Nectary
A term that has been used in a somewhat vague way either (1) to denote any appendage in a flower that does not belong to one of the series of parts recognised by botanists as forming ordinary flowers (e.g., the glands in front of the petals in Parnassia palustris), without reference to their function and use to the plant, or (2) to indicate that the part so named produces a sweet fluid (see Nectar), which is retained in the cells on its surface, or soaks out through the walls of the cells or through the stomata (see Stoma), which many of the Nectaries possess in their surface layer of cells. The word is now almost restricted to this latter use, and is employed to denote simply the nectariferous character of the part, whatever may be its structure, and whatever the nature of the part of which it is a modification. The relation of the sugary secretions to growth is discussed below, and from this relation it must evidently be present frequently in growing tissues. Flowers present conditions that render rapid growth a necessity at certain times, inasmuch as, just before opening, the various parts of the flower increase much in size, and, after fertilisation, the fruits and seeds usually take on rapid growth. Hence, Nectar may be expected to be largely present in flowers. The many careful observations that have been made of late years by botanists in various countries have shown, among other interesting facts connected with the existence of Nectar in plants, that sugar is present in considerable amount in the tissues of many flowers in which there is no trace of Nectar on any part of the surface; and, also, that Nectaries are frequently present on vegetative organs of plants, e.g., on stipules of Beans, on the small glandular swellings on the leaf-stalks of species of Prunus, and of the Castor-oil plant, and on the secondary leafstalks of various ferns. Since, then, there is a tendency to produce sugar in the flowers, and since the Nectar containing the sugar tends, like other fluids, to soak through the cell-walls, and to appear on the outer surface of the part in which it exists, we can perceive that insects would probably be induced to visit the flowers to collect the Nectar, just as bees visit the stipules of Beans for this purpose. But, in visiting the flowers, the insects are apt to transfer pollen from the anthers of one flower to the stigma of the next of the same kind visited by them, and they thus aid in securing cross-fertilisation; a result which experiments show to be productive, in many plants, of more numerous seeds, and healthier and stronger seedlings, than follow self-fertilisation. It is thus an advantage to such plants to have frequent insect-visitors at their flowers. In many unisexual flowers, the seeds could not be fertilised in the absence of insects, since they produce pollen unsuited for conveyance by the wind to the stigma of the male flowers. Thus, the presence of Nectar is advantageous to both flowers and insects; and there seems good reason to believe that the habit, in insects, of visiting flowers for Nectar, has brought about, in a great degree, the vast diversities of structure and form in both flowers and flower-frequenting insects. It is necessary for the well-being of the plant that the Nectar shall be so placed as to insure that any insect able to reach it shall transfer pollen from the ripe anthers of one flower to the ripe stigma of a flower (usually older) subsequently visited. Hence arise most of the irregularities and peculiarities in the form and in the position of the various parts of flowers; some to insure the access of the suitable insects by the right path, and others to prevent the access of visitors that would remove the Nectar without effecting pollination in repayment of the benefit. The position of the Nectary or Nectaries in flowers, and the organs of which they are modifications, differ with the kinds of insects for which they are suited: some lie almost on the surface of the flower, e.g., in Carrot, Elder, Ivy, &c.; but most are situated in the deeper recesses of the flowers. The position of Nectaries is also affected by the fact that exposure to water, in the form of rain or dew, injures the Nectar, and renders it unsuited for attracting insect visitors. Hence, the Nectaries have to be protected against this danger also.
