Nature and Cause of the Potato Disease. 
305 
water, and are generally considered to be the laboratories of the 
products as well. 
In the solar rays the leaves throw off both oxygen and water; 
in the shade and at night, carbonic acid only. The light of the 
sun, therefore, causes secretion, and its absence retards or an- 
nihilates it. A plant like the potato cannot exist without leaves, 
and the leaves without the sun cannot perform their functions. 
While the solar rays act on the leaves, the plant increases in the 
amount of its products, and, consequently, in substance ; but in a 
continuous shade it decreases by oxidation of its carbon and the 
liberation of the carbonic acid thus formed. The solar rays 
therefore produce a vital action, and their absence a chemical 
one ; for secretion is the act of vitality, and the formation of 
carbonic acid the result of chemical action. 
I have thus, as briefly as I could, given an epitome of the 
structure and functions of such parts of the plant as I shall have 
occasion hereafter to refer to in the course of the inquiry into the 
nature and cause of the disease. I shall now therefore proceed 
to notice the other matter connected with this subject, adhering 
to the plan of sections or separate heads, as affording more ready 
reference to any particular subject than when continuously carried 
on and intermingled with each other. 
5. Evaporation — its Laws. Exhalation from Plants and the 
Earth considered. 
The thermometer ranges in Britain from 0° to 136°. In the 
shade it varies from 0° to 89°, and in the sun it has not been 
observed to exceed 136°, which may therefore be considered its 
highest point. The difference of heat between shade and sun- 
shine equals 57°, a variation of about 9° beyond the mean tem- 
perature of the year. The exhalation of water from plants, the 
earth, and surfaces in general, depends on the temperature of the 
air, the force and direction of the wind, and the hygrometric state 
of the air itself. If the temperature be low, and the air still and 
moist, evaporation is at its lowest point ; but with rapid motion, 
dry air, and high temperature, it is at its maximum. The wind 
from N.E. to S.E., on account of its dryness, is more favourable 
for evaporation than from any other points of the compass, and ex- 
halation is increased by about ^^-^ when the wind blows from these 
points, beyond what it is when it blows with the same force from 
any other part of the compass. If a still air represent 1 as 
the evaporating power of a surface, modeiate motion increases it 
to \\, and a gale to \\, As the motion, temperature, and satu- 
ration of the air govern evaporation, exhalation is directly as its 
warmth, dryness, and motion, and inversely as these are receded 
from. 
