46 Vegetable Diseases. 
VEGETABLE DISEASES— EFFECT OF LIGHT AND HEAT ON 
VEGETATION. 
The element of light, as well as that of heat, is necessary to 
vegetation. They seem in general to be injurious only by their 
defects, and seldom by their excess. But as light acts as a stimu- 
lus on the more irritative or sensitive parts of plants, which ap- 
pears by the expansion of many flowers, and of some leaves, 
when the sun shines on them; and by the mutation of the whole 
flower, as of the sun-flower; and by the bending of the summits 
of all plants confined in houses towards the light; there may be 
diseases, owing to the excess of such stimulus, which have not 
been attended to; to prevent which the flowers of salsify, and of 
other plants close about noon. Other- unobser\^ed diseases may be 
owing to a defect of the stimulus of light; as the sensitive plant, 
{mimosa,) which, confined in a dark room, did not open its foli- 
age, though late in the day, till many minutes after it was exposed 
to the light. 
The excess of light has been observed to be attended by vege- 
table diseases in these more northern latitudes; but the disease 
produced by the deficiency of it, which is termed blanching, has 
been successfully used to render some vegetable leaves and stalks 
esculent by depriving them of much of their acrimony, and of 
their cohesion, as well as of their color; as is seen in blanching 
cellery, endive, &c., &c. 
The excess oi heat is seldom much injurious to the vegetation 
of this country, unless it may contribute to increase the dryness of 
the soil, when there is a scarcity of moisture. But the defect of 
the element of heat, or excess of cold, is frequently destructive to 
the early shoots, or to the early blossoms of many fruit trees, such 
as apples, pears and apricots; as these are either more succulent, 
or have less irritability, or more sensibility; on both which ac- 
counts they are more liable to be diseased by cold. 
The blight occasioned by frost generally happens in the spring, 
when cold nights succeed to warm sunny days, as the living power 
of the plant has been previously exhausted by the stimulus of 
heat, and is therefore less capable of being excited into the ac- 
tions, which are necessary to vegetable life, by the greatly dimi- 
nished stimulus of a freezing atmosphere. — Darwin. 
The value of the anthracite and bituminous coal, sent to mar- 
ket in 1847, from the mines of Pennsylvania, is estimated at 
$40,000,000. 
