176 
THE FLORIST AND POMOLOGIST. 
[ August, 
resolves itself into two layers, tlie inner one being termed the endospore, and the outer, which 
in Peronospora infestans is almost black in colour and strongly warted, the exospore. The 
latter resembles in outward aspect, instead of one spore, a dense concreted mass of minute 
brown-black bodies. The antheridia are shown at N N. The perfected resting-spores are 
slightly egg-shaped, and on an average are one-thousandth of an inch in diameter. 
The oosphere is fertilised by the contact of the antheridium; when the two bodies accident¬ 
ally touch, the latter fixes a small branch or tube, called a pollinodium or fecundating tube, 
into the wall of the oogonium, and discharges part of it contents into the protoplasm of the 
infant resting-spore; when these latter are mature the mycelial threads soon vanish, and 
the spores are free. When I read my notes before the Royal Horticultural Society I had not 
been able to detect this fecundating tube, but since then I have seen it. 
“ After the Potato plant has been badly attacked and destroyed by the fungus, every part 
of the plant and its parasite perishes, except the dark brown warted resting-spores just de¬ 
scribed, and these find their way into the earth and hibernate. When they awake to 
renewed life in the summer they must germinate in the damp earth, and if no Potato plants 
are near they perish, as the earth cannot support them. In this they are not unlike the 
seeds of germinating Dodder, for if they cannot find a proper host they die. But if Potato 
plants happen to be near the corrosive mycelium, it at once penetrates and enters the tuber 
or haulm. The tuber cannot produce simple "or swarm-spores if buried, but in the haulms 
the mycelium doubtless soon grows and produces both these forms of fruit. These are at 
once carried by the air into the breathing-pores, and the wdiole history of the fungus here 
described is re-enacted.” 
Since the foregoing remarks were published, Mr. Smith has given an account 
of his further observations in the journal already quoted (1875, ii., 101) 
“I now give in conclusion an illustration of the perfectly mature resting-spore of Perono¬ 
spora infestans , as seen imbedded in the substance of the Potato leaf. These resting-spores, 
which carry on the winter life of the fungus, are not restricted to the leaves, for I find them 
sparingly in both haulm and tuber, although I have at present seen the best specimens in the 
leaves. Fig. 3 shows a transverse section through a black spot of one of the leaves from 
Chiswick; the resting-spore is there seen at A, nestling in amongst the cells of the leaf. An 
antheridium, b, and two oogonia (c, c) from which such resting-spores arise, may be seen, 
and the old common form of the fungus will be noticed breaking through a hair on the upper 
surface of the leaf, which is a very uncommon occurrence. The situation of the resting-spores 
can generally be ascertained on the leaves by noticing the slightly thickened and very dark 
spots, for the bodies are commonly in these spots. It is, however, an extremely doubtful 
matter either to get them out, or, indeed, to see them when imbedded, for, when mature, they 
are black-brown in colour, and only a little larger in size than the leaf-cells. These leaf-cells 
are also intense brown-black in colour from contact with the hurtful mycelium, and almost 
as hard as wood. The best way to see the resting-spores is to macerate the leaves for several 
days in water, and then, by crushing the spot between two slips of glass, to set them free. 
The presence of the fungus in the leaf makes the cells very thick and woody as well as black, 
so that in crushing the leaf-cells the resting-spore is not uncommonly crushed at the same 
time. With care, however, they can be got at, when they will be seen, as at D, covered with 
warts or coarse reticulations, and beautifully regular and perfect in outline; w T hen young they 
are of a pure warm sienna-colour, and when perfectly mature, brown-black and shining. They 
are spherical, or slightly egg-shaped, and measure on an average about one-thousandth of an 
inch in diameter. At E is shown a semi-mature resting-spore with pollinodium attached, 
accidentally half washed out of its coating of cellulose by maceration in water. 
“ There is a marked analogy in size and habit on the one hand between the oogonia and 
the swarm-spores, and on the other hand between the simple-spores and the antheridia. I 
consider that the oogonia and antheridia are merely the intercellular condition of the swarm- 
spores and conidia, which latter are the aerial state of the former. The facts which point in 
this direction are these:—Sometimes there is no differentiation in the contents of the swarm- 
spores, but the plasma is discharged in one mass, and not in the zoospore condition ; the 
swarm-spore then resembles the oogonium. At other times the oogonium shows a distinct 
differentiation in its contents, and matures from one to three resting-spores, which to me 
shows an approach to the condition of the swarm-spore.” 
In his first paper referred to above, Mr. Smith remarks that he is disposed to 
think that the fungus which produces the Potato disease is aquatic in one stage 
of its existence, and in this stage the resting-spores are formed; and he adds :—• 
u The reason these resting-spores have evaded previous search is that no one has 
