G6 
THE FLOEIST AND POMOLOQIST. 
[ March, 
during hot moist summer weather, spreading more or less rapidly over it, and in 
some way or other finding its way to the tubers, those which are infected by it 
soon becoming putrescent. The Peronospora^ like other fungi, is propagated by 
means of spores, which are minute reproductive germs taking the place of seeds. 
These spores are of three kinds, called bud-spores, moving-spores, and resting- 
spores, the two former being regarded as somewhat analogous to the buds, and the 
latter to the seeds, of a flowering plant. The resting-spore is so called because 
it does not grow till some time after it is formed ; and when it does grow, it 
does not, like the others, produce the mycelium or spawn, but bears new bud- 
spores and moving-spores, from either of which the spawn may be developed. 
The exact mode in which these different kinds of spores act in the propaga¬ 
tion of the Potato Disease is not positively known. The observations of Mr. J. M. 
Barnes, of Levens, tend to show that the attack is to be traced back to the ger¬ 
mination of the resting-spores, which takes place as soon as the ground becomes 
sufiSciently heated, usually about the middle of July, and from these the bud- 
spores and moving-spores are soon evolved. 
After many years’ observation, Mr. Barnes finds that in old gardens the 
fungus is always first met with on leaves lying flat upon the ground, and from 
these as centres of infection it spreads rapidly to the plants round about. In 
this way arises the patched appearance which is presented by a plot of Potatos 
for a short time after being attacked by the disease. In the case of new ground, 
where the patched look is seldom or never seen, he concludes that the spores are 
brought by the winds, and scattered indiscriminately over the whole crop. A 
few days after the germination of the spores, the fungus arrives at maturity, and 
the bud-spores, set free from the filaments, float in immense multitudes through 
the atmosphere, or are washed down by the rains till they reach a spot on which 
they can undergo development. Wherever they settle, on leaf, stem, or tuber, 
they commence at once to germinate, and the infection spreads more or less 
rapidly, according to the humidity of the season. Without rain it makes but 
slow progress in the haulm, and does not reach the tubers at all; while, on the 
other hand, the more rain there is during the time the fungus is active, the more 
is the crop affected.” 
The assumption that the disease is communicated to the tuber by the mycelium 
descending through the interior of the stem has, in Mr. Barnes’ opinion, done 
more than anything else to blind people as to the remedy for the evil. If this 
were the case, there would be some moderate degree of regularity in the way in 
which its effects presented themselves there, and its track through the Potato 
would be permanently marked. But there is not the slightest regularity as to 
the part of the tubers attacked; very frequently they are affected in several 
places at once, but in these instances no communication can be discovered between 
the various patches, which do not radiate from the point where the tuber is 
fastened to the stem, as they would do were the disease to enter that way; besides 
which, until the disease has made great progress, they are always separated from 
each other by portions of sound and healthy tuber. 
These considerations induced Mr. Barnes, in 1849, to enter upon observations 
and experiments which at length convinced him that the disease was carried to the 
tuber by the drip from the leaves. As Potatos are generally grown, every shower 
of rain carries down the active-spores to the tubers below, and as a natural conse¬ 
quence, they fall a sure prey to the fungus. Hence, he concluded, it is not to the 
mj^'celium in the stem, but to the spore-bearing branches outside, that we owe the 
destruction of our crops, and that if by any means the spores could be kept from 
reaching the tubers, we should have no disease in them, however virulently it 
