BACTEEIAL DISEASES OF VEGETABLE KINGDOM 129 



is oxalic acid. It was found that the inoculation of the plants with 

 this acid produced the same effects as their inoculation with the 

 bacteria. The destruction of plant tissues by such organisms is effected 

 by the secretion of ferments, some of which change the cell-walls 

 into a pulpy mass, whilst others change insoluble into soluble and 

 more digestible substances. Three of these ferments are known to 

 be secreted by Pseudomonas destructans. One called cytase softens 

 the cell-wail, causing it at the same time to swell up, and also 

 destroys the middle lamella. Another called diastase changes starch 

 into sugar, whilst the third is one which must be able to decompose 

 proteids, for in gelatine cultures of the organism, liquefaction of 

 the gelatine takes place. These bacteria gain an entrance into the 

 turnip through wounds caused by snails, slugs, larvae, etc., which 

 bore holes or feed upon certain portions of the plant. The secretion 

 of oxalic acid is a common phenomenon in plant diseases, not only 

 in those caused by bacteria, but also in those caused by the higher 

 fungi as well. It is known, for example, to be secreted by Peziza 

 sclerotiorum, Aspergillus niger, and Penicillium glaucum when these 

 organisms are parasitic on plants. 



Another disease, entirely due to bacteria, is one which affects the 

 tomato, the egg-plant, and the Irish potato. The foliage, and later the 

 stem and leaves, become discoloured, and eventually are destroyed 

 altogether. When the plant is in this condition, the cells are found 

 to be full of bacteria, and if a tiny cut be made in the plant, small 

 drops of a dirty white and yellowish colour ooze slowly out. In 

 potatoes the bacteria make their way to the tubers, causing a brown 

 or black rot. As in most other plant diseases, wounds must first be. 

 caused before the bacteria can gain an entrance. In America, where 

 this disease is very prevalent, the wounds are made by an insect called 

 the Colorado potato-beetle, which bites the leaves and, of course, the 

 bacteria first settle on the part of the leaf left exposed by the wound. 

 In recent years this disease has become prevalent in the North of 

 England and in Scotland. The affected tuber is characterised by a pale 

 brown ring inside the tuber at some distance from the outside : the 

 ring later becomes broader and darker. In this way the whole tuber 

 is destroyed, after which the skin withers and many millions of the 

 bacteria contained inside the tuber pass into the soil ready to infect 

 other plants of the same kind. The microbe-parasite is called Bacillus 

 solanacearum. The wounds are caused by insects, so the only way of 

 preventing this disease from spreading is by the use of an effective 

 insecticide. 



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