VEGETABLE PHYSIOLOGY. 75 



will be dispersed like a cloud of black smoke ; and if a portion 

 of the powder is wetted by a drop of water and put under the 

 microscope, it will be found to consist of millions of minute and 

 transparent globules, which seem to be composed of a clear and 

 glairy fluid, encompassed by a thin and skinny membrane. 



This disease does not affect the whole body of the crop, but 

 the smutted ears are sometimes very numerously dispersed 

 throughout it. Some have attributed it to the soil in which the 

 grain is sown, and others have attributed it to the seed itself, 

 alleging that smutted seed will produce a smutted crop. But in 

 all this there seems to be a great deal of doubt. Willdenow 

 regards it as originating in a small fungus, which multiplies and 

 extends till it occupies the whole ear. But Mr. F. Bauer of 

 Kew, seems to have ascertained it to be merely a morbid swelling 

 of the ear, and not at all connected with the growth of a fungus. 

 It is said to be effectually prevented by steeping the grain 

 before sowing in a weak solution of arsenic. 



But besides the disease called smut, there is also a disease 

 analogous to it, or a different stage of the same disease, known 

 to the farmer by the name of bags, or smut balls, in which the 

 nucleus of the seed only is converted into a black powder, while 

 the ovary, as well as the husk, remains sound. The ear is not 

 much altered in its external appearance, and the diseased grain 

 contained in it will even bear the operation of threshing, and 

 consequently mingle with the bulk. But it is always readily 

 detected by the experienced buyer, and fatal to the character 

 of the sample. It is prevented as in the case of smut. 



3. Mildew. Mildew is a thin and whitish coating with which 

 the leaves of vegetables are sometimes covered, occasioning their 

 decay and death, and injuring the health of the plant. It is 

 frequently found on the leaves of the Humulus, SupuJus 

 and the white and yellow dead nettle. It is found also on 

 wheat in the shape of a glutinous exudation, particularly when 

 the days are hot and the nights without dew. Willdenow says 

 it is occasioned by the growth of a fungus of great minuteness, 

 the Mucor erisyphe of Linnaeus ; or by a sort of whitish slime 



