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ROTTENNESS OF TURNIPS AND SWEDES. [Sept. 1896. 



regarded as saprophytes, and in this state may be found living 

 upon dead vegetable matter ; they can also be cultivated with 

 ease upon various nutrient solutions. But Botrytis can also 

 undoubtedly live as a parasite, and it would seem to have 

 gradually acquired a parasitic habit. This is to some extent 

 borne out by the experiment upon the action of the frost 

 described later, showing more rapid development as a saprophyte 

 than as a parasite, and Professor Marshall Ward has drawn 

 attention (Groonian Lecture, 1890) to the fact that the infection 

 takes place more easily in living tissues when the hyphse have 

 been invigorated by saprophytic nutrition for a short time upon 

 adjacent dead tissue. A favourable point of infection is thus 

 presented by a wound or bruised surface such as is frequently 

 found in the roots, and the rottenness can often be traced to 1 

 such an origin. But having once established itself as a parasite 

 upon any particular host, the Botrytis attacks plants of the 

 same kind with gradually increasing vigour. Kissling has 

 shown that the conidia from a saprophytic mycelium at first 

 only develope slowly within the living tissue, but the second 

 generation developed from the conidia of this parasitic mycelium 

 grows more vigorously in a plant of the same kind, and the 

 succeeding generation produces a still more virulent mycelium ; 

 that is, if one may so express it, the faculty is improved by 

 cultivation and the fungus becomes gradually educated to the 

 parasitic habit. The conidia, therefore, produced from the 

 hyphse parasitic upon a swede will effect an entrance more 

 readily into another swede and destroy it more rapidly, and so 

 the fungus proceeds invigorated by its life in each successive 

 swede attacked. 



Certain conditions are necessary for the germination of the 

 conidia, namely, a sufficiency of moisture, temperature, and 

 oxygen. Provided moisture and air are present, germination 

 may take place slightly above the freezing point. A conidium 

 falling upon a freshly-cut surface of swede is supplied with 

 moisture for its germination and provided the atmosphere is 

 sufficiently damp to prevent evaporation from the cut surface, 

 its germ-tube can then perforate the first cell-wall and gain an 

 entrance into the plant, infecting it in the course of a few hours 

 as previously shown. But if the cut surface has become dry, 

 the conidium would in all probability fail to infect it. The 

 supply of moisture would be inadequate for germination, and 

 also in a comparatively short time a wounded surface is healed 

 over by the formation of a corky layer ; the suberized cell- wall 

 would then have more power of resisting the action of the 

 ferment secreted by the germ-tube, and infection would be 

 rendered more difficult. 



The swedes in the pits are often found to be attacked at the 

 ends, and the examination of numerous specimens shows that, 

 while the central part is often sound, the extremities are 

 rotten, as if the disease were advancing towards the centre. In 

 the experiments previously described, the conidia were always 



