Flower Structure

The flower is the reproductive structure for flowering plants (angiosperms). Flowers are extremely diverse in size, shape, color, and so on. This makes them excellent tools for distinguishing plants. As a physiologist, I can identify a few plants from just leaf and stem, but most of the time I need a flower to identify a plant.

The flower is a short branch (stem with leaves). The nodes of this branch are very close together; the internodes are typically extremely short. The leaves of this branch are of four types: sepals, petals, stamens, and carpels. The short branch is called the receptacle and the four kinds of leaves are attached to this receptacle. In most flowers there are more than two leaves of each kind on the flower, so the leaves are in a whorled arrangement (more than two leaves per node). We'll consider the flower from bottom to top:

The lowest whorl on the receptacle is called the calyx. It is composed of a few or many sepals. In some species, sepals are green and photosynthetic. In other species, they are showy and almost indistingishable from petals.

The next whorl on the receptacle is called the corolla. It is composed of a few or many petals. Please note the spelling of petal; it is not a bicycle part! The petals are typically showy and brightly colored. They serve to attract pollinators for many species. Sometimes they are extremely fragrant. They may also exude nectar (typically at the base of the petal or in a special nectar spur) to reward the pollinator. Color patterns might include nectar guides to point the way to the reward, or a "bulls eye" target among the petals might get the flying pollinator to notice the flower.

The calyx and corolla are collectively named the perianth. This literally means "around the flower" and indicates that these whorls are the sterile parts of the flower...they surround the "business" parts of the flower. A flower whose perianth is missing either of these two whorls is called incomplete. A flower with both calyx and corolla has a complete perianth.

The next whorl on the receptacle is called the androecium (literally the male household). It is composed of a few or many stamens. The stamens are specialized leaves with two distinct sections: the filament (a long stalk) and the anther (usually four sacs containing pollen grains). The function of the filament is to lift the anther to a position to effectively release pollen grains into/onto the pollinator; the filament also serves to provide the anther with xylem and phloem connections to the rest of the plant. The anther serves to produce pollen grains. The pollen grains ultimately make sperm cells; thus the idea of stamens as a male unit.

The top whorl on the receptacle, in the center of the flower, is the gynoecium (literally the female household). It is composed of a few or many carpels. Note the spelling of carpels; make no bones about it! The carpels may be fused together into a single, compound pistil. Note the spelling of pistil; it is not a handgun! Carpels consist of three parts, a swollen base called the ovary, a stalk called the style, and a tip called the stigma. The ovary contains a chamber called a locule, and inside the locule is one or more ovules. The ovules contain an embryo sac, and the embryo sac contains the egg. The carpel is thus a female unit.

Sometimes a picture is worth a thousand words.

A flower can be radially or bilaterally symmetric. A sea urchin or a starfish has radial symmetry...this asterisk * has radial symmetry. There are several planes by which you can divide the organism (or flower) into essentially equal halves. A human is nominally bilaterally symmetric; there is only one plane of symmetry. You divide a human down the forehead, the end of the nose, the chin, the navel, and between the legs to get two equal sides (bi-lateral). Of course you are probably aware that the two sides of a human are not exactly equal. You have two eyes, but they're not the same size (to contradict the Sesame Street song). Your ears may not be attached at the same level on the sides of your head. Your heart is on one side, you liver is mostly on the other side. The appendix and spleen are found on only one side. In women, the mammary glands are not the same size. In men the testes are different sizes and are suspended differently to permit bipedal locomotion (walking on two legs). Thus a human is really asymmetric (lacking a plane of symmetry). The letter T is bilaterally symmetric. The longitudinal section of a flower shown above cannot reveal enough information to decide about symmetry, even though it might hint at least one plane of symmetry.

The flower shown above has an ovary in the superior position (the other flower parts are attached below the ovary on the receptacle). Other flowers might have the ovary sunken into the receptacle so deeply that the other flower parts appear to be attached on top of the ovary; in that case the ovary is in the inferior position.

The flower parts attached below a superior ovary are called hypogynous (below the female) while flower parts attached above an inferior ovary are called epigynous (above the female). A flower with parts attaching around the sides of the ovary are called perigynous (around the female). The ovary in that case is neither superior nor inferior in position...we generally say half-superior (or half-inferior).

A flower with both male and female parts is called perfect or bisexual or hermaphroditic. Such a flower might be able to use its own egg and sperm to reproduce...this would be called a self-pollination or a self-cross. On the other hand, such flowers might produce pollen when the stigma is not receptive to pollen, thus ensuring out-crossing or cross pollination. Sometimes the stigma can recognize a pollen grain as its own and prevent it from growing in the style; this process is called self-incompatibility.

Some plants have unisexual (imperfect) flowers: staminate (male) flowers and pistillate (female) flowers. These can be on the same plant (monoecious) or on two different plants (dioecious). A begonia is an example of a plant that has unisexual flowers but is monoecious (one household). Holly is an example of a shrub that has unisexual flowers and is dioecious (two households). Thus in planting holly, you need to take precautions. First, you need to position the female holly plant where you want a shrub that will have the red fruits on it. Male shrubs never produce fruits! Second, you need to put a male holly somewhere in your landscape so that the females will be able to receive pollen to produce the fruit. You might put the fruitless male holly in a less conspicuous spot but near the females. Remember if you are at the nursery and select only shrubs with red fruits on them (a common error), those will be the last fruits you will see! You need at least one male...it "takes two to tango!"...to produce fruits the next year.

Thus the range of sexuality in plants is very broad: self-crossing hermaphrodites to out-crossing hermaphrodites, to unisexual flowers, to unisexual plants. But that is really only the beginning. Plants can change sex too. Cucumbers are famous for changing from male to bisexual to female and then to parthenocarpic as they grow. What is parthenocarpic? It means literally virgin fruit. The last flowers on some cucumbers do not need to be pollinated to produce a fruit...they make the fruit on their own!

Another interesting example is the Abelmoschus (hibiscus) in the greenhouse. The flowers last only one day and are bisexual. The flower hedges its bets. In the early morning the female parts stick out beyond the stamens to be pollinated with pollen from some other plant. If this happens, fine, but if it doesn't happen by afternoon, the styles curl backwards and push the stigmas against the stamens in the flower; it is a self-pollination. By evening the flower senesces and fruits begin to develop.

There is much more to this...later!


This page © Ross E. Koning 1994.



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