428 Massee . — The 'Spot' Disease of Orchids. 
the temperature is exceptionally low, and the roots copiously 
supplied with water. 
The effect of the chill produced by the drops of water 
under the above-mentioned conditions, is to cause plasmolysis 
of the cells of the leaf underlying the drops ; this is followed 
by the precipitation of tannin and other substances, and 
eventually the complete disintegration of the cells. 
‘ Spot ’ in the broadest sense of the term, which would 
include the effects of exceptional meteoric conditions on the 
living parts of plants, more especially the leaves, when growing 
in a state of nature, is, in the case of cultivated orchids, mainly 
if not entirely caused by the three following conditions : — 
(1) too high a temperature ; (2) too much water, and not 
sufficient air in contact with the roots ; (3) watering or 
spraying with a falling instead of a rising temperature. 
EXPLANATION OF FIGURES IN PLATE XV. 
Illustrating Mr. Massee’s paper on the ‘spot’ disease of orchids. 
Fig. 1. Leaf of Eria rosea showing the appearance produced by the ‘spot’ 
disease. Nat. size. 
Fig. 2. Section through portion of a diseased spot on the leaf of Eria rosea. 
The cells at the periphery of the diseased spot have the protoplasm only slightly 
tinged brown, and the tannin-vesicles are still small and not at all vacuolated, 
as at a ; at b, the tannin- vesicles are larger, granular at the centre, and appear as if 
radially striate, due to the commencement of a fine-meshed vacuolation. At c, the 
tannin-vesicles have reached the extreme stage of vacuolation, their substance 
being reduced to a thin film which soon collapses. In the preparation, owing 
to being mounted in water containing only a trace of glycerine, the brown 
protoplasmic contents of the cells, which were plasmolysed and free from the 
walls, have in many instances become again expanded so as to fill the cells. 
The tannin-vesicles are stained with a saturated aqueous solution of potassium 
bichromate. x45odiam. 
Fig. 3. Tannin-vesicles in various stages of development in leaf of Eria 
rosea. The other cell-contents are omitted, x 450. 
