40 
ON MILDEW, OR. MELDEW. 
expansion occasioned by increase of temperature produce con 
gestion, and burst the vessels of circulation, and discharge 
their contents into the cellular texture, destroy the vitality of 
the plant, and leave the hull only partially filled with farina. 
Heat, air and moisture, the agents of putrefaction, being pre- 
sent, decomposition begins, and the surface of the plant soon 
displays signs of decay. This destructive process first shows 
itself in the smallest part of the stalk, near the head, which is 
of most recent formation, and consequently mostsucculent and 
tender, and most liable to rupture. The rust is probably oc- 
casioned by the heat and internal pressure enlarging the pores 
of exhalation and discharging the sap on to the surface of the 
stalk, and when evaporation carries off the moisture, the resi- 
duum displays itself somewhat like the rust of iron. After the 
rupture and discharge of the sap-vessels, the surface of the 
plant is covered with mucous which is adhesive, and this will 
account for the seeds of fungi, which are supposed to be float- 
ing in abundance in the atmosphere, taking root and vegeta- 
ting in the decaying structure ; and hence the supposition, 
that fungi are the cause of mildew. 
The presence of animalcula may be accounted for on the 
same principle, for nature is ever economical, and wherever 
animal or vegetable substances are in the progress of decay, 
mouths are found ever ready to convert dead matter into food 
for living things, so as to perpetuate the largest possible amount 
of animated existence. On the death of the plant, the tender 
succulent fibres of the roots immediately decay, and on draw- 
ing them from the ground, the appearance of them has led 
many to suppose that they had thus discovered the true, cause 
of the disease of the plant, when, in fact, it was only the effect 
of its previous dissolution. 
It may be objected that if the mildew is the result of a ge- 
neral saturation of the atmosphere with moisture at a particu- 
lar period, that all wheat should be equally injured by it ; 
when the fact is well known, that of contiguous fields one 
will be destroyed and the other remain uninjured. This ap- 
parent contradiction is accounted for by the uninjured wheat 