From the fact that sugar is present in all growing structures of flowers, and that it is most abundant in the receptacle, in the neighbourhood of the ovary, we should expect to find the Nectaries very generally developed in this region; and such is very often the case. The chief structures that may be modified to form Nectaries are the following: The receptacle often produces Nectar, either over the whole surface (where not occupied by parts of the flower), as in Marsh Marigold (Caltha palustris), or on special outgrowths, forming what is sometimes called the disk; and this latter may form a complete ring between any two successive series of parts, e.g., in the Maples; or may be broken into portions surrounding the bases of particular organs, e.g., in cruciferous plants, round the bases of the short stamens. Any of the organs of the flower may be modified to form Nectaries in different plants. In Poplars, the stigma acts as a Nectary; in Umbelliferae, and in many other plants, the Nectary is closely adherent to the base of the style; in many Solanaceae, it is at the base of the carpels. The stamens may abort, and may be changed into Nectaries, e.g., in Scrophularia, &c., or they may bear nectariferous spurs, as in Viola (see below), or outgrowths from the filament, or from some point of the connective. On the petals and sepals they often appear, usually as small pits on the inner surface, e.g., in Fritillaria, either uncovered, or, as in some species of Buttercups, covered with a small flat scale, behind which the Nectar is formed. In a good many plants, the petals (and less often the sepals) are tubular or spurred, as in Columbine, Hellebore, Aconite, &c., and the inner end of the organ is the Nectary; but in some (e.g., Violet) the spur merely serves to receive the Nectar. In Viola, one of the petals is thus extended backwards, and curious appendages (n c, B, Fig. 662) on two anthers pass into the cavity provided, and there secrete a sweetish fluid. Perhaps no flower presents equal advantages with this to the microscopic tyro who would study Nectar cells; for not only are these large (n c, A, Fig. 662), but they lie on the outside of the process (their protection being derived from the covering afforded by the spur-like petal previously mentioned), and, consequently, the difficulties of section cutting are, in their case, altogether avoided. Much has been written upon the nature of Nectaries in the leading European languages; but even the enumeration of the principal works would exceed our space, and we shall content ourselves with naming the following books written in English or translated from German, which are replete with information on the mutual actions of the plants and insects: H. Muller's "Fertilisation of Flowers by Insects" (Clarendon Press, Oxford); Lubbock's "British Wild-flowers in their Relation to Insects"; Kerner's "Flowers and their Unbidden Guests"; Darwin's "Cross and Self Fertilisation of Plants," and various papers by Rev. G. Henslow and others in the publications of the Linnean Society, in the "Popular Science Review," and elsewhere. In regard to the microscopic structure of the Nectary, the nectar-producing tissue is usually made up of small thin-walled cells that contain abundant protoplasm, a nucleus, and cell-sap, rich in sugar. As a rule, the Nectary shows a number of pores or stomata in the surface layer of cells, and through these the Nectar is poured on to the surface of the organ, whence it is sucked up by the visitors to the flowers. There is usually only a thin cuticle, or it is even absent practically in some plants, over Nectaries; and frequently, the Nectar soaks out through the thin walls of the cells to the surface; but it may be retained inside the surface layer, in cells so thin walled as to be easily pierced by the proboscis of the insects suited to convey pollen to the stigma.
The Nectary has also been microscopically studied by Mr. Cheshire, some of whose results and illustrations (engraved from his own drawings on the wood), as given in "Bees and Bee-Keeping, Scientific and Practical," are here, by permission, introduced. Taking a recently-expanded blossom of the common scarlet Pelargonium of gardens, which is selected because it is at command, in most places, and at every season of the year, we find, running down the flower stalk, and immediately under the uppermost and broadest sepal, an enlargement of the stalk itself, marked off by inconspicuous grooves, and terminating in a small bulbous expansion a little below the line b, Fig. 663, and which is often purplish in colour. This is the Nectary; and, if we now remove the petals, and look at the calyx from the front, we shall see into its opening (n, B). Making cross sections through the lines a and b, we find the Nectary wider above, as at D, and narrow below, as at E. A keen razor, dipped in methylated spirit, will take off slices sufficiently thin for microscopic examination under a cover glass in water. Cutting D longitudinally, so that the Nectary is divided, and then removing a thin slice from that which forms the upper part of the figure, and magnifying about 200 diameters, we find the outside to consist of cuticular cells, carrying glandular hairs (gh, Fig. 664), which secrete a resinous body of strong odour. The cells on the opposite side of the section are not unlike those of the external cuticle, although they constitute the lining of the upper part of the Nectary, for they have here no secretory function. Taking a section from the face of E, which lies in the line b (A, Fig. 663), we discover the hairs and cuticle to be of precisely the same character as those previously noticed; but the lining cells (nc) of this part of the Nectary are totally different, extending inwards by almost pointed prominences. The structure of the pointed cells is quite special, their contents, as seen under high magnifying power, being granular, especially near the cell-wall, which, at the prominence, is excessively thin, and has, lying immediately within it, a globular mass of highly refractive protoplasm (n, Fig. 666), containing a distinct nucleus. This is the active agent in accomplishing the secretive act, and the surface of the cells here, in healthy plants, and in proper conditions of the atmosphere, will always be found to be coated with a layer of Nectar.
