AN ENCYCLOPADIA 
a 
OF HORTICULTURE. 81 
Peronospora—continued. 
a time, when on a suitable part of the plant, they pu 
out tubes, and these pass through the stomata into the 
interior of the plant, and there reproduce the Fungus, 
This is the usual process in summer. The conidia and 
the zoospores, however, are unfit to resist intense cold; 
hence, another process of reproduction occurs in autumn, 
resulting in the formation of the oospores in the tissues 
of the host-plants. This latter process is as follows. 
On two neighbouring branches of mycelium-threads, 
inside the tissues of the host, two projections begin 
to form. One of these (oosphere) is globular; the 
other (antheridium) is oval, or clubbed, and much 
smaller, and a cell is cut off from the rest, at the end, 
by a partition. The antheridium comes to lie against the 
oosphere, and sends into it a kind of beak, which pierces 
its wall, and fertilises the contents. These contract, 
x 500 * 0 
Fic. 91. PERONOSPORA NIVEA a, Part of Conidiophore, with 
Seia 6, Conidium, inclosing five Zoospores; c, 
pore. 
become covered with a cell wall, and form the oospore (see 
Fig. 91, c). The outer wall of the oospore often bears 
ridges, or outgrowths, characteristic of the species. 
The oospores remain unchanged during winter, and in 
spring they reproduce the Fungus. : 
Remedy. Owing to the fact that the Fungi live 
inside the host-plants, and only produce their conidia 
outside them, it is impossible to destroy the parasite 
without destroying the host. Efforts should be directed 
against the spread of the disease, since a cure of the 
diseased plants is practically hopeless. All tissues 
attacked by Peronospore are liable to communicate the 
disease to healthy plants; hence, the former should, if 
Possible, be burned, that being the only certain means 
of preventing the spread of the disease. Damp places, 
and confined air favour the Fungi; hence, drainage is 
of value, and the plants liable to injury should be sepa- 
so far as can be done, to permit free circulation of 
air. Any means to strengthen the plants will probably 
render them better able to resist the evil influence of the 
Fungi. Warm, close, showery weather is peculiarly 
favourable to the spread of the latter, since it promotes 
the production of conidia, and aids the zoospores in 
diffusing themselves over neighbouring healthy plants 
by means of the drops of rain or dew. 
In the case of the Potato-disease Fungus (P. infestans), 
numerous remedies have been proposed, e.g., cutting off 
stems and leaves as soon as disease appears, Jensen’s 
method of earthing-up the drills, &c. See Phyto- 
phthora. 
The species of Peronospora are distinguished by 
differences in the erect branches that bear the conidia, 
and also in the markings of the outer coat of the 
Cospores; but the latter bodies have, as yet, not been 
detected in some of these Fungi. The genus has been 
Vol. III. é 
Peronospora—continued. 
divided into fonr sections by Professor de Bary, of 
Strasbourg, who has employed, as the basis of his 
arrangement, the modes of development of the zoo- 
spores, or mycelium, from the conidia, in forming new 
plants. This arrangement, however, is not of much 
assistance in making out the species of such Fungi as 
may be injuring any cultivated plants; and for ease of 
reference, therefore, in the following descriptions, the 
nature of the branching of the erect, fertile stems, and 
the markings of the outer coat of the resting spores, 
are chiefly employed. 
1. P. parasitica, abundant on, and destructive to, many 
ernciferous plants, including Turnips and Cabbages. On 
the larger kinds of Crucifere, the Fungus is often con- 
fined to spots on the leaves. It has large, branched 
haustoria, or suckers, which often nearly fill the cells 
of the host-plants, from which they draw nourishment. 
The fertile stems are rather stout below, and five to 
eight times bifurcated; the branches are rather crowded, 
and end in slender, short branches, each of which bears 
on its tip an elliptical, colourless conidium. The conidium 
shows no papilla, and, in germination, emits a mycelium 
tube from any point of its surface. The oospore is 
globular, and has a thin, nearly smooth, outer coat. 
2. P. Vici grows on Peas and Vetches, often doing 
considerable damage. The fertile stems are rather slender, 
six to eight times bifurcated, with the branches rather 
stiff and spreading, and the terminal branchlets rather 
longer than in most of the other species, and very slender, — 
The conidia are grey, with a pale violet tint, but otherwise 
` 
much as in P. parasitica. The oospore is globular, pale 2 
brown, with a network of ridges on the outer coat. — 
3. P. effusa attacks Spinach, and other Chenopodiacea, 
causing the leaves to become thickened, and to speedily 
decay. The fertile stems are rather short and thick, 
and bifurcate from two to seven times, ending in short, 
straight or hooked branches. They are grey, with a 
tinge -of violet, as are also the elliptical conidia. The 
latter germinate as in P. parasitica. The oospore is 
pale brown, with irregular, brown ridges which render 
it, though globular, decidedly angular in ontline. This 
species forms very dense tufts on the lower surface of 
the leaves. 
4. P. Schleideniana grows on Onions and their allies, 
The fertile threads are rather stout, not, or scarcely, 
jointed, giving off branches along the sides alternately; 
branches usually three to five times bifurcated, and 
ending in short, hooked or spreading. branchlets; conidia 
large, obovoid or nearly pear-shaped, all parts reddish- 
The conidia germinate from any 
part of their sides. The oospores are globular, brown, | 
grey or dull violet. 
smooth, except for an open network of 0 
5. P.: 5 lives on Poppies only, vr Pegged 
is of no very great practical import in fain. ] 
often causes distortions in the plants affected by it. The 
fertile stems are rather longer and more slender than 
usual, and are seven to ten times bifurcated; and the 
branches end in spreading, or hooked and very slender 
tips. The conidia are nearly round, small, colourless, and 
germinate from the side. The oospores are globular, 
brown, marked with a network of ridges. 
6. P. sparsa is one of the few Peronospore on woody 
plants. It grows, as a delicate grey mould, on the 
under side of the leaves of Roses, in conservatories. 
The leaves become spotted with brown, and soon shrivel 
up. The fertile stems of the Fungus bifurcate several 
times, and end in curved tips, bearing round conidia, 
which germinate from the side. The oospores of this 
species have not been observed. : 
7. P. nivea has been already described. The spreading 
branches, bifurcated from one to three times; the ovoid, 
rather small conidia, with a papilla at the tip, and pro- 
ducing, not a germinal mycelium tube, but four or five 
a M 
