AN ENCYCLOPADIA 
OF HORTICULTURE. 
PLEIOS. A term which, used in Greek compounds, 
signifies more than one; e.g., Pleiophylins, several- 
leaved. * 
PLENUS. Full; double. Applied to flowers in 
which the number of petals, &c., is abnormally multi- 
plied. 
PLEOCNEMIA. 
(which see). 
PLEOMELE. A synonym of Dracæna (which see). 
PLEOMORPHISM. See Pleomorphy. 
PLEOMORPHY, or PLEOMORPHISM (from 
pleion, several, and morphe, form; in allusion to the 
variability in the spores). A term used to express 
the condition observed in several groups of Fungi (see 
Oidium, Peronospora, Pleospora, Puccinia, and 
Pyrenomycetes), in which bodies of two or more forms 
are produced to effect reproduction of these plants under 
‘varying conditions; one form is, in some groups, known 
to be the result of sexual reproduction, e.g., zygospores 
of Peronospora, ascospores of Peziza and Pyrenomycetes, 
&c., and to such the term “spore” is, by some botanists, 
restricted in theory, thongh this is scarcely adhered to 
in practice. All the other forms of bodies specialised 
for reproduction fall under the two types of conidia 
and sclerotia. In many plants, conidia of two or more 
kinds occur, either simultaneously or in succession, and 
in many Fungi (e.g., Hymenomycetes, Pucciniei, &¢.) no 
_ sexual form has yet been detected. à 
Polypodium 
PLEOPELTIS. Included under 
(which see). j 
PLEOSPORA. A genus of parasitic Fungi, belong- 
ing to the group of Pyrenomycetes, and to the sub-group 
Spheriacee, in which the perithecia are globular or 
- flask-shaped, and open by a circular pore or mouth to 
permit the escape of the spores. The perithecia are 
Included under Nephrodium 
ibe 
borne upon a mycelium, which penetrates the tissues of - 
the host-plant; but they are quite separable from this 
mycelium. The genus is one of a section in which the 
perithecia are at first covered by the epidermis of the 
host-plant, through which, usually, they ultimately burst, 
The mycelium does not form an evident layer or mass 
(stroma). The perithecia are not very thick-walled, are 
dark brown; and generally smooth; the opening, or neck, 
usually projects from the stratum in which the bodies 
are sunk, Another form of spore (conidia) is usually 
produced on the surface on the same host. The spores 
produced in the asci, inside the perithecia, are divided 
- by numerous cell-walls, crosswise and lengthwise, so as 
to resemble the arrangement of bricks in a wall, and 
they ure usually some shade of brown, seldom colour- 
less. A number- of species of Pleospora exist, some 
parasitic on one plant, some on another; but most of 
them seem to be found in the perfect condition only on 
dead or dying stems and leaves, and are thus, in this 
stage, not hurtful to garden or field produce. A large 
proportion of the species have been found only on wild 
plants or on grasses; and there is considerable doubt 
as to the number of really distinct species, since many 
of the named forms are only varieties of the extremely 
common P. herbarum. It will be well to give here a 
brief description of the conclusions of mycologists with 
regard to the life-history of this species, as it is believed 
to be injurious to various garden plants in its earlier 
conditions; and several of these stages differ sq much 
from one another, and from the mature condition, that 
they have been described as distinct species under widely- 
separated groups. This Fungus is supposed to be the 
cause of a disease of Potato-plants, characterised by 
retardation of growth and curling of the leaves, which 
become yellowish-green. On the le 
there 
eS 
share f-stalks and stems — 
appear brown spots, at first round, but widening 
Pleospora—continued. 
out, and, after a time, the whole stalk, with its leaves, 
withers and dies. The formation of tubers is but small. 
Various other garden and field plants, and many wild 
plants, present diseased conditions that are generally 
referred to the action of P. herbarum. Some botanists 
are of opinion that experimental researches, by means of 
cultivation of the Fungus, show that two distinct species 
have been confounded under the name P. herbarum, and 
that these two can be distinguished in the earlier, though 
not easily, if at all, in the mature, state; and they have 
been named, by Gibelli, P. Sarcinule and P. Alternarie. 
Practically, to gardeners, it matters little whether there 
are two species or only one, as both forms are common, 
and they agree in the mode of life. 
The Fungus can seldom, if ever, be observed in plants of 
quite a healthy appearance; but it undoubtedly exerts its 
action some time before it is externally visible, and, in this 
period, the food-plant becomes penetrated by its colourless, 
branched mycelium. On this mycelium, near and on the 
surface of the plant, are formed the reproductive organs, 
- in the form of conidia of two or three kinds, succeeded by 
the pycnidia and the perithecia. Both the latter are 
globular or flask-shaped bodies, with a narrowed neck, and 
a membranous, leathery or brittle, cellular coat. They 
are usually scattered plentifully on the surface of dead 
stems and leaves. The pycnidia are full of minute, 
é 
9009 
oy am 
Fic. 201. PLEOSPORA HERBARUM, — hint nove es 
- Phoma herbarum)—a, Pyenidia in transverse section, x 20, one 
opened; b, Cli eit on the Stalks, x 400; c, Conidia free 
aker falling off the Stalks, x 400. . é 
elliptical, transparent, one-celled bodies (see Fig. 201), 
borne on slender stalks from the inner surface of the 
walls. The perithecia are larger and darker than the 
pycnidia, and differ, from these in the spores contained 
in them being inclosed in large, transparent, elongated, 
cells (asci). In each of these are eight spores (see 
a 
a 
— 
— 
a 
8 
— 
j 
bis 
Ki 
* 
s 
8 
—— 
[È O 
w 
ey 
3 ipe Ascus, x 200; d, Ripe Ascus, x 200, inclosing eight 
ie ete: S — Spores, x 400, s Be h 
form, and divisions, _ 
Fig. 202). The latter are some shade of brown, elliptical, 
narrowed in the middle, and show numerous partitions, 
_ of which seven are across the length, and others divide | 
the spaces so formed into smaller spaces or cells. They _ 
are very much larger than the spores contained in the _ 
pycnidia. The conidia are formed, not in special recep- — 
_ tacles, as in the former cases, but exposed on the surface — 
of the plant. One of the earliest Fungi to appear on 
sickly plants in general is a bluish or greenish-grey coat 
