820 . BULLETIN OF THE BUSSEY INSTITUTION. 
rot in 1842, it is not at all likely that it then appeared for the first 
time, but we must suppose that some of the vaguely described epidem- 
ies of the last century were of the same nature. 
It is with such sudden and violent outbreaks as those of 1842 and 
1845, that in the public mind the potato rot is associated. Asa rule, 
however, the disease is of a milder type. Instead of a sudden destruc- 
tion of the crop, there appear on the leaves and stem brown spots, 
which gradually extend. After a while, certain plants are found to be 
rotting, and this process may keep on until a whole field is involved. 
This case, as far as the farmer is concerned, is very different from the 
other, inasmuch as, having seen that some plants are rotting, he can 
then, by harvesting the rest, save a portion of the crop. 
In whichever form the rot occurs, it makes its appearance always 
about the same time, as before mentioned, about the first of August, 
and always in damp weather. There is no case reported as occurring 
in a dry season, and a moist condition of the atmosphere is absolutely 
necessary to its production to any decided extent. Damp, muggy 
weather is, however, quite as favorable to its development as heavy 
rains. 
As was just remarked, the disease is first recognized by the brown 
spots appearing on the leaves. What is the structure of these brown 
spots? But, first, a word on the normal structure of the potato leaf. 
If we make a section through a healthy green leaf, and examine it 
with a moderately high power of the microscope, we find that it is 
composed of a number of cells or sacs packed together in an orderly 
sort of confusion, if one can say so. On the upper and lower surfaces, 
respectively, we find the cells arranged in a single layer, known as the 
epidermis. ere the cells are almost colorless, and shaped like flat 
tiles or the bricks of a sidewalk. On the upper surface they are 
nearly continuous; on the lower we find certain breaks, known as 
breathing-pores, where there is a communication between the internal 
part of the leaf and the external air. The internal cells are much 
more nearly spherical or ovoidal than the external, and are full of 
roundish green bodies which are called chlorophyl-grains. It is these 
bodies, seen in mass, which make the whole leaf look green to the 
naked eye, although the outside epidermis cells are ‘colorless. The 
internal cells are packed tolerably closely together near the upper 
surface of the leaf, but below they are arranged loosely, so that there 
