156 
THE DICTIONARY OF GARDENING, 
Plasmodiophora Brassice—continued. 
Microscopic inspection of a thin slice from a diseased 
portion of a root, shows numerous cells of the tissue of 
the host-plant hardly altered in any way; but, inter- 
mixed with these, there are many others, from four to six 
times their diameter, which are occupied by the parasite ; 
and it is these enlarged cells that alter the colour of the 
root, and make its surface in a transverse section, when 
a little magnified, look mottled. In the newly-diseased 
roots, these enlarged cells inclose a slimy, yellowish, 
granular plasm, in which are usually numerous cavities, 
filled with cell sap or with air. If examined at a later 
period, these enlarged cells are found filled with myriads 
of the minute, globular, thin-walled, transparent spores 
of P. Brassice. ‘These remain uninjured during the 
keenest frosts of winter, and form a large part of the 
powdery material that fills up the decayed root between 
the woody bundles, and they become widely scattered 
through the soil by the action of wind and rain. When 
the weather becomes warmer, in the following spring, they 
begin to germinate, the cell-wall splits, and the proto- 
plasmic contents pass out, and creep about through the 
damp soil like Amebe, and also by the help of a slender 
hair or cilium at one part. When they come into contact 
with others like themselves, they unite completely, and 
form larger plasmodia. If they meet with young roots of 
Cabbages, Turnips, Charlock, or Wild Mustard, they make 
their way into them, and produce the disease in them also. 
This disease has, of late years, been spreading far and 
wide, and has proved very destructive in Turnip fields; 
so much so, that, in some districts, Turnips are no longer 
a profitable crop. It also attacks Turnips and Cabbages in 
vegetable gardens, though the less extended diffusion of 
the host-plants renders the attacks less fatal, and remedies 
more readily applicable. Frequent experiments have shown 
that, in soil in which diseased roots have been allowed to 
= rot, a new crop of Cabbages, or other host-plants,- will 
certainly suffer from disease if sown in that soil in the 
ensuing year. Even after a year has elapsed, the crop 
_ is liable to suffer from this cause, though to a less extent. 
_ Yet farmers almost always, and gardeners frequently, 
leave the diseased roots in the soil, as not repaying the 
labour of pulling them up. Experiments have been con- 
ducted by Mr. T. Jamieson, to ascertain the result of 
different manures on this disease, and have led him to 
the conclusion that the use of superphosphates, or soluble 
p. , is followed by a great increase of disease ; 
and that ground coprolites, and ground or steamed 
bone-flour, are the manures that best enable the 
plants to resist it; but these conclusions are opposed by 
other experimenters. If the effects on Charlock, Wild 
Mustard, and Cabbages, are compared with those on 
cultivated Turnips, it is at once seen that the last plant 
~ suffers far more severely. The cause seems to be that 
the Fungus attacks the cellular tissue only, and this in 
Turnips is very largely developed, as it is this that 
renders the roots for food, and, accordingly, it 
has been promoted by the selection of those varieties 
for propagation that produce the largest roots with least 
woody fibre in them. The former plants are crippled, 
bat are generally able to produce some seeds; but the 
_ Turnips are, in general, destroyed before they reach the 
stage to form seeds. 
Remedies. All diseased roots ought to be collected at 
S early a stage as possible, and destroyed by burning, 
a if this is practicable. Ground where Turnips have become 
3 diseased should not have Turnips or Cabbages grown in 
it for at least two years, to permit any spores in the 
soil to germinate, and thus to starve and kill the plas- 
upon the effect of different manures are desirable, but, in 
the meantime, they point to ground coprolites, and to 
| in a well-drained, sandy soil, 
Plasmodiophora Brassice—continued. 
ground or steamed bone-flour, as the best artificial 
manure for Turnips. Unfortunately, no means are 
known for the cure of plants already attacked, pre- 
vention alone being practicable in any way. 
