Book I. DISEASES OF VEGETABLES. 193 



consist of thousands of small globules collected into groups beneath the epidermis, which they raised up 

 in a sort of blister and at last burst Some of the globules seemed as if imbedded even in the longitudinal 

 vessels of the blade. They were of a yellowish or rusty brown, and somewhat transparent. But these 

 groups of globules have been ascertained by Sir J. Banks to be patches of a minute fungus, the seeds of 

 which, as they float in the air, enter the pores of the epidermis of the leaf, particularly if the plant is 

 sickly ; or they exist in the manure or soil, and enter by the pores of the root. (Sir J. Banks on Blight, 

 1805.) This fungus has been figured by Sowerby, and by F. Bauer, and Grew. It is known among farmers 

 by the name of red rust, and as it affects the stalks and leaves only it does not materially injure the crop. 

 But there is another species of fungus known to the farmer by the name of red gum, which attacks the 

 ear only, and is extremely prejudicial. In the aggregate it consists of groups of minute globules inter- 

 spersed with transparent fibres. The globules are filled with a fine powder, which explodes when they are 

 put into water. It is very generally accompanied with a maggot of a yellow colour, that preys also upon 

 the grain, and increases the amount of injury. The only means of preventing or lessening the effect of 

 any of the different varieties of blight mentioned is proper culture. Palliatives are to be found in topical 

 applications, such as flower of sulphur, and where the disease proceeds from, or consists of, innumerable 

 minute insects, it may occasionally be removed. Grisenthwaite conjectures that in many cases in 

 which the blight and mildew attack corn-crops, it may be for want of the peculiar food requisite for per- 

 fecting the grain ; it being known that the fruit or seeds of many plants contain primitive principles not 

 found in the rest of the plant. Thus the grain of wheat contains gluten and phosphate of lime, and where 

 these are wanting in the soil, that is, in the manured earths in which the plant grows, it will be unable to 

 perfect its fruit, which of consequence becomes more liable to disease. (New Theory of Agr. &c.) 



879. Smut is a disease incidental to cultivated corn, by which the farina of the grain, 

 together with its proper integuments and even part of the husk, is converted into a black, 

 soot-like powder. If the injured ear is struck with the finger, the powder will be dis- 

 persed 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 glary 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. (Princip. of Bot. p. 356.) 

 But 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. (Smith's Introd. p. 348.) It 

 is said to be 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, whilst 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 con- 

 sequently mingle with the bulk. But it is always readily detected by the experienced buyer, 

 and fatal to the character of the sample. It is said to be prevented as in the case of smut. 



880. Mildew is a thin and whitish coating with which the leaves of vegetables are some- 

 times covered, occasioning their decay and death, and injuring the health of the plant. It 

 is frequently found on the leaves of tussilago farfara, humulus lupulus, corylus avellana, 

 and the white and yellow dead-nettle. It is found also on wheat in the shape of a glu- 

 tinous exudation, particularly when the days are hot and the nights without dew. Will- 

 denow 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 which some species of aphides deposit 

 upon the leaves. J. Robertson (Hort. Trans, v. 178.) considers it as a minute fungus 

 of which different species attack different plants. Sulphur he has found the only specific 

 cure. In cultivated crops mildew is said to be prevented by manuring with soot. 



881. Honey-dew is a sweet and clammy substance which coagulates on the surface of 

 the leaves during hot weather, particularly on the leaves of the oak-tree and beech, and is 

 regarded by Curtis as being merely the dung of some species of aphides. This seems to 

 be the opinion of Willdenow also, and it is no doubt possible that it may be the case in 

 some instances or species of the disease. But Sir J. E. Smith contends that it is not al- 

 ways so, or that there are more species of honey-dew than one, regarding it particularly 

 as being an exudation, at least in the case of the beech, whose leaves are, in consequence 

 of an unfavorable wind, apt to become covered with a sweet sort of glutinous coating, 

 similar in flavor to the fluid obtained from the trunk. 



882. It is certain, however, that saccharine exudations are found on the leaves of many plants, though 

 not always distinguished by the name of honey-dew ; which should not perhaps be applied except when 

 the exudation occasions disease. But if it is to be applied to all saccharine exudations whatever, then we 

 must include under the appellation of honey-dew, the saccharine exudations observed on the orange-tree 

 by De la Hire, together with that of the lime-tree which is more glutinous, and of the poplar which is 

 more resinous ; as also that of the cistus creticus, and of the manna which exudes from the ash-tree of 

 Italy and larch of France. It is also possible that the exudation of excrement constituting honey-dew 

 may occasionally occur without producing disease ; for if it should happen to be washed off soon after by 

 rains or heavy dews, then the leaves will not suffer. Washing is therefore the palliative : judicious culture 

 the preventive. 



883. Plants are also liable to a disease which affects them in a manner similar to that 

 of the dropsy in animals, arising from long continued rain or too abundant watering. 



