1875. ] 
THE FUNGUS OF THE POTATO DISEASE. 
173 
that the new Potato disease was no other than the old enemy in disguise,—the 
Peronospora infestans in an unusual and excited condition. Under the microscope, 
two distinct forms of spore-like bodies were observed, the smallest of which was 
rightly determined to be the antheridia of the Potato fungus, and the larger the 
oospores of the same fungus—these latter, it is now ascertained, being the 
resting-spores so long sought for in vain. 
Mr. Smith’s account of his discovery was read by him at the meeting of the 
Scientific Committee of the Eoyal Horticultural Society on July 7, and was pub¬ 
lished in the Gardeners'' Chronicle (1875, ii., 35) : subsequently further observa¬ 
tions of his, accompanied by illustrations from his own pencil, have appeared in 
the same journal (1875, ii., 68), and from this source we borrow the following 
illustrations, and Mr. Smith’s remarks thereupon, somewhat condensed:— 
“ Figure 1 shows (greatly enlarged) a transverse section through the leaf of a Potato 
plant. The two great bodies at A A represent two minute hairs on the leaf, and at BB are 
seen the individual cells of which the leaf is constructed. When these hairs and cells are 
compared with the fine thread at c, which represents a branch of the Potato fungus coming 
out of a breatliing-pore of the leaf, it will be seen how very minute the fungus is in com¬ 
parison with the dimensions of the leaf. This fine thread is no other than a continuation of 
a thread of spawn or mycelium which lives inside, and at the expense of the assimilated 
material of the leaf. When the thread emerges into the air, as here shown, it speedily 
ramifies in different directions, and bears at the tips of the branches, as at dd, fruits which 
are termed simple-spores , or conidi a, because from their smallness they are dustlike; these 
bodies are capaljle of germinating, and reproducing the species in the same way as any seed 
is capable of so doing. The Potato fungus has another method of reproducing itself in the 
larger swarm-spores shown at E, f. These are so called because, on the application of moisture 
(as supplied by dew or rain, or when applied artificially) they set free a swarm of from six 
to fifteen or sixteen other bodies known as zoospores , so named because they are furnished 
with two lasli-like tails, and are capable of moving rapidly about like animalcules. This 
rapid movement usually lasts for about half-an-hour, and, like the dust-like conidia or 
simple-spores before-mentioned, the swarm-spores generally enter the breathing-pores of the 
leaf, and there germinate. So potent, however, is the contents of these bodies when set free, 
that it is capable of at once corroding, boring, and entering the epidermis of the leaf, or 
even the stem itself. These zoospores are best seen when within the swarm-spore f, where 
they arise from a differentiation of the contents, but when once set free (g) they are, from the 
extreme rapidity of their movements, very difficult to make out. In about half an hour they 
cease to move, their lash-like tails (cilia) disappear, and having burst at one end, a trans¬ 
parent tube is protruded, which is a similar mycelium in every respect with that produced by 
the simple-spore, and which grows, branches, and fruits in a precisely similar manner. These 
simple-spores and swarm-spores are lost in the production of the mycelium or spawn, and this 
fine thread-like material cannot, of course, survive the frosts and rains of winter, but must 
utterly perish with the perished leaves and haulm. 
“ A study of other species of Peronospora allied to the one which produces the Potato 
disease reveals the fact of a third mode of reproduction. The simple-spores and swarm- 
spores are termed asexual, because they are without sex, as distinguished from other bodies 
called oospores, which are produced by the contact of two sexual spore-like bodies, known as 
the antheridium (h), the male, analogous with the anther, and the oogonium, (j) the female, 
analogous with the ovary of a flower. The oospores, not till now seen for certain in the 
Potato disease, are the true resting-spores. Instead of being transparent and unenduring, as 
are the simple and swarm-spores, these bodies are dense in substance, black-brown in colour, 
and covered externally with reticulations or warts. They are produced from the mycelium, 
by the contact of the antheridium and oogonium, in the substance of the decaying plant; 
they are washed into the earth, and there they rest till a certain set of conditions makes them 
germinate in the year following their production, just as a seed falls and rests in the autumn 
and starts again into life during the following spring. The terms here used will be better 
understood if it is borne in mind that the oogonium is analogous with a pod; the oosphere 
within answers to the ovule, and the oospore (or resting-spore) to the matured seed. The 
antheridium with its contents is analogous with the anther and its pollen. 
“In various other fungi nearly allied to the Potato fungus these rcsting-sporesliave been 
