284 DISEASES OF VEGETABLES. 



plants of wheat early in May, but it is not seen to fix 

 injuriously on the culms till about the 20th of July. 

 Should the weather at this last mentioned date be 

 dry, and at a high temperature, no ill effects from 

 rust take place ; but should wet weather then set in, 

 chilling- the air, and checking- the exhalations from 

 the ground, immediately the straw is struck, and sud- 

 denly changes from a bright yellow to a dingy hue, a 

 certain sign that the blight, as it is commonly called, 

 has seized the crop. About this time the grain is 

 just arriving at perfection ; if the attack takes place 

 before this is effected, it never fills ; if afterwards, 

 less damage is sustained ; the straw may be injured 

 but not the grain. In looking over a blighted field 

 of wheat we observe that the lowest and richest 

 stations, or where the crop is thin on the ground, 

 receive the pestilence more severely than the higher 

 situated, or poor portions of the field. In some 

 seasons the crop is only partially, or locally affected ; 

 in others not a culm escapes. 



These circumstances prove that the evil is always 

 commensurate with the susceptibility of the plant to 

 receive it, and to that critical state of the atmosphere 

 which favours the vegetation of the fungi. 



Some writers have imagined the Pucinia graminis 

 (the name now given to this fungus by botanists) is 

 the effect, not the cause of the malady. The evil, 

 say they, proceeds from exuberant growth, and a sur- 

 charged state of the sap vessels, which first rupture 

 the cuticle, whence the sap flows out, forming an 



