CAUSES OF RUST IN WHEAT. 181 



the wheat any more than the larvae of the flesh fly have 

 the healthy skin of the sheep. 



Any one who will examine the stalks of wheat growing 

 on a luxuriant, rank soil, at short intervals, about the time 

 of its first showing the swelling of the ear, will perceive 

 the vessels to become ruptured, either from the luxuriant 

 flow of the sap up the tender tops of the plants, being 

 checked by cold winds, or an unhealthy overfulness, or 

 some other casual obstruction, and the sap being thus sud- 

 denly checked will rupture the vessels and ooze out through 

 little slits or longitudinal fissures ; the discharged matter 

 will soon assume the appearance of a white jelly as it dries, 

 it will become yellow, and then brown, and of a hard tex- 

 ture; and in proportion as the sap-vessels are injured and 

 destroyed, and this exudation takes place, the plant must of 

 course, more or less, fail in its supply of nourishment to 

 the grain. In some cases, the strongest stalks will not be 

 able to push the ear beyond the leaf, and the corn conse- 

 quently will be starved. Whilst the season continues dry 

 and cold the exuded sap will remain like dry gum, but as 

 it advances, and the weather becomes warm and moist, the 

 gum becomes moist, soft, and putrefying, and then forms 

 and affords a nutritive bed for the mould or fungus, which 

 grows and increases until it is deprived of moisture, or is so 

 reduced as to be insufficient to sustain it, when it dies ; 

 and according as the season is favourable or unfavourable 

 to its growth, it produces a brown or black powdery sub- 

 stance in a proportional quantity. Thus, then, the founda- 

 tion or cause of the rust or fungus is the putrefying matter 

 discharged from the ruptured sap-vessels of the plant ; and 

 although the ruptures may be occasioned by a contraction 

 or obstruction of the vessels by atmospheric influence, the 

 overfulness or over-luxuriance of the plant produced by 

 surfeit, or the being glutted with rank and unwholesome 

 food, and its incapacity of digestion and unhealthy ob- 

 structions renders it more liable to such injuries, and may 

 therefore be considered as the general cause of the disease, 

 blight, or rust. 



