178 



THE DICTIONARY OF GARDENING, 



Polianthes continued. 



Tuberose. ORD. Amaryllidece. 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 snccessional 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 bo 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 

 year. 



P. tnberosa (tuberous). /. white, delightfully fragrant, showy, 

 disposed in a long, terminal, simple raceme ; perianth funnel- 

 shaped and incurved; stamens affixed at the throat. Autumn. 

 I. radical or on the lower part of the stem. Stem or rhizome 

 short, tuberous, erect, simple, h. 3ft. to 4ft. Mexico (cultivated 

 in American, Asiatic, and European gardens), 1629. (B. M. 

 1817; B. R. 63.) The double-flowered form is that principally 

 grown. There are several varieties, such as DOUBLE;AFRICAN, 

 DOUBLE 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 Jtmipems communis 

 cracovia. 



FOLIUM. Included under Tencrinm (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- 

 cognise 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 

 usually absent in a line near the thin partition be- 

 tween the loculi on each side ; and the wall of each 

 space is weakest where they are absent. Hence, when 

 the spaces burst from pressure exerted on the walls of 

 each in growth, the opening usually forms along the 

 lines left unstrengthened. The amount and arrangement 

 of the fibre cells vary greatly in different anthers, and 

 the modes of bursting vary in agreement with these. 



The Pollen grains are formed in the locnli as follows : 

 In each of four places in the young anther, a group of 

 cells becomes different from those lying around them 

 in the larger size of the individual cells, which form 

 others in the ordinary method by division. At last, a 



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 tho 

 walls of the parent cells, since these remain and bind 

 the Pollen grains together. The grains possess two coats 



A ^^ = -^' B 



FIG. 216. POLLEN OF EPILOBIDM ANGUSTIFOLIUM. 



A, Section of Pollen Grain e, Extine ; f, Intine ; ti, Thick Intine ; 

 /, Fovilla. B, Growing Point of Pollen Grain e, e, Extine ; 

 t, i, Intine ; /, 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 tho 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- 

 truded; 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 

 little value as an indication of affinities between plants, 



