Seeds and Seed Dispersal
For a seed plant species to be successful, the seeds must be dispersed, or scattered out in various directions. If the seeds are distributed in a variety of areas, there is a better chance that some of the seeds will find suitable conditions for growth. Furthermore, for plants to establish themselves in new areas, such as areas formed after a glacial retreat, the seeds must somehow reach that new site. To aid with seed dispersal, some plants have evolved special features to encourage movement of their seeds over long distances.
One such strategy is to allow the wind to carry the seeds. With special adaptations in the seeds, the seeds can be carried long distances by the wind. For example, you might have noticed how the “fluff” of a dandelion moves swiftly in the breeze. Each piece of fluff carries a seed to a new location. Or if you look under the scales of pine cone, you would see tiny seeds with “wings” that allow these seeds to be carried away by the wind. Maple trees also have specialized fruits with wing-like extensions that aid in seed dispersal, as shown in Figure below.
Figure 10.18
Maple trees have fruits with wings that help the wind disperse the seeds.
Another common seed dispersal strategy that some flowering plants utilize is to produce a fleshy fruit around the seeds. Animals that eat the seeds aid in the dispersal of the seeds inside. Berries, citrus fruits, cherries, apples, and a variety of other types of fruits are all adapted to be attractive to animals (Figure below). Their seeds can pass through an animal’s digestive tract unharmed and germinate after they are passed out with the feces.
Figure 10.19
Fleshy fruits aid in seed dispersal since animals eat the fruits and carry the seeds to a new location.
Some non-fleshy fruits are especially adapted for animals to carry them on their fur. You might have returned from a walk in the woods to find burrs stuck to your socks. These burrs are actually specialized fruits designed to carry seeds to a new location.
Gymnosperms
Plants with “naked” seeds, meaning they are not enclosed by a fruit, are called Gymnosperms. Instead, the seeds of Gymnosperms are usually found in cones. There are four phyla of gymnosperms:
Coniferophyta, common name conifers
Cycadophyta, common name cycads
Ginkgophyta, Ginkgo trees
Gnetophyta, common name gnetophytes
The Conifers, members of the phylum Coniferophyta, are probably the gymnosperms that are most familiar to you. The conifers include pines, firs, spruces, cedars, and the coastal redwoods in California that are tallest living vascular plants. The name of this group signifies that the plants bear their reproductive structures in cones, but this is not a characteristic unique to this phylum (Figure below). Conifer pollen cones are usually very small, while the seed cones are larger. Pollen contains gametophytes that produce the male gamete of seed plants. The pollen, which is a fine to coarse powder-like material, is carried by the wind to fertilize the seed cones (Figure below).
Figure 10.20
A red pine, which bears seeds in cones, is an example of a conifer.
Figure 10.21
The end of a pine tree branch bears the male cones that produce the pollen.
The Conifers are important to humankind since they have many uses. They are important sources of lumber and are also used to make paper. Resins, the sticky substance you might see oozing out of a wound on a pine tree, are collected from conifers to make a variety of products, such as the solvent turpentine and the rosin used by musicians and baseball players. The sticky rosin improves the pitcher’s hold on the ball or increases the friction between the bow and the strings to help create music from a violin or other stringed instrument.
The Cycads, in the phylum Cycadophyta, are also Gymnosperms. They have large, finely-divided leaves and grow as short shrubs and trees in tropical regions. Like the conifers, they produce cones, but the seed cones and pollen cones are always on separate plants (Figure below). One type of cycad, the sago palm, is a popular landscape plant. During the Age of the Dinosaurs (about 65 to 200 million years ago) the cycads were the dominant plants. So you can imagine dinosaurs grazing on cycad seeds and roaming through cycad forests.
Figure 10.22
Cycads bear their pollen and seeds in cones on separate plants.
Ginkgo trees, in the phylum Ginkgophyta, are unique because they are the only species left in the phylum, although there are many other species in the fossil record that have gone extinct (Figure below). Therefore, the Ginkgo tree is sometimes considered a “living fossil”. The Ginkgo tree survived as it was widely cultivated in China, especially around Buddhist temples. The Ginkgo tree is also a popular landscape tree today in American cities because it is very tolerant of pollution. The Ginkgo tree, like the cycads, has separate female and male plants. The male trees are usually preferred for landscaping because the seeds produced by the female plants smell rather foul as they ripen.
