422 Mas see. — The ‘ Spot ’ Disease of Orchids. 
precautions to be entirely under the control of the cultivator. 
My previous note 1 announcing that 6 spot ’ was due to the 
presence of an organism called Plasmodiophora orchidis , was 
based upon the absolute agreement with the microscopic 
details of two diseases of vine leaves described by Viala 
and Sauvageau 2 , and attributed to the presence of two 
organisms called Plasmodiophora vitis and Plasmodiophora 
calif ornica respectively. Returning to the orchid disease ; 
the minute pale spots already alluded to, which, it may be 
remarked, are not at all conspicuous, and are likely to escape 
observation unless specially looked for, soon assume a pale 
brown colour and gradually increase in size, retaining an 
irregularly circular outline, until they attain a diameter vary- 
ing from four to eight millimetres. When the spots are 
numerous and near together they frequently run into each 
other, forming irregular blotches of variable size and form. 
As the disease progresses the spots become darker in colour, 
and owing to the collapse of diseased cells beneath the 
epidermis, the surface of the spot becomes depressed below 
the level of the surface of the leaf (Fig. i). In many instances 
the disease passes completely through the leaf, forming a 
corresponding brown depressed spot on the under surface. 
Microscopic examination shows the following details. The 
first indication of disorganization is plasmolysis, followed by 
the complete disappearance of the chloroplasts from the 
palisade-cells of the leaf, hence the pale colour of the spots 
during the first stage of the disease. This is immediately 
followed by the appearance of a highly refringent, hyaline, 
oleaginous looking sphere in each cell. During the early 
period of formation, the centre of the sphere presents a 
granular appearance, and when broken up and treated with 
a solution of iodine, the granules prove to be minute starch 
grains, which were liberated from the disintegrated chlorophyll 
grains, and engulphed during the formation of the sphere 
1 Annals of Bot., Vol. ix, p. 170 (1895). 
2 La Brunissure et la Maladie de Californie. Journ. de Bot., Tom. vi, pp. 355 
and 378, PI. XII (1892). 
