Flowers

Flowers represent a unique refinement that plants have evolved to increase seed set. They are intimately coupled with fruit and seed production. Flowers that secrete large amounts of nectar or pollen are highly attractive to insects, and those that do not produce much nectar or pollen are rarely visited. In the floral cross section at right, note the developing ovules within the gynoecium at the base of the flower, the anthers, producing the yellow pollen at the top. Insects help to pollinate flowers by transferring pollen from one flower to another. This increases the out-breeding of plants, making them more fertile in the long run. Nectar is secreted from floral nectaries to attract the insects, or birds, or even some mammals.   The stages of flower development have been studied for several species.   The development of tobacco flowers was elequently described by Koltunow et al (1990). Floral development is divided into 12 distinct stages. A description of each of these stages is presented below.      Stage  Description  Flower  Carpel  1  Anthers and pistil fully differentiated and green  2  Calyx opens slightly at top of bud  3  Corolla emerges from calyx  4  Sepals completely separated at top of calyx  5  Corolla tube bulge just inside calyx  6  Corolla tube bulge at tip of calyx  7  Corolla tube bulge above calyx; petals closed  8  Corolla elongating; petals green and slightly open  9  Corolla tube bulge enlarging; petal tips becomming pink  10  Corolla limb beginning to open; petal tips pink  11  Corolla limb halfway open; stigma and anthers visible  12  Flower open; anthers dehisced; corolla limb fully expanded and deep pink

	Depending upon the species, some plants produce either male only or female only flowers. Squash is a good example of such a diecious plant. Squash flowers are either produce only anthers and filaments and no ovary. Alternatively, other squash flowers produce only an ovary, style and stigma and no anthers. Silene virginica, Fire Pink Stein's Virtual Herbarium
	However, most plants produce both structures. The anther, borne by the filament, produces pollen which can fertilize the ovules within the ovary. When the pollen lands on the stigma, the pollen grain germinates and extends a growing pollen tube that works its way down to the ovary, where fertilization of the ovules occurs.
	A specific tissue termed the nectary secretes the nectar into the region surrounding the ovary. The nectar is rich in carbohydrates, amino acids, vitamins, lipids, and lots of other good things. The nectar attracts insect, avian, or mammalian pollinators to feed on the rich fluid. When animals come to feed, they indavertantly transfer pollen from one flower to another, thereby increasing cross-pollination, thereby reducing the level of inbreeding among plants. Lobelia cardinalis, Cardinal Flower Stein's Virtual Herbarium
	Lobelia siphilitica, Great Lobelia Stein's Virtual Herbarium
	Related Links Identifying plants and Flowers Texas A&M University Bioinformatics Working Group Prarie Flowers Program  How Plants Grow -- Kidport Reference Library Floral Links Stein's Virtual Herbarium

Last updated 4/6/01 -- Robert Thornburg -- thorn@iastate.edu