BACTERIAL 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 cyiase softens 



the cell-wall, 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 qr 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. 



I 



