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Pollination (continued) Ideally, each pollen grain forms a pollen tube, which grows down through the slender style (stalk of the pistil) toward one of the ovules enclosed in the basal part or ovary. Here, one of the two sperm cells within the pollen tube fuses with the egg cell of the ovule, making possible the development of an embryo, whereas the other sperm cell combines with the two polar nuclei of the ovule, thus starting the formation of endo- sperm (a tissue rich in reserve food). After this double fertiliza- tion, the ovule enlarges and transforms into a seed. Self- and cross-pollination In most plants, self-pollination is difficult or impossible. For example, in dichogamous flowers, the pistils and stamens reach maturity at different times: in protogyny, the pistils mature first; and in protandry, the stamens mature before the pistils. Selfing is also impossible in dioecious species, such as date palms and willows, where some plants bear flowers that have only pistils (pistillate or female flowers), whereas other individuals have flowers that produce only pollen (staminate or male flowers). In monoecious species, such as hazel, where pistillate and staminate flowers are found on the same plant, self-breeding is at least reduced. Heterostyly, as exemplified by certain prim- roses, is another device that promotes outbreeding (the union of gametes from organisms that are not closely related). Here, some flowers (pins) possess a long pistil and short stamens, whereas others (thrums) exhibit the reverse condition; each plant individual bears only pins or only thrums. Pollen transport from stamens to pistils of the same length is easy; however, in the other cases, it is difficult. Flower attractants As immobile organisms, plants normally need external agents for pollen transport. These agents can be insects (Fig. 2), wind, birds, mammals, or water, roughly in that order of importance. For example, in certain arum lilies, Dutchman's pipes, orchids, and other plants, the pollinators are simply trapped. In the large majority of cases, though, the flowers offer one or more rewards to pollinators, such as sugary nectar, oil, solid food bodies, perfume, sex, an opportunity to breed (as in figs), a place to sleep (Serapias orchids), or some of the pollen itself. For the attraction of pollinators, flowers provide either visual or olfactory signals. Color includes ultraviolet, which is perceived as a color by most insects and at least some hummingbird species. Fragrance is characteristic of flowers that are pollinated by bees, butterflies, and hawkmoths, whereas carrion or dung odors are produced by flowers that cater to cer- tain beetles and flies. By using a combination of olfactory and visual signals, a few orchids (including Ophrys and Cryptostylis species) mimic females of certain bees or wasps so successfully that the corresponding male insects will try to mate with them, thus achieving pollination (pseudocopulation). At close range, pollinating insects are guided to the nectar in many cases by special color patterns, lines, or dots, known as nectar or honey guides. Olfactory honey guides and osmo- phores (particular flower parts specialized for odor production) may be equally important. For better evaporation of odors, a number of flowers may also release considerable heat through a special process known as thermogenic respiration (this occurs in certain arum lilies and water lilies). Animal pollinators Although some flowers are generalists, catering to a whole array of different animals, others (for example, Ophrys) are highly specialized, being pollinated by only a single species of insect. Extreme pollinator specificity is an important factor in maintaining the purity of plant species in the field, even in those cases where hybridization can easily be achieved artificially in a greenhouse or laboratory, as in most orchids. The almost incredible mutual adaptation between pollinating animal and flower that can frequently be observed exemplifies the idea of coevolution (Fig. 2). Bird pollination Hummingbirds have good red vision and feed on the wing. Flowers pollinated by these birds are often orange and red, and they hang down freely or are exposed (Fuchsia); in addition, they are sturdy, lack a landing platform, are open in the day- time, have little or no odor, and offer a not-too-concentrated Fig. 2: Flowers of Pelargonium suburbanum are probed by the South African meganosed fly (Moegistorhynchus longirostris). Pollinaria (sacs of pollen) from an earlier visit to a nearby orchid dangle from the fly's proboscis. The proboscis length closely matches the length of the flower tubes of a guild of plants that have coevolved with this specialist pollinator. (Credit: Steven D. Johnson) + ward ' s science