Figure 10.23
Ginkgo trees are gymnosperms with broad leaves.
The Gnetophytes, in the phylum Gnetophyta, are a very small and unusual group of plants. Ephedra is an important member of this group since this desert shrub produces the ephedrine used to treat asthma and other conditions. Welwitschia produces extremely long leaves and is found in the deserts of southwestern Africa (Figure below). Overall, there are about 70 different species in this very diverse phylum.
Figure 10.24
One type of gnetophyte is .
Angiosperms
Angiosperms, in the phylum Anthophyta, are the most successful phylum of plants and vastly outnumber the individuals in other phyla (Figure below). The feature that distinguishes the angiosperms is the evolution of the flower, so they are also called the flowering plants. Angiosperms inhabit a variety of environments; a water lily, an oak tree, and a barrel cactus are all angiosperms.
Figure 10.25
Angiosperms are the flowering plants.
Even though flowers may differ widely in their appearance, they do have some structures in common. The outermost structure is the sepals, collectively known as the calyx, which are usually green and protect the flower before it opens. The petals, collectively known as the corolla, are often bright and colorful to attract a particular pollinator, an animal that carries pollen from one flower to another. The next structure is the stamens, consisting of the stalk-like filament that holds up the anther, or the pollen sacs. The pollen is the male gametophyte. At the very center is the carpel, which is divided into three different regions: the sticky, knob-like stigma where the pollen lands, the slender tube of the style, and the enlarged base known as the ovary. The ovary is where the ovules, the female gametophytes, are found. When the ovules are fertilized, the ovule becomes the seed and the ovary becomes the fruit. Some flowers have all these parts and are known as complete flowers (Figure below), while others may be missing one or more of these parts and are known as incomplete flowers.
Figure 10.26
A complete flower has sepals, petals, stamens, and one or more carpels.
Flower part Definition
sepals Outermost layer of the flower that is usually leaf-like and green.
calyx The sepals collectively; outermost layer of the flower.
corolla The petals of a flower collectively.
stamens The part of the flower consisting of a filament and an anther that produces pollen.
filament Stalk that holds up the anther.
anther The pollen-containing structure in a flower.
carpel “Female” portion of the flower; consists of stigma, style, and ovary.
stigma The knob-like section of the carpel where the pollen must land for fertilization to occur.
style Slender tube that makes up part of the carpel.
ovary Enlarged part of the carpel where the ovules are contained.
(Source: Jessica Harwood, License: CC-BY-SA)
Many plants can self-pollinate, meaning that pollen falls on the stigma of th
e same flower. Cross-fertilization is often favored and occurs when the pollen from an anther is transferred to a stigma of another flower on another plant. This can be accomplished two ways, by wind or by animals. Flowers that are pollinated by animals such as birds, butterflies, or bees are often colorful and provide nectar, a sugary reward, for their animal pollinators.
Angiosperms are important to humankind in many ways, but the most significant role of angiosperms is as food. Wheat, rye, corn, and other grains are all harvested from flowering plants. Starchy foods, such as potatoes, and legumes, such as beans, are also angiosperms. And as mentioned previously, fruits are a product of angiosperms to increase seed dispersal and are also nutritious foods. There are also many non-food uses of angiosperms that are important to society; for example, cotton and other plants are used make cloth, and hardwood trees to make lumber. The flowering plants are dominant in the environment and are important resources for humans and all animals.
Lesson Summary
Seeds consist of a dormant plant embryo and stored food.
Seeds can be dispersed by wind or by animals that eat fleshy fruits.
Gymnosperms, seed plants without flowers, include the Conifers, the Cycads, the Gingko tree, and the Gnetophytes.
Angiosperms are flowering plants.
Seed plants provide many foods and products for humans.
Review Questions
Why are seeds an adaptive feature for seed plants?
What is the purpose of a plant developing a fruit?
What are two ways that plants disperse their seeds?
How do Gymnosperms and Angiosperms differ?
What are some examples of Gymnosperms?
What are some uses that seed plants have for humans?
