124 
Certain Fungi Parasitie on Plants. 
the mildew is confined to certain varieties. The black Hamburgh, 
for example, was not affected at all by it, although growing side by 
side with mildewed vines. The green wood is always more injured 
by the Oidium than the ripe ; consequently, as some varieties of 
vines ripen sooner than others under the same conditions, so the 
green branches of the later varieties will probably be more affected 
than those of the early. It was shown by my paper on the fungus 
of the American grape-vine, in the Annual Keport of the Depart- 
ment for 1871, that the early spring leaves of American grape- 
vines are not affected by the mildew (Peronospora viticola) during 
the summer months, under ordinary conditions, although the leaves 
that sprout in summer, particularly during rainy weather, when 
sappy and of a very light-green colour, are very liable to be 
affected with the mildew, particularly some varieties. 
In the fall of 1872 I selected several vine leaves from the 
foreign Department grapery, having on their surface patches of 
mildew intermixed with perithecia of the Erysijphe Tucheri. 
Having removed portions of them, I placed them on glass slides, 
and secured them in position with gum water, over which I placed 
a thin glass disk. While viewing them under a power of about 
100 diameters, I applied pressure on the disk in order to burst the 
perithecia. I used great care in my manipulation, but failed to get 
sporangia out of them. I then laid the leaves aside until Novem- 
ber, 1874. In consideration of recent successful experiments on 
perithecia of black-knot fungus, I resumed my experiments on those 
of the foreign grape-vine mentioned. I removed a small portion 
of the leaves procured in 1872, containing the perithecia, placed it 
in a capsule and poured over it concentrated ammonia with the view 
of softening its albuminoid matter. To another portion I added 
nitro-muriatic acid, and neutralized the acid by ammonia. This 
latter method has the advantage of bleaching the perithecium, 
which is naturally opaque, but when partially bleached is of a 
translucent Yandyke-brown colour. Under either treatment the 
perithecia become soft and pliable, and the proper degree of pressure 
may be given during the operation while viewing them under the 
microscope. In this way I have succeeded in bursting them and 
forcing out their sporangia in perfect form. I had previously failed 
in this experiment, probably for the reason that the sporangia had 
not matured sufficiently, and in consequence of the thinness of 
their cell-walls they burst with slight pressure, and a grumous mass 
was all that I obtained. The sporangia of some of the forms of 
MierosjjJiwria are easily removed, and seem to bear more pressure 
without breaking the cell-walls of the sporangia than those of the 
vine, judging from my experience thus far. 
During the last four years I have examined many hundreds of 
specimens of the Oidium form of the vine fungus, but in no case 
