AN ENCYCLOPADIA 
OF HORTICULTURE. 203 
Posoqueria—continued. 
with a terete tube sometimes more than lft. in length, 
a glabrous or villous, hardly dilated throat, and a five- 
parted limb; stamens five, a little exserted. Leaves 
opposite, on short petioles, coriaceous. For culture of the 
species described below, which are those best known in 
gardens, see Gardenia. 
P. formosa (beautiful). fi. white, erect, very long, showy, 
sweetly scented ; inflorescence terminal, cymose. July. l oppo- 
site, oval, petiolate ; stipules intrapetiolar, oblong-triangular, 
entire, at len th deciduous. h. 15ft. to 20ft. Caraccas, 1815. 
(F. d. S. vi. 5875 L. & P. F. G. i tia) 
P. — ⸗ (very fragrant).* fl. white, ver t, 
with a slender, cylindrical tube, 6in. long, and — e iptic: 
oblong segments, — in sub- corymbose panicles at the ends 
of the branches. l. opposite, ovate-oblong, coriaceous, shining 
radi the principal veins yellow. Branches yellow Brazil, 
871. A very handsome plant. (I. H. ser. iii. 27.) 
P. gracilis (slender). fl. white, four or five in a co; 
with a curved tube and an irregular limb, September. 
lanceolate ; stipules oblong. h. 5ft. to 6ft. Guiana, 1825. 
P. longiflora (long-flowered). fi. white, with the tube 6in. long, 
very much incurved, nutant at the apex, green at the base ; 
hairs in the throat very long; corymbs from six to twelve- 
flowered. Summer. 1. oblong, acuminated, acute at the base. 
h. Sit. to 6ft. French Guiana, 1820. 
bs —— 
l oval- 
Fia. 256. FLOWERING TWIG OF POSOQUERIA MULTIFLORA. 
P. aiaia iiia ad" * —— nt, with a slender 
tube 4in. long, and a spreading, sta; limb 24in. across ; 
l. broad ——— ng, coriaceous, velvety, 
Brazil, 1866, A magnificent plant. See 
Fig. 256. (I. H. 597.) 
bf — pona fl. white, disposed in dense corym 
corolla with a straight tube, a villous throat, and an pret 
limb. April. J. elliptic-ovate, each we ce ores date 
point, and having revolute margins. h. Sft t. to 6ft. — 
P. versicolor (various-coloured). fl. Teei chang- 
ing — white to crimson through pink, long, pendulous, fra- 
— ; segments of corolla not much ge than the stamens. 
quai l. oval-lanceolate, acuminate at porh Py Ss, — 
R Cuba, 1840. A handsome shrub. (B. R. 
OR. Applied to that part 1. an axillary 
flower which is placed next the axis of inflorescence. 
POSTICOUS. On the posterior sidoj an adnate 
anther is said to be Posticous when it faces the petals. 
POTAMOGETON (from potamos, a river, and geiton, 
a neighbour; alluding to the natural place of growth). 
Pond Weed. ORD. Naidacew. A genus comprising abont 
species (with many sub-species and varieties) of 
aquatic — with creeping rhizomes, widely dis- 
tributed. They are of no horticultural, — Twenty- 
one species are natives of Britain. : i 
— A compound of oxygen with a metallic 
i ogg tomer pt oa by Sir Humphrey 
—— in the century, and named by 
ae a a 
i ala to preparation from Pola | 1 
Potash—continued. : 
because of the difficulty of analysing it, and of separating 
the oxygen in it from the potassium. This latter is an 
essential element in the chemical composition of plants; 
as may be inferred from two facts, viz.: (1) it is always 
present in the ash that remains when well-dried plants 
are burned; and (2) plants grown in soils or fluids of 
known composition, from which potassium is entirely 
withheld, remain stunted. Among the chief commercial 
sources of potassium are wood-ashes. These are washed, 
and carbonate of potassium and various other sub- 
stances’ are separated from them in this operation; 
the water in which they are dissolved is then heated 
until it is mostly driven off in the form of steam, and 
the less soluble substances can no longer be kept in 
solution, but fall to the bottom of the vessel. The car- 
bonate of potassium remains dissolved after the other 
compounds have separated out. The solution is poured 
off, and, on being heated till all the water is driven off, 
the carbonate remains in a somewhat impure state, known 
as pearl-ash. From this are prepared, by appropriate 
chemical processes, the element potassium, and its 
various compounds. The ashes of leaves, and, in fact, 
of all parts of plants, yield a considerable proportion of 
pearl-ash; indeed, it is more abundant in the green 
parts than in the wood. It is plentiful also in seeds. 
Phosphate and chloride of potassium are also found in 
the ashes of plants. The compounds of potassium, taken 
together, amount very frequently to one-half, or even 
more, of the total weight of the ashes. Potassium pro- 
. bably exists in plants combined with organic acids, 
formed in the plants during growth, e.g., as potassic 
tartrate, potassic oxalate, &c.; but these compounds are 
broken up when the dried plants are burned; and they 
are replaced in the ashes by the compounds already men- 
tioned. Plants obtain the potassium that they require 
from the soil, which is seldom, if ever, wholly devoid of 
the element. Its exact uses to plants are still some- 
what uncertain, as the results of experiments do not 
entirely agree among themselves; but there is little 
doubt in regard to certain conclusions, which are founded 
alike on analyses of ash of plants and on experiments. 
It has been found that plants from which potassium: is 
entirely withheld cease to form new food for themselves, 
although supplied with every other element required for 
their nutrition. The tissnes and organs of the plants 
remain healthy for a time; but they do not increase in 
size. On supplying a solution of any compound of potas- 
sium to them, they begin to grow again, and they con- - 
tinue to do so if the supply is kept up. It has been 
ascertained, by experiment, that the compounds of 
potassium most useful to plants are the chloride and 
nitrate; the phosphates and sulphates being less effective 
in promoting increase in size. 
In reference to the mode of action of potassium on 
plants, its presence seems necessary to permit of starch 
being formed in the green tissues, by means of the 
chlorophyll. But, in addition to starch being formed in 
the green parts, it must be transferred from the tissues, 
where it is formed, to those in which it is to be made 
use of, or to be stored; and this seems to require the 
presence of chloride or ‘of nitrate of potassium in the 
tissues. When the sulphate alone is supplied, the leaves 
become gorged with starch grains, and fleshy, and 
look sickly. The same result follows, though less 
markedly, when the phosphate is used: instead of the 
sulphate. 
POTATO (Solanum tuberosum). The product of 
this valuable and well-known plant may safely be de- 
 signated, as an article of food; one of the most important 
and essential of any obtained from the vegetable kingdom; — 
indeed, it would be difficult to imagine how the present 
population could, for any length of time, be adequately 
fed and provided for without it. The species from which oe 
