INJURY CAUSED BY NON-PARASITIC ORGANISMS 65 



that the disease is of a non-parasitic nature, the prevention 

 of which, by the exercise of proper precautions, is entirely 

 under the control of the cultivator. The minute spots on the 

 leaves, indicating the incipient stage of the disease, soon 

 assume a pale brown colour and increase in size. As the 

 disease progresses the spots become darker in colour, and 

 owing to the collapse of the cells beneath the epidermis, the 

 surface of the spot sinks below the general level of the surface 

 of the leaf. In many instances the injury passes quite 

 through the substance of the leaf, and shows a corresponding 

 brown, sunken spot on the opposite side. Microscopic 

 examination reveals the presence of a large, refringent, 

 oleaginous-looking sphere in each cell of the browned tissue. 

 When placed in water these spheres undergo vacuolation and 

 constantly change their shape, after the manner of the 

 movements shown by the vegetative or plasmodial phase of 

 Plasmodiophora brassicae. These spheres correspond to the 

 bodies discovered in diseased vine leaves, and called Plas- 

 modiophora vitis by Viala. The proof that these bodies are 

 not independent organisms, but simply globules, consisting 

 for the most part of oils and fatty substances, is proved by 

 the reactions shown when treated with a solution of alcannin; 

 or with a one per cent, solution of osmic acid, when the 

 spheres become blackened, but vacuolation is not checked so 

 long as water is present. Tannin is also present in quantity. 

 Mr. Watson, curator, Royal Botanic Gardens, Kew, suggested 

 a sudden chill as the cause of the disease. Acting on this 

 hint, a young healthy plant of Habenaria Susannae (R. Br.), 

 perfectly free from 'spot,' which had been growing in a 

 temperature ranging between 75 and 80 R, was selected for 

 experiment. Minute particles of ice were placed at intervals 

 on the uninjured epidermis; 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 from a tap was allowed to flow over 

 the bell-jar for twelve hours. During this period the tempera- 

 ture inside the bell-jar ranged between 41 and 45 F. The 

 plant was now removed from under the bell-jar and returned 

 to its proper house. Within twenty-four hours pale spots 

 appeared on the leaves at the points occupied by the particles 

 of ice, and within four days characteristic 'spot' was pro- 

 duced. The disease was afterwards produced, when minute 

 drops of water were used instead of ice. Check plants 

 submitted to the same conditions, except that the leaves were 



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