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Biology 110 - Basic Concepts and Biodiversity

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Plants III - Vascular Flowering Plants

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In order for a flowering plant to reproduce sexually, the male pollen must reach the female stigma in a process called pollination. Some plants pollinate via the wind (Fig. 3); this is also the mechanism of pollination for most gymnosperms. You have seen the silk coming out of an ear of corn. These are the styles and stigmas of the female flowers. The male pollen is located at the top of the plant in the tassels, appropriately located for the wind to distribute the pollen.

Some plants are pollinated by animals (e.g., bees and hummingbirds). Their flowers are specially colored and shaped to attract a specific pollinator. Bees are attracted to flowers that are white or yellow. Read this article (http://www.thenakedscientists.com/HTML/content/interviews/interview/950/). What is different about the vision of a bee versus a human?  Compare the photographs of the Mimulus flowers shown on that web page.  What becomes evident when the flower is placed under different light?  In addition, flowers pollinated by bees tend to have sturdy petals to support the weight of a bee, and short stamens and carpels so that the pollen can transfer to and from the bee while it drinks the nectar.  Hummingbird-pollinated flowers tend to be red, and the petals usually form a long tubular-shaped corolla. The hummingbird is attracted to the red color. When the hummingbird reaches its long beak into the corolla to eat the nectar, its forehead or throat contacts the stigma and/or anther to transfer pollen.

The various relationships between flowers and their pollinators are good examples of coevolution. That is, the character of the flower is typically uniquely suited for a particular pollinator. One striking example occurs in flowers that are pollinated by flies. One flower, found in Malaysia, gives off a putrid smell of rotting meat that flies find quite appealing. This flower is not frequented by bees, nor do flies frequent the flowers that have smells that bees (and humans) prefer.

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Figure 3.  Flowers of Black Birch, a wind pollinated, monoecious plant.  The staminate flowers (left) shed pollen that is carried by the wind to carpellate flowers (right). (http://www.missouriplants.com/Catkins/Betula_nigra_page.html)

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Transcript for Plants IV - Part I

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The Life Cycle of Flowering Seed Plants

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Once inside the ovule, one sperm cell fuses with the egg and the other fuses with the large cell that has two nuclei. Recall from Tutorial 23, this is known as double fertilization. The diploid zygote forms from the fusion of the sperm and the egg. A triploid cell forms from the fusion of the other sperm and the central cell. This triploid cell will then divide by mitosis to produce endosperm, which provides nourishment for the developing embryo.  The zygote grows within the seed and eventually the seed germinates and grows into a mature, diploid, sporophytic plant.

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Figure 5.Pollination, Germination and Double Fertilization. (Click image to enlarge and play the animation)

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Transcript for Plants IV - Part II

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How can plants prevent self-fertilization?

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