178 THE DICTIONARY 
OF GARDENING, 
Polianthes—continued. 
Tnberose, Orp. Amaryllideæ. A monotypic genus. 
The species is a splendid, half-hardy, bulbous plant. 
Tuberoses may be had in flower throughout the greater 
part of the year by potting successional batches of bulbs. 
They are imported at the latter end of the year, but a 
portion may be kept for successions. Loam, with a little 
manure or leaf mould intermixed, is a suitable compost, 
and 5in., or at the most 6in., pots are large enough. The 
bulbs may be inserted singly or three in a pot, and plunged 
at once in a bottom heat of from 60deg. to 70deg.; water 
should be withheld until the leaves appear, unless the 
soil becomes very dry; afterwards, it may be given freely. 
The flowers are pure white, and very highly perfumed; 
when detached singly, they are very useful for buttonhole 
and other bouquets. As the plants naturally grow tall, 
they should be kept in a light position, to induce them 
to keep as dwarf as possible. They will succeed during 
summer in any cool house, or may be planted in an 
open border. ‘The bulbs are not usually kept after one 
tuberosa (tuberous).* ite, delightfully fragrant, showy, 
gfe in Ay ren : aly simple raceme 5 = Ay parts 
shaped and incurved; stamens affixed at the t t. Autumn. 
4. radical or on the grad pr aa * a — or — 
‘short, tuberous, erec b, . . le e: cl iv 
in American, Asiatic, me § p odiaba gardens), 1629.. (B. M. 
1817; B. R. 63.) The double-flowered form is that principally 
— There are several varieties, such as DOUBLE AFRICAN, 
BLE AMERICAN, DOUBLE ITALIAN, and PEARL, and, of these, 
the last-named is most preferable, it being not so tall in growth 
as the others, 
POLISH JUNIPER. See Juniperus communis 
cracovia. 
POLIUM. Included under Teucrium (which see). 
POLLEN. The coloured dust found in all mature 
flowers, except the few that are entirely female. It is 
- found in the anthers, or thick heads of the stamens, and 
is set free, in the form in which it is best known, by the 
_ bursting of the walls that surround the spaces in which 
it is formed, and in which it is retained till ripe. In 
order to render this account of Pollen more clear, it is 
necessary to give a short account of the development 
and structure of anthers. The anther is the essential 
part of each stamen. In most cases, it is supported on 
a stalk or filament. It is at first made up of a mass of 
small cells, almost alike in form and size; but changes 
go on during its growth, and, when mature, one can re- 
eognise in it the various structures described below. 
The whole anther is covered with an outer layer of 
cells known as the epidermis. In the centre lies a 
column of thin-walled cellular tissue, called the con- 
nective, with a fibro-vascular bundle in the middle of it. 
At each side of this are two spaces or loculi, in which. 
lie the Pollen grains till the spaces burst. Each is lined 
by a thin, dark layer of disorganised cells, known as the 
_ endothecium. Between these and the epidermis lies a 
tissue, known as the mesothecium, generally composed of 
several layers of cells. These cells, called “fibre cells,” 
are usually peculiar in having the walls thickened with 
_ deposits, variously arranged in spirals, rings, networks, 
arches, and several other figures. The fibre cells are 
little value as an indication ication of affiniti ies betwee: en. plants, 
Pollen—continued. 
considerable number is formed, and they are called the 
“parent ceils of the Pollen.” In each parent cell, the 
contents group themselves together, and form four cells, 
the Pollen grains. ‘There are differences in detail in 
different plants in the development of Pollen; but the 
usual course is that the walls of the mother cell waste 
away, and, it is believed, assist to nourish the grains, 
and to form the spines on the exterior of many kinds of 
Pollen. The Pollen grains at last lie in the loculi 
like a powder. The endothecium is, at first, a layer of 
thin-walled cells, with abundance of protoplasm; but the 
Pollen is nourished, in part, at the expense of these 
cells also; and there remains, to indicate its former 
existence, only the thin layer already noticed. 
Pollen grains are usually free, but, in many plants, 
development seems arrested early; e.g., in Heaths, the 
four cells developed from each “parent cell” remain 
united together. In some Acacias, the Pollen grains 
are made up of from eight to thirty-two united cells. 
In Orchids, the grains in each loculus often stick 
together in pyriform masses, called pollinia. These 
peculiarities are the result of incomplete solution of the 
walls of the parent cells, since these remain and bind 
the Pollen grains together. The grains possess two coats 
Fig. 216. POLLEN OF EPILOBIUM ANGUSTIFOLIUM. 
A, Section of Pollen Grain—e, Extine ; 7, Intine ; ti, Thick Intine; 
J, Fovilla. B, Growing Point of Pollen Grain—e, e, Extine ; 
i, i, Intine ; f, Fovilla ; pt, Pollen Tube. 
(extine and intine) (see Fig. 216, A). The inner consists of 
cellulose, is, in general, thin, and can be stretched, 
especially so in the form of a tube (see Fig. 216, B), 
protruded from the grain, when it lies on the stigma 
of the same species of plant, or is placed in a drop 
of weak solution of sugar. There are no openings 
in this coat. The outer coat differs from the inner, 
inasmuch as it is not extensible, and consists of a sub- 
stance like cuticle in its chemical composition. This 
coat is entirely absent from the Pollen grains of Zostera, 
and of a few other plants that flower under water. It 
is occasionally uniformly spread all over the grain, and 
must be burst off before the Pollen tube can be pro- 
traded; but, in general, it is pierced by pores, or slits, 
of definite form and number for each species of plant. 
Through these openings one or more Pollen tubes are 
pushed when conditions favour their growth. The sur- 
face of the extine is smooth in many Pollen grains, but 
in most it bears characteristic outgrowths in the form 
of ridges, e.g., in many Composite, or of spines, e.g., 
in Mallow, Mistletoe, or of granules, as in many 
Dicotyledons. The nature of the surface in different 
Pollen grains is closely connected with the modes in 
which the Pollen is conveyed from the anthers to the 
stigma (see Pollination). In form, the Pollen grains 
differ very greatly in different plants. The most com- 
mon forms are spherical, and oval with rounded ends; 
but many others exist, such as cubical, triangular, cylin- 
drical, and polygonal. The form seems rather constant 
within the limits of genera, but varies greatly within 
certain families. Hence, the form of Pollen grains is of 