To understand the presence of Nectar in a plant, and the uses to which it is put, a short explanation is necessary in regard to the use of sugar to plants. There is reason to believe that sugar is one form in which part of the food, formed by plants for themselves from that taken in by them, is retained for a short time in a state that is readily available for use in forming new cell-walls in growing organs, or other substances of the same general composition. Wherever growth is active, sugar is present in the tissues, and gives its characteristic results when tested for. Hence, sugar is present, one may say, in the tissues of all flowers, whatever the mode of their fertilisation; but in some, the sugar is retained in the tissues, while in others it is contained in the surface cells, or oozes out on the surface of certain parts called Nectaries, which serve as the attraction to insects. Besides floral Nectaries, or those in the flowers, there are also in some plants (e.g., in some Ferns and in the common Bean) extra floral Nectaries. In the Bean, they are on the stipules, and form a great attraction to bees in the search for honey. In the Bracken Fern (Pteris aquilina), the Nectar flows from small, pale swellings at the bases of the secondary petioles. It has been found that emission of water vapour into the atmosphere, and emission of Nectar on the surface of the Nectary, are so related that what favours the one, retards the other. In the flowers, it is usually emitted most abundantly in the early morning, diminishes till afternoon, and again increases towards evening. It is generally found to be more abundant in flowers of the same kind, the colder the climate.
The position of the Nectary (n, B, Fig. 663) now demands attention. It lies above the anthers and stigmas, and an insect, in seeking sweets, would insert the tongue with the body in such a position that its hairs would dust off the pollen, or else rub against the stigmatic faces. It may be observed, in a Pelargonium truss, that recently-opened blossoms have their anthers already shedding their pollen, while the stigmatic faces are held firmly in mutual contact, so that fertilisation is impossible; but that older blossoms, from which the pollen has all, or nearly all, disappeared, have their stigmatic surfaces exposed, since they have separated and curled back upon the top of the style, as at s, C, Fig. 663--clearly pointing to an effort to secure cross-fertilisation. If an insect visits a young flower with stigmas not yet receptive, it nevertheless secures pollen on its breast, which it transfers to the stigmas of older flowers, when seeking their Nectar. The enormous importance of insects' visits has not, until recently, been realised. See Hybridising To mention only a few instances, our orchard and fruit crops, and leguminous seeds, forming together no inconsiderable fraction of human food, are very largely dependent upon insect agency, and the fee paid for professional attendance on the part of the insect inoculator, is Nectar. Let us take, as an illustration, a common Raspberry. The nectar glands have their tiny openings (C, Fig. 667) set in a ring just within the very numerous anther filaments. The stigmas (S) of the various drupels (D) need the pollen to be passed from the anthers (A) to the surfaces of the former, but the interval between the two is considerable. A bee settles, and, in applying her tongue consecutively to the circularly-disposed sources of supply, makes a revolution. The side of the body is thus dusted with pollen; but this is not transferred to the stigmas. Flitting to a neighbouring blossom, she generally revolves the body in the opposite direction, so as to rest the legs previously most exercised, and so transfers the pollen before gathered to the waiting stigmas, thus securing cross-fertilisation. If the stigmas be not in this way pollinated, the drupels do not develop, and we get, on part of the Raspberry, shrunken greenish-grey abortions, of which two are seen in the section. These failures are common late in the season, in consequence of imperfect insect action.