308 
Nature and Cause of the Potato Disease. 
mitted both night and day, and the plants continued healthy 
during the whole period. 
In Hale's experiment on the sunflower, his mean evaporation 
was 15 grains for every square inch daily. This experiment was 
carried on in July and August for a period of 1 5 days, and eva- 
poration is at its maximum in August, so that Hale's experiment 
was made at a period when transpiration was at its highest point. 
On the 3rd of March, however, the polyanthus transpired nearly 
8 grains for every square inch, which, considering the season, is a 
large quantity. Transpiration increases progressively from March 
to August, after which period it declines. It is greater from 
sunrise till noon, after which hour it lessens. In my experi- 
ments I have noticed considerable evaporation from the earth, 
and transpiration from the plants as well, from 9 till noon, and I 
have also observed that it rapidly decreases from that period. 
From the results of the experiments I have stated, it appears 
that the evaporation from the earth exceeded by five times that of 
the transpiration of the plants. Now, if this ratio should con- 
tinue until we reach the highest point in August, and we take 
Hale's mean of 15 grains for the potato, we shall have 81 grains 
for the earth as a mean for each square inch of its surface, which 
gives an evaporating power for every acre of land of 318 tons 
daily during the months of July and August. And if this acre 
contain 26,800 plants at 1 foot apart, and 15 inches between the 
rows, and each plant an area of 400 square inches, we have for 
exhalation from the plants rather over 10 tons daily. This sum 
of transpiration added to the evaporation of the earth, gives a 
total of 328 tons as the average amount of water vaporized from 
1 acre of plants daily during the months of July and August. 
Let us now briefly consider 
G. Tlie Lifluence of Light on Plants. 
The solar rays stimulate the organs of plants and increase their 
action. In the direct rays of the sun, plants decompose carbonic 
acid, and throw off oxygen. In the shade and at night the vitality 
of the plant is so lessened that it absorbs oxygen and liberates 
carbonic acid. The absorption of carbonic acid and the liberation 
of oxygen is a vital action, but the absorption of oxygen and lil)e- 
ration of carbonic acid is a chemical one. 
Vital action increases the weight of the plant by assimilation 
of carbon, while chemical action decreases its substance by the 
loss of it. Light forms the green colouring matter of plants 
(chlorophylle), and also changes the nature of the sap by its action 
on the organs of the plant, and thus imparts life and vigour to 
the system. Wiliiout the solar rays the juices of a plant become 
attenuated, and its secretions and circulation impeded ; and if the 
