Flowers are the reproductive organs of angiosperm plants. A flower is a sophisticated pollination machine. It employs bright colors to attract the attention of insects (or birds or small mammals), nectar to induce the insect to enter the flower, and structures that coat the insect with pollen grains while it is visiting. Then, when the insect visits another flower, it carries the pollen with it into the flower.
The basic structure of a flower consists of four concentric circles, or whorls, connected to a base called the receptacle:
If you were to watch insects visiting flowers, you would quickly discover that the visits are not random. Instead, certain insects are attracted by particular flowers. Insects recognize a particular color pattern an odor and search for flowers that look similar. Insects and plants have coevolved so that certain insects specialize in visiting particular kinds of flowers. As a result, a particular insect carries pollen from one individual flower to another of the same species. It is this keying to particular species that makes insects pollination so effective.
Of all insect pollinators, the most numerous are bees. Bees involved soon after flowering plants, some 125 million years ago. Today there are over twenty thousand species. Bees locate sources of nectar largely by odor at first (That is why flowers smell sweet) and then focus in on the flower’s color and shape. Bee-pollinated flowers are usually yellow or blue. They frequently have guiding stripes or lines of dots to indicate the position in the flower in the flower of the nectar, usually in the throat of the flower, but these markings may not always be visible to the human eye. Yellow flowers look very different through ultraviolet filter. The UV rays show a dark area in the middle of the flower, where the nectar is located. Why have hidden signals? Because they are not hidden from bees that can detect UV rays. While inside the flower, the bee becomes coated with pollen. The bee leaves this flower and visits another, it takes the pollen along for the ride, pollinating a neighboring flower.
Many other insects pollinate flowers, butterflies tend to visit flowers of plants like phlox that have ”Landing platforms” on which they can perch. These flowers typically have long, slender floral tubes filled with nectar that a butterfly can reach by uncoiling its long proboscis (a hose-like tube extending out from the mouth). Moths, which visit flowers at night are attracted to white or very pale-colored flowers. Often heavily scented, that are easy to locate in dim light. Flowers pollinated by flies, such as members of the milkweed family, are usually brownish in color and foul smelling.
Red Flowers, interestingly, are not typically visited by insects, most of which cannot ”see” red as a distinct color. Who pollinates these flowers? Hummingbirds and sunbirds! To these birds, red is a very conspicuous color, just as it is to us. Birds do not have a well-developed sense of smell, and do not orient to odor, which is why red flowers often do not have a strong smell.
Some angiosperms have reverted to the wind pollination practiced by their ancestors, notably oaks, birches, and, most important, the grasses. The flowers of these plants are small, greenish, and odorless. Other angiosperm species are aquatic and while some have developed specialized pollination systems where the pollen is transported underwater or floats from one plant to another, most aquatic angiosperms are either wind pollinated or inset pollinated like their terrestrial ancestors. Their flowers extend up above the surface of the water.
The seeds of gymnosperms often contain food to nourish the developing plant in the critical time immediately after germination, but the seeds of angiosperms have greatly improved on this aspect of seed function. Angiosperms have greatly improved on this aspect of seed function. Angiosperms produce a special, highly nutritious tissue called endosperms within their seeds, here is how it happens. The angiosperm life cycle is presented but there are actually two parts to the cycle., a male and a female part, indicated by the two sets of arrows at the top of the cycle. We will begin with the flower of the sporophyte on the left side of the cycle. The development of the male gametophyte (the pollen grain) occurs in the anthers and is indicated in the upper set of arrows.
The anthers are shown in cross section. The microspores mother cells that develop into pollen grains. The pollen grain contains two haploid sperm. Upon adhering to the stigma at the top of the carpel (the female organ in where the egg cell is produced), the pollen begins to form a pollen tube. The yellow pollen tube grows down into the carpel until it reaches the ovule in the ovary. The two sperm (small purplish cells) travel down the pollen tube and into the ovary. The first sperm fuses with the egg (the green cell at the base of the ovary). As in all sexually reproducing organisms, forming the zygote that develops into the embryo. The other sperms cell fuses with two other products of meiosis, called polar nuclei, to form a triploid (three copies of the chromosomes) endosperm cell. This cell divides much more rapidly than the zygote, giving rise to the nutritive endosperm tissue within the seed (the tan material surrounding the embryo). This process of fertilization with two sperm to produce both a zygote and endosperm is called double fertilization. Double fertilization forming endosperm is exclusive to angiosperms.
In some angiosperms, such as the common pea or bean, the endosperm is fully used up by the time the seed is mature. Food reserves are stored by the embryo in swollen, fleshly leaves called cotyledons, or seed leaves. In other angiosperms, such as corn, the mature seed contains abundant endosperm, which is used after germination. It also contains a cotyledon, but its seed leaf is used to protect the plant during germination and not as food source. Some angiosperm embryos have two cotyledons, and are called dicotyledons, or dicots. The first angiosperms were like this. Dicots typically have leaves with netlike branching of veins and flowers with four to five parts per whorl. Oak and maple trees are dicots, as are many shrubs. The embryos of other angiosperms, which evolved somewhat later, have a single cotyledon and are called monocotyledons or monocots. Monocots typically have leaves with parallel veins and flowers with three parts per whorl. Grasses, one of the most abundant of all plants, are wind pollinated monocots. There are also differences in the organization of vascular tissue in the stems of monocots and dicots, which will be compared in more detail later.
In angiosperms, are in gymnosperms, the sporophyte is the dominant generation. Eggs for within the embryo sac, inside the ovules, which, in turn, are enclosed in the carpels. The carpel is differentiated in most angiosperms into a slender portion, on style, ending in a stigma, the surface on which the pollen grains germinate. The pollen grains, meanwhile, are formed within the anthers. And complete their differentiation to the mature, three-celled stage either before or after grains are shed. Fertilization is distinctive in angiosperms, being a double process. A sperm and an egg came together, producing a zygote, at the same time, another sperm fuses with the two polar nuclei, producing the primary endosperm nucleus, which is triploid. The zygote and the primary endosperm nucleus divide mitotically, giving rise, respectively, to the embryo and the endosperm. The endosperm is the tissue, almost exclusive to angiosperms, that nourishes the embryo and young plant.
Just as a mature ovule becomes a seed, so a mature ovary that surrounds the ovule becomes all or a part of the fruit. This is why fruit forms in angiosperms and not in gymnosperms. Both types of plants have tissue that surrounds the egg, called the ovule. The ovule becomes the seed In both. But in the sperm life cycle, you will notice that the ovule is surrounded by another layer of tissue, the ovary, which develops into the fruit. A fruit is a mature ripened ovary containing fertilized seeds. Fruits provide angiosperms with a second way of dispersing their progeny than simply sending their deeds off on the wind. Instead, just as in pollination, they employ animals. By Making fruits fleshy and tasty to animals, like the berries. Angiosperms encourage animals to eat them.
The seeds within the fruit are resistant to chewing and digestion. They pass out of the animal with the faces, undamaged and ready to germinate at a new location far from the parent plant. Although many fruits are dispersed by animals, some fruits are dispersed by water, like the coconut, and many plant fruits are specialized for wind dispersal. The small, non-fleshy fruits of the dandelion, for example, have a plume-like structure that allows them to be carried long distances on wind currents. The fruits of many grasses are small particles, so light wind bears them easily. Maples have long wings attached to the fruit, that allows them to be carried by the wind before reaching the ground. In tumbleweeds, the whole plant breaks off and is blown across open country by the wind, scattering seeds as it moves. Fruits will be discussed in more detail later.