Tlie Potato Disease. 
251 
The mycelium does not naturally fruit on the upper surface 
of the leaf, on the stem, or on the tuber of the potato, as these 
parts are either destitute of stomates or but partially furnished 
with them, and the mycelium does not send its fruiting branches 
through continuous epidermal structures ; but when any of these 
parts of the potato, attacked by the parasitic fungus, are cut and 
placed in a moist atmosphere, the fruiting branches speedily 
appear. 
Some of the oval heads which terminate the branches are 
larger than the others, 
and contain within them Fig. 4. — Siwres 0/ Peronospora infestans. 
from six to sixteen 
jninute bodies. When 
water is applied either 
artificially or naturally, 
the outer covering 
bursts and the contents 
are liberated. Each of 
the little spores thus 
set free moves about in 
the water by the aid of 
two cilia. In a short 
time the motion ceases, and if a proper nidus exists, the spore 
germinates. 
The minute fungus belongs to the genus Peronospora, and has 
leceived the name Peronospora infestans, Mont. Another method 
of reproduction has been noticed in other species of this genus, 
but it has not yet been detected in the case of the species causing 
the potato disease. In the other species the mycelium buried in 
the tissues of the supporting plant 
produces two kinds of cells, which 
have the same relation to each 
other that the ovule and the pollen 
grain have in flowering plants. 
The small cell, representing the 
pollen grain, when it comes into 
contact with the larger cell, 
pushes out a tube which pene- 
trates its outer wall, and on reach- 
ing the inner wall induces changes 
which produce a ripe spore, called an oospore 
is full of small granules, which are liberated, as in 
A. Large spores borne on the mould of rei on >tpora in- 
festans; the contents of the cell divided. B. The small .-.pores 
(.zoospnies) escaping from the cell. C. A zoospore with its 
two cilia, n. A zoospore deprived of cilia, and germinating. 
(Magnified 300 diameters.) 
Fi?;. 5. 
A. Oospore of Ptronospora 
rarum, Casp. T!. Small autheridian 
(Magnified 300 diameters.) 
umbelUfe- 
spore. 
The oospore 
the fruit 
already described, on the application of water, and being fur- 
nished with cilia they move about for some time. Although 
these minute spores abound on and in the soil around the diseased 
plant, it appears that they never attack healthy plants through 
VOL. IX. — S. s. S 
