The Potato Disease. 
249 
Spores of Peio- 
nofpora inffStanh, 
magnified 30U dia- 
meters. One of the 
spores germinating 
a deterioration of the plant, liave been conclusively set aside. 
Nor can it be held that the microscopic fungus, which is known 
to be invariably found in diseased potatoes, is the result of the 
■disease and not its cause, since De Bary has produced the disease 
by placing the spores of the fungus on the leaves and tubers of 
healthy potatoes. 
Beginning this narration with De Bary's experiment,* we may 
trace intelligently the history of this baneful para- 
site, and notice the nature and progress of the 
injury it produces in the potato. The seeds, or 
more properly spores of the fungus, are minute 
ovoid bodies, so small that the greatest diameter is 
not more than the eight-hundredth of an inch 
long. When a spore rests on the under surface of 
a leaf, and- there is sufficient moisture, it pushes 
out a slender tube, through a ruptured opening in 
its coat. This tube penetrates the epidermis on the spot where 
germination takes place or finds its way to one of the innumer- 
able openings or stomates which 
abound on the lower surface of 
the leaf, and passing through 
the opening enters the tissues. 
The slender tubular root, called 
the mycelium, rapidly grows, push- 
ing its way everywhere through 
the substance of the leaf. It 
branches and rebranches freely ; 
the brown colouring matter con- 
tained in it gives the spotted ap- 
pearance to the leaves, which indi- 
cates to the eye the existence of 
the disease. The mycelium sends 
out, through the stomates, branches 
into the air, that give a mouldy aspect to the under surface of 
the leaf. The ultimate branches of this external growth are 
somewhat interruptedly swollen, and many of them bear minute 
oval bodies at their extremities. These are the spores. The 
mycelium passes down the leaf-stalk into the stem ; through this 
it obtains access to the other leaves as well as to the underground 
branches, and through them to the potatoes themselves, which are 
indeed only enlarged and shortened portions of the underground 
stem. De Bary placed some spores on the leaves of a healthy 
A spore which has penetrated the epi- 
dermis of the stem of a potato, and its 
mycelium root is penetrating the tissues of 
the stems. 
* The elaborate Paper by De Bary on this and allied parasitic fungi will be 
found in the ' Annales des Sciences Naturelles, Partie Botanique,' 4th Series, 
Tol. XX. (1863) pp. 1-148. Plates I.-XIII. 
