Book I. DISEASES OF VEGETABLES. 261 



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 through- 

 out 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 origin- 

 ating in a small fungus, which multiplies and extends till it occupies the whole ear 

 (Princip. of Bot. p. '656.) : 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. 282.) 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 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 said to be prevented as in the case of smut. 



1694. 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 Tussilago jFarfara, Humulus Lupulus, 

 Corylus avellana, 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. J. Robertson (Hort. Trans, v. 178.) considers it as a minute fungus of 

 which different species attack different plants. Sulphur he has found to be a specific 

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

 though by some this is denied, and soot, by rendering the crop more luxuriant, is said to 

 be an encourager of mildew, the richest parts of a field being always most infected by it. 

 As it is least common in airy situations, thinning and ventilation may be considered as 

 preventives. 



1695. 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 

 always 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 unfavourable wind, apt to become covered with a sweet sort of glutinous coating, 

 similar in flavour to the fluid obtained from the trunk. 



1696. It is certain, however, that saccharine exudations arc found on the leaves of many plants, though 

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

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

 must include under the appellation of honev-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 

 Italv and larch of France It is also possible that the exudation or 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 d'ews, then the leaves will not suffer. Washing is therefore the palliative ; judicious cul- 

 ture the preventive. 



1 697. Dropsy. Plants are also liable to a disease which affects them in a manner similaj 

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

 Willdenow describes it as occasioning a preternatural swelling of particular parts, and 

 inducing putrefaction. It is said to take place chiefly in bulbous and tuberous roots, 

 which are often found much swelled after rain. It affects fruit also, which it renders 

 watery and insipid. It prevents the ripening of seeds, and occasions an immoderate pro- 

 duction of roots from the stem. 



1698. In succulent plants this disease generally appears in consequence of excessive waterings, and is 

 for the most part incurable. The leaves drop, even though plump and green ; and the fruit rots before 

 reaching maturitv. In this case the absorption seems to be too great in proportion to the transpiration ; 

 but the soil when' too much manured produces similar effects. Du Hamel planted some elms in a soil 

 that was particularly well manured, and accordingly thev pushed with great vigour for some time ; but 

 at the end of five or six vears thev all died suddenly. The bark was found to be detached from the wood, 

 and the cavity filled up with a reddish -coloured water. The symptoms of this disease suggest the palli- 

 atives ; and the preventive is ever the same — judicious culture. 



1699. Flux of juices. Some trees, but particularly the oak and birch, are liable to a 

 great loss of sap, which bursts out spontaneously, owing to its superabundance, or issues 

 from accidental wounds : sometimes it is injurious to the health of the plant, and some- 

 times not. 



1700. There is a spontaneous extravasation of the sap of the vine, known by the naire of the tears of the 

 vine, which is not always injurious. As it often happens that the root imbibes sap, wb'Ch the leaves are 

 not vet prepared to throw off, because not vet sufficiently expanded, owing to an inclement season, the 



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