PLATANACEA:. A very small natural order of 
usually tall trees, with flaking bark; they inhabit „the 
temperate or sub-tropical regions of the Northern hêmi- 
sphere, two are natives of East Europe or Asia, and the 
rest are American. Flowers moncecious, in unisexual, 
globose, densely-crowded heads; “the stamens in the 
males, and the ovaries in the females, are mixed, with- 
out definite order, with scales, which may be bracts, 
perianth segments, or staminodes, or arrested ovaries” 
(J. D. Hooker). Fruit a nut. Leaves alternate, petio- 
late, broad, palmately nerved and lobed; petioles dilated 
at base; stipules membranous, caducous. The only genus 
of the order—Platanus—comprises only five or six species, 
which are mainly valuable for their timber and orna- 
mental appearance. ` 
PLATANTHERA. 
(which see). 
PLATANUS (Platanos, the old Greek name, from 
platys, broad; referring to the foliage). Plane-tree. The 
only genus of ORD. Platanacew (which see for characters, 
&c.). The two species described below are magnificent 
trees for parks and similar situations. Both require a 
deep, rich, soft soil, and generally attain the greatest 
size where their roots have access to water. They 
require shelter, but must not be confined. Propagated 
by seeds, or by layers. The former are contained in 
round balls, which require to be broken, and should be 
sown in March; merely pressing them into the surface of 
the soil is sufficient, but they must be kept moist and 
shaded. The quickest way to propagate is from layers. 
P. occidentalis (Western).* Button-wood; Western Plane, 
jl. greenish. May. fr. brownish ; ripe in October and November. 
l. five-angled, obsoletely lobed, dentate, wedge-shaped at the 
t. to 80ft. Atlan 
beneath. tic and Western 
from P. orientalis in its 
being solitary on the long peduncles. It is rare in 
Included under Habenaria 
fertile cai 
. British gardens, and tly not so hardy as the common 
species. (E. T. S. M. 2, 261.) i 
v 
orientalis (Eastern).* Oriental or Common Plane. fl. greenish- 
yellow. pean Jr. brown; ripe in October, and persistent for 
the greater part of the winter. J. five-lobed, palmate, wedge- 
shaped at the base, the divisions lanceolate, sinuated ; stipules 
nearly entire. h. t. to 80ft. Levant, previous to 1648. — 
W. D. B. 101.) A beautiful tree, ae es variety of 
dsome forms, which differ chiefly in the shape and lobing of 
the leaves. The variety acerifolia (Maple-leaved) is the com- 
monest in cultivation, frequently ing the name of P. occi- 
dentalis, from which it may be readily distinguished when in 
fruit by the peduncles bearing more than one ball, and 
frequently many. It is the form known as the London Plane, 
on account of its being generally planted in the parks ; and is 
an erect-growing tree, with usually three-lobed leaves, or, if five- 
lobed, less deeply so than in the typical form. (W. D. B. ` 
under name of P, occidentalis.) The typical orientalis is a more 
spreading tree, with very large, deeply — leaves, cordate 
or truncate at the base. The variety cuneata has the leaves dis- > 
tinctly wedge-shaped at the base ; laciniata, very deeply, much- 
divided leaves ; and variegata, variegated foliage. 
A plant in gardens, with fine bold foliage, called P. californica 
has not yet fruited in this country ; it is doubtful whites: really 
is the Californian P. racemosa (with which the true P. cali- 
| fornica is synonymous). F 
Spa al a Included under Fumaria (which 
see). 
PLATYCARPHA (from platys, broad, and carphe, 
; in allusion to the broad, chaffy scales of the in- 
volucre). ORD. Composite. A genus consisting of a 
couple of species of stemless perennials from South 
Africa. Flower-heads purple, densely crowded, many- 
flowered, homogamous, sessile. Leaves numerous, stalked, 
spreading like a star upon the ground, pinnately divided, 
with coarsely-toothed lobes and pungent teeth, P. 
glomerata is a pretty and interesting plant; it thrives — 
