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Chapter 31: 9-15

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Plant Growth and Reproduction

Web Lecture

Reproductive Structures

Flower Parts

A typical flower has three kinds of parts. Notice that the names of the parts can vary, depending on the type of discussion (gardeners versus biologists!). Some flowers have a single carpel, as shown in the diagram; others may have multiple carpels within the same petal structure.

Flower Variations

Flower shapes can vary greatly, depending on the climate and the methods of pollination (insect, bird, wind). Solitary flowers grow singly at the end of a stem, such as daisies, roses, and dandelions. Others grow in clusters that can form different shapes, from umbel (umbrella-like) to racimes to spikes, like those at the end of wheat stalks. Some flower variations can make it difficult to identify which structures are stamen or pistils, or even which are flowers. Note the lily below: it's central stalk is a collection of multiple flowers.

The variation in flower structures allow the angiosperm to function optimally in a given location, where amount of sunlight, availability of water, growing season, temperature, and available pollinators all influence likelihood of survival. Some flowers are specialized to attract specific species of bees or birds, through a process of co-evolution, where the flower species and the pollinator species become codependent.

Seeds and Propagation Methods

Seed formation

When pollen is transfered from the anthers to the stigma by wind, rain, insects, or birds, a new tubular structure develops inside the style. Pollen grains move down through the pollen tube to the ovary which contains two polar nuclei and an egg nucleus, the product of the four gametes from meiosis.

When the pollen tube reaches the embryo sac in the ovule, it release two sperm into the sac. One of the sperm fertilizes the two polar nuclei, u then develop into a triploid endosperm that will protect and nourish the developing embryo. The second haploid sperm fertilizes the haploid egg nucleus, which now forms a diploid zygote that begins to divide and grow into an embryonic plant, complete with leaves, stem, and root segments. Protected by the seed coat and nourished by the endosperm, a dormant seed can remain in a suspended state for years to decades, restarting growth only when temperature, soil, and water conditions are right.

Seed Dispersal

Once seeds have formed, the flower structure withers and decays, leaving the fertilized seed behind. It may simply fall to the ground, or it may be shaped so that it can be spread by wind. Many seeds have burrs that allow it to stick to the fur of passing animals; others form nuts or fruits which will be eaten by animals and deposited as part of animal waste products far from the parent plant.

Fruits have many different forms, depending on how the seed is embedded, and whether one or more seeds are embedded. These include:

Propagating Plants without Seeds

Because plant differentiation can revert so that plants can generate new leaves, stems, or roots from the same tissues, plants can propagate without seeds. Some plants produces specific structures, such as bulbs, corms, tubers, and rhizomes, which, like the endosperm, contain nutrients to support an individual plant once the structure has soil and water nutrients. Most plants, whether seed-producing or not, can be split into segments (divison); if enough of the plant is present in each, it will generate missing structures. A stem will generage new roots and leaves, for example.

Methods of seedless propagation are all asexual, and the resulting new plant is a genetic clone of its parent.

Because of the variety of forms, seed production, seed dispersal, and even seedless propagation, angiosperms have become the dominant plant. We say they are species-rich, because there are more angiosperm species than mosses, ferns, or gymnosperms. We consider them more successful, because they are found in more habitats than other plant types, and are able to survive in a wider range of climate and resource conditions.

Because they store nutrients in nuts, fruits, and seeds, angiosperms are also the main source of plant nutrition for herbivores and omnivores. The flowers and seeds of grasses such as wheat, barley, oats, corn, and rice form the staple foods of most people around the world. The seeds of legumes such as peas and beans provide proteins and essential nutrients animals are not able to make for themselves. Supporting angiosperm success and diversity is a major priority to ensure adequate food supplies.