ti 
found, but as it is incapable of obtaining a footing on a healthy leaf it 
can do no harm. 
The moral of the above will be obvious to everyone, and Orchid growers 
will be grateful to Mr. Massee for the light he has thrown on this much- 
debated question. It makes all the difference to know whether we have an 
infectious disease to deal with, or whether it is only a question of maintaining 
a healthy atmosphere by careful stoking, ventilating, watering, and damping 
down, especially in the early part of the year. A point now remaining to 
be settled is whether all the spot-like diseases of Orchids can be assigned 
to the same cause, 
! 
