330 
Nature and Cause of the Potato Disease. 
cold to contend with as well. Now, what appearances would the 
plant, under such circumstances, present ? 
1st. The colour of the foliage would become darkened by the 
liberation of carbonic acid. 
2ndly. The foliage of the plant would droop as its powers 
declined. 
3rdly. Putrescence and fungi would appear. 
The organs being the part attacked, putrescence would com- 
mence internally, and fungi might appear externally before the 
disease had made any outward show. 1 have satisfactorily ascer- 
tained that plants, in the absence of the sun, transpire carbonic 
acid, which is also authenticated by Saussure, Ingenhoutz, and 
others. A plant, therefore, in the shade and at night, undergoes 
a chemical action the reverse of that which vitality induces; and 
as the potato was placed in a condition during the last season to 
favour this action, it must have ensued, and the continuance of it 
would necessarily produce fatal results. The changes, however, 
that would take place in the juices of the plant would be gradual, 
and its powers would gently decline as the vitality of its juices 
and the disorganization of its vessels proceeded. The absence of 
the sun only, if other circumstances had been favourable to the 
plant, could not have produced putrefaction ; because if the soil 
and air had been dry, exhalation would have so far solidified the 
juices as to prevent putrefaction, for without water this action 
cannot proceed. 
Fungi have been considered the cause of the disease, and so also 
has the use of the tuber for seed ; I shall therefore make a few 
remarks upon these heads before closing this secticm. 
With regard to fungi being the cause, and not an effect, I shall 
refer the reader in the first place to section 7 ; and, with that 
fresh in our memories, proceed to relate some new matter, making 
such use of that in section 7 as may be suited to our present 
purpose. 
I selected an apparently healthy potato plant, the top of which 
I cut off and immersed the stem of it in a vessel of common water, 
cemented to an ordinary table plate. I covered the plate with 
lime water, and the plant with a glass receiver. The glass 
receiver I cemented with Venice turpentine to the plate, and in 
tliis state I left it in the shade for about four weeks, and observed 
the changes it underwent; which were as follows: — 
1st. The colour of the leaves changed to a deep but clear 
green ; this remained for about ten days. 
2ndly. The lower leaves became gradually brown and drooped. 
3rdly. At the end of throe weeks the whole of the leaves, 
except at the apex and stem, also were reddish-brown, and an 
abundance of fungi was now apparent. 
