In the flowers of dicotyledonous and monocotyledonous plants, there are special glands - nectaries, which secrete a sugary liquid - nectar. Nectar as an isolation of plants attracted the attention of botanists at the end of the 16th century, and nectars began to be studied since the time of Linnaeus. In 1753 Linnaeus proposed the term nectarium to denote the "melliferous parts of plants". Greek "nectar" is a drink of the gods that gives immortality and eternal youth. This name is due to the fact that nectar is a source of honey, which really has valuable healing properties. Nectar serves as food for many insects, including bees, which process it into honey, storing it for future use. The first descriptions of melliferous plants were given by Aristotle and Theophrastus, and the honey of wild bees was known to man and was used by him in ancient times. It has long been known in Russia for buckwheat and linden honey, the names of which give an exact indication of melliferous plants, and the so-called flower honey collected from various herbs.
By extracting nectar and pollen, insects cross-pollinate the flowers. The nectar of some tropical and subtropical plants also attracts birds. More than 2,000 plant species from 50 families are known, which are pollinated by birds using different parts of the flower for food, mainly nectar. These are various types of eucalyptus, aloe, bankxia, protea, banana, mallow, sage, acacia, grevillea, bauginia, etc. Among the birds that feed on nectar, the most famous are hummingbirds and flower girls in America and sunbirds, honey suckers and white-eyed in the Old World.
The significance of nectaries is not limited only to this. Experiments with the introduction of radioactive isotopes and vital (vital) dyes have shown that nectar is used by the flower itself, since it is absorbed by parts of the flower. At the time of pollination and pollen germination, nectar labeled with C14 is found in the stigma region, and after fertilization - in the ovule region, where the embryo begins to develop at this time. In addition, the absorbed substances move to other parts of the plant (leaves, roots), to other flowers of the same plant, and even are released with their nectar. It is assumed that nectaries synthesize some hormonal substances necessary for the processes of fertilization, development of the ovary, fruit and seed. Such substances, possibly, are steroid hormones. Recently, they have been found in a number of higher plants (beans, perilla, marin) and their participation in reproductive processes has been established. Nectar also has bacteriostatic properties, which is partly responsible for the medicinal use of honey.
Fig. 20. Some types of nectaries (n): 1 - caustic buttercup (Ranunculus acer), 2 - Thunberg barberry (Berberis thunbergii), 3 - Amur vesicle (Physocarpus amurensis), the stamens are not shown on the left, 4 - meadow geranium (Geranium pratense), 5 - blue cyanosis (Polemonium coeruleum), 6 - the softest lungwort (Pulmonaria mollissima), the perianth and stamens are not shown, 7 - common weasel (Aegopodium podagraria), 8 - shadow saxifrage (Saxifraga umbrosa), 9 - hybrid violet (Viola) , 10 - night violet (Hesperis sp.), 11 - American avocado (Persea americana), a - fertile stamen, b - sterile stamen (nectary)
The main components of nectar are glucose, sucrose and fructose, the ratio of which varies from plant to plant; in addition, there are other sugars, amino acids, proteins, vitamins and other organic and inorganic substances.
The amount of nectar varies widely, from subtle traces to tens of milligrams in one flower. The level of nectar in flowers of carnation, prickly, many legumes, labiates, and Compositae rises to the top of the ovary. One prince flower accumulates up to 90 mg of nectar. A particularly large amount of nectar is formed in the flowers of representatives of the tropical flora.
The composition and concentration of nectar components, the intensity of nectar release vary depending on the plant species and environmental conditions.In different plants, the concentration of sugars reaches 30 - 60%, a very high content of sugars in the flowers of cornflower rough, black currant, clover, Veronica, some Proteaceae, Doriantes and many others.
The amount of nectar does not remain constant throughout the day. In most plants, the greatest amount of nectar is released in the morning (linden, dead nettle, oregano, thistle, vetch), in others, the maximum is in the daytime (phacelia, willow herb, loosestrife), in others - in the evening (cyanosis, rank, lungwort).
Nectars are very diverse in size and shape, in origin and location on parts of the flower. Most often, nectaries arise from epidermal and subepidermal cells, which become meristematic, divide repeatedly, forming glands of various shapes (they are called nectaries-emergence). They can develop from the epidermal and subepidermal cells of the perianth (perigonal nectaries), receptacles (thoral nectaries), on stamens (staminate, or staminal, nectaries), on the ovary. Emergent nectars form multicellular hairs, papillae, ridges, discs and tubercles. Let's consider some examples of nectaries of this type.
