An Encyclopedia of Horticulture. 



203 



Fosoc[Tieria — continued. 

 with a terete tabe sometimes more than 1ft. 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), rf. white, erect, very long, -showy, 

 sweetly scenteil ; inriorescehce terminal, c.viuose. July. I. oppo- 

 site, oval, petio!at« ; stipules intrapetiolHr, oblong-triangular, 

 entire, .it length deciduous, h. 15ft. to 20ft. Caraccas, 1815, 

 (F. A. s. vi. 587 ; 1,. iS P, F, U. i. lU.) 



P. fragrantisslma (very fragrant).* jt. white, very fragrant, 

 with a slender, cylindrical tul>e, 6in. long, and reflexed, elliptic- 

 oblong segments,' disposed in sub-coryuibose panicles at the ends 

 of the branches. L opposite, ovate-oblong, coriaceous, shining 

 gieeu, the principal veins yellow. Branches yellow Brazil, 

 1871. A very handsome plant. (I. H. ser. iii. 27.) 



P. gracilis (slender). Ji. white, four or five in a corymb ; corolla 

 with a curved tube anil an irregular liml>. September. I. oval- 

 lanceolate ; stipules oblong, h. bit. to 6ft. (iuiana, 1625, 



P, longiflora (long-flowered), ff. white, with the tube 6in, long, 

 very nnich inc rved. nutant at the apex, green at the base ; 

 hairs in the throat very long ; corymbs from six to twelve- 

 flowered. Summer, I. oblong, acuminated, acute at th-j base, 

 h. 5ft, to 6ft. French (Iuiana, 1820. 



Fig, 256, Flowering Twig of PosoyiERU miltiflora, 



P. mnltiflora (many-flowered).* .//. white, fragrant, with a slender 

 tube 4in. long, and a spreading, star-shaped limb 2iin. across ; 

 cymes teruunal. I. broad, oval-oblong, coriaceous, velvety, 

 villous beneath. Brazil, 1866. A magnificent plant. See 

 Fig. 256. (I. H. 597.) 



P, revoluta (revolute). ff. white, disposed in dense corymbs ; 

 corolla with a straight tube, a villous throat, and an irr. gular 

 limb. April. I. elliptic-ovate, each ending in a short, cuspidate 

 point, and having revolute niargius, h. 5ft- to 6ft, Brazil, 1852, 



P. versicolor (various-coloured), Fi. various-coloured, chang- 

 ing from white to crimson throuj^h pink, long, pendulous, fra- 

 grant ; segments of corolla not much longer than the stamens, 

 August, /, oval-lanceolate, acuminate at both ends, glabrous, 

 h. 6ft, Cuba, 1810, A handsome shrub, (B. R, 1841, 26-) 



POSTERIOR. Applied to that part of an axillary 

 flower which is placed next the axis of inflorescence, 



POSTICOUS. On the posterior side ; an adnate 

 anther is said to be Posticou.s when it faces the petals, 



POTAMOGETOIT (from pofamnn^ a river, and rjeifon, 

 a neighbour ; alluding to the natural place of growth). 

 Pond Weed, Ord, Naidacew. A genus comprising about 

 fifty species (with many sub-species and varietie.s) of 

 aquatic herbs, with creeping rhizomes, widely dis- 

 tributed. They are of no horticultural value. Twenty- 

 one species are natives of Britain, 



POTASH. A compound of oxygen with a metallic 

 element, first separated, in the pure state, by Sir Humphrey 

 Davy, in the ijeginning of this century, and named by 

 him potassium, in allusion to its preparation from Potash. 

 The latter was itself long regarded as an element, 



Fotasli — continued. 

 becatxse of the difliculty 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 oil' 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 

 p.arts 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 o e-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.y., as potassio 

 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 tissues 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 atarch 

 being formed in the green tissues, by means of the 

 chlorophyll. But, in addition to starch being formed in 

 the green parts, it must bo 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 



