124 The Besting Spores of the Potato Fungus. By W. G. Smith. 
herewith (Fig. 6) shows a transverse section through a black spot 
of one of the leaves from Chiswick, and the resting spore is seen at 
A nestling in amongst the cells of the leaf. An antheridium, B, 
and two oogonia (C, C), from which sach resting spores arise, may 
be seen in the cut, 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 rest- 
ing spores can generally be ascertained on the leaves by noticing 
the slightly thickened and very dark spots, for the bodies are com- 
monly in these spots. It is, however, an extremely difficult 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 set 
them free by crushing the spot between two slips of glass. 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 : when young they are of a pure warm 
sienna colour, and when perfectly mature, brown-black and shining. 
They are sphserical or slightly egg-shaped, and measure on an 
average about one-thousandth of an inch in diameter. I consider 
it worthy of special note that these resting spores are almost 
exactly the same in size, conformation and colour with Peronospora 
arenari^, Berk., an allied species found parasitic on Arenaria 
trinervis. In looking for these bodies care must be taken not to 
confound them with corroded cells, granules of starch injured by 
the disease, or foreign bodies. 
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. 
I may say as an addendum that to me there is a marked 
analogy in size and habit on the one hand between the oogonia and 
the vesicles which contain the zoospores, 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 
vesicles which contain the zoospores and conidia, which latter are 
the aerial state of the former. 
The facts which point in the direction just indicated are 
these : Sometimes there is no differentiation in the contents of the 
vesicles, but the plasma is discharged in one mass and not in the 
z(;ospore condition, the vesicle then resembles the oogonium. At 