On the petals of buttercups, swimsuits and the catchment area, nectaries look like small depressions (the so-called honey pits), covered with a kind of tongue. In other plants, thickenings or ridges form on the petals: two oval bright yellow glands are clearly visible at the base of barberry petals. In flowers with a perianth modified into a spur, nectaries are located on a spur or covered with a spur, where it often flows down and where nectar accumulates (aconite, nigella, larkspur, nasturtium). The described nectaries are rather primitive, with a poorly developed secretory function. On sepals, nectaries are formed less frequently than on other parts of the flower. They can be seen in representatives of linden, mallow, sterculia, nasturtium. Morphologically, they are very diverse and can be hollow, flat, scaly, funnel-shaped, and not even shaped. They are characterized by a slight release of nectar.
A large number of plants have nectaries on a receptacle or flower tube. In some plants, the glandular tissue completely lines the flower tube and the nectary has the appearance of a bowl, a circle or a low thickening (eucalyptus, linden, raspberry, apple, plum, blackberry, quince, strawberry, pomegranate, buckthorn, fireweed, bauginia). In other plants, the nectary surrounds the base of the ovary with an annular ridge, continuous or intermittent. This type of glands is widely represented in species of various families (lungwort, norichnik, blueberry, lingonberry, tobacco, dope, deaf nettle, sage, grevillea, citrus, many representatives of bignoniaceae and euphorbiaceae). Maple has a flat disc-shaped nectar ring located between the petals and stamens. The term "disc" is often used to refer to nectaries of any kind and origin. A ring of glandular tissue of small protuberances between the stamens and around the ovary develops in the cistus.
The nectaries are very diverse among representatives of the cruciferous and buckwheat families: they form spherical thickenings, oblong outgrowths and papillae at the base of the filaments or around them. With such an arrangement, it is difficult to decide whether they are derivatives of the receptacle or the stamens.
The formation of nectaries may be associated with members of the androeum. In the laurel family, they are arranged symmetrically by two glands in the lower part of the filaments. In a violet, a nectary grows into a nectary, forming a long curved hook-shaped outgrowth. Glands of various shapes are formed on staminate filaments in flowers of cloves, poppies (subfamily of smoky flowers), and flax. At the base of the fused filaments, a convex nectar ring is formed in most legumes (Judas tree, cesalpinia, ceratonia).
Finally, nectaries can appear on the ovary, and there is a transition of nectaries from the base of the carpels (magnolia, marigold, gentian) to its upper part (bastard, currant, gooseberry). With the formation of the lower ovary, the secretory tissue moves to the upper part of the ovary, located around the base of the column. Such is the wide flat nectary of bellflower, many umbellate, Compositae, nectaries of female flowers of pumpkin.
The examples considered show a variety of nectaries as outgrowths of epidermal and subepidermal cells of flower parts.
A number of plants have nectaries that are not morphologically expressed in any way. In this case, the function of nectar formation and nectar secretion is performed by the epidermal and subepidermal cells themselves of certain parts of the flower. For example, nectar is secreted by the bases of staminate filaments and tea petals, the walls of the ovary of jasmine, forsythia, olive, pittosporum, magnolia, Eurya, Actaea, prince staminode, clematis.
Finally, there is a group of nectaries arising from the rudiments of underdeveloped organs - petals (willow), stamens, ovaries. Most often, they develop in unisexual flowers, where a reduced gynoecium or androecium becomes a nectary (meadow dawn, holly, male pumpkin flowers, many umbrella, laurel, boxwood, papaya).
The types of nectaries described above are characteristic of dicotyledonous plants. Histologically typical nectaries are represented by multi-layered small-cell tissue, the cells of which are rich in cytoplasm and organelles. The nectar-bearing tissue contains various pigments, due to which nectaries can be yellow, green, pink and even blue.
Many nectaries are equipped with conductive bundles through which various substances used to form nectar are supplied to them. Nectar formation is a complex process associated with the activity of the entire cell and with enzymatic transformations of incoming substances. The secretion of nectar occurs in a diffuse way or through modified stomata.
In monocotyledonous plants, nectaries, consisting of only one layer of the glandular epidermis, develop at the place of fusion of carpels, the so-called septal nectaries (lily, iris, amaryllis and other plants).
Considering the origin, morphology and topography of nectaries, it is possible to trace a certain direction of evolution from nectaries developing on numerous tepals in primitive plants, to a ring nectary located around the ovary, and, finally, to a nectary on an ovary or a column. These progressive changes in nectaries are in good agreement with general changes in flowers when plants adapt to cross-pollination (increased zygomorphism, a decrease in the number of flower members, the appearance of "honey marks", etc.).