for the Growth of Undiseased Potatoes. 391 
nterior of the plant as long as they are external to it. In July 
)r August, when the warm atmosphere drinks up to saturation 
he rain that has fallen, any slight reduction of the temperature, 
ike that caused by the setting of the sun, sets free a certain 
imount of water vapour ; the spores are able to appropriate what 
hey require of this vapour, and begin their active life by 
lushing out a small process or thread. Of all the spores which 
hus germinate only those that are in close relation to the host- 
ilant are able to maintain their life. Stone and earth afford no 
lourishment to the young plant of the Peronospora, nor even 
he leaves and stems of any of the plants on the surface of which 
t may germinate, unless these belong to the potato. But 
vhen the small thread from the spore has pushed its way 
hrough a stomate or penetrated the skin, it obtains possession 
)f a supply of food and moisture suited to all its needs, and 
peedily developes, destroying the plant in its progress, and 
hrowing into the air myriads of new spores to spread the 
nalady among the neighbours of its host. 
The successful cultivation of the potato in relation to disease 
s then, judging from our previous knowledge and from the 
esults of last year's experiments, really the problem of com- 
)ating the free atmospheric moisture, a battle whose issue is not 
mcertain. And as we cannot cope with this adversary, we 
.hall, yielding to circumstances, probably see the cultivation of 
he potato travelling eastwards to the districts in our island 
vhere the rainfall is small, and the soil is naturally or artifi- 
ially well-drained. Whenever the crops in the west can be 
■ecured before the July rains set in, there is no risk. At Ayr 
VIr. Wallace grows early potatoes extensively for the markets in 
he centre of England, and no crop could be more safe or pro- 
Itable when thus managed. But the same field which yielded, 
"arly in the season, . a heavy crop of perfectly healthy but 
mmature tubers, produced in the autumn a crop of ripe potatoes 
n which, in one of the varieties, three-fourths were diseased, 
vhile the whole six varieties were injured on the average to 
. he extent of very nearly one quarter. 
^ The experiments of last year may supply information as to 
he best conditions under which the potato can be grown. Does 
he exact information contained in the schedules of the growers 
md the judges throw any light on the great differences in the 
ictual yield of each hundredweight of seed, or in the estimated 
vield per acre ? The results of the experiments in relation to 
he amount of crop produced is exhibited in the Table (VII.) 
nserted opposite page 3<S5. 
The most obvious fact on the face of this Table is that, with 
ew exceptions, the varieties selected by each grower for his 