Firs, spruces, and pines belong to what group of Gymnosperms?
Why is the Ginkgo tree considered a “living fossil”?
Where is the pollen stored in a flower?
How are plants pollinated?
Further Reading / Supplemental Links
http://home.manhattan.edu/~frances.cardillo/plants/intro/plantmen.html
http://www.ucmp.berkeley.edu/seedplants/seedplants.html
http://hcs.osu.edu/hcs300/gymno.htm
http://biology.clc.uc.edu/Courses/bio106/gymnospr.htm
http://www.biologie.uni-hamburg.de/b-online/e02/02d.htm
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookflowers.html
http://en.wikipedia.org/wiki
Vocabulary
angiosperms
Another name for flowering plants.
anther
The pollen-containing structure in a flower.
calyx
The sepals collectively; outermost layer of the flower.
carpel
“Female” portion of the flower; consists of stigma, style, and ovary.
complete flowers
Flowers that contain all four structures: sepals, petals, stamens, and one or more carpels.
conifers
Group of gymnosperms that bear cones; includes spruces, pine, and fir trees.
corolla
The petals of a flower collectively are known as the corolla.
dormant
Halting growth and development temporarily.
ginkgo
Tree known as the “living fossil” because it is the only species left in the phylum Ginkgophyta.
gnetophytes
Diverse group of gymnosperms that includes Ephedra and Welwitschia.
gymnosperms
Seed plants in which the seeds are not encased in a fruit.
incomplete flowers
Flowers that are missing one or more structures: sepals, petals, stamens, or carpels.
ovary
Enlarged part of the carpel where the ovules are contained.
sepals
Outermost layer of the flower that is usually leaf-like and green.
stamens
The part of the flower consisting of a filament and an anther that produces pollen.
stigma
The knoblike section of the carpel where the pollen must land for fertilization to occur.
Points to Consider
Do you think plants can sense their environment? Why or why not?
Can you think of an example of a hormone?
Do you think that plants have hormones?
How do you think trees know when it’s time to lose their leaves?
Lesson 10.4: Plant Responses
Lesson Objectives
List the major types of plant hormones and the main functions of each.
Define tropism and explain examples of tropisms.
Explain how plants detect the change of seasons.
Check Your Understanding
Why do pants need sunlight?
Introduction
Plants may not move, but that does not mean they don't respond to their environment. Plants are constantly responding to their surroundings. Plants detect and respond to stimuli such as gravity, light, touch, and seasonal changes. For example, you might have noticed how a house plant bends towards a bright window. Plants must be able to detect and respond to changes in the direction of light. You probably also have noticed that some trees lose their leaves in the autumn, so plants must be able to detect the time of year. Plants are able to respond to stimuli through the help of special chemical messengers, called hormones. The various ways that plants respond to their environment and the hormones that control these responses will be the focus of this section.
Plant Hormones
In order for plants to respond to the environment, their cells must be able to communicate with other cells. The chemical signals that travel through cells to help them communicate are called hormones. You might be familiar with the term hormones since animals, including humans, also depend on hormones, such as testosterone or estrogen, to carry messages from cell to cell. Animal hormones will be discussed in the Controlling the Body chapter. In both plants and animals, hormones travel from cell to cell in response to a stimulus and also activate a specific response.
Each plant hormone has a specific function. Hormone Function
Ethylene Fruit ripening and abscission
Gibberellins Break the dormancy of seeds and buds; promote growth
Cytokinins Promote cell division; prevent senescence
Abscisic Acid Close the stomata ; maintain dormancy
Auxins Involved in tropisms and apical dominance
(Source: Jessica Harwood, License: CC-BY-SA)
Ethylene is the plant hormone involved in ripening fruit and with abscission, the dropping of leaves, fruits and flowers. When a flower is done blooming or a fruit is ripe and ready to be eaten, ethylene stimulates the production of enzymes that allow the petals or fruit to separate from the plant (Figures below and below). Ethylene is an unusual plant hormone because it is a gas. That means it can move through the air, and a ripening apple can cause another to ripen, or even over-ripen. That’s why one rotten apple spoils the whole barrel!
Figure 10.27
The hormone ethylene is signaling these tomatoes to ripen.
Figure 10.28
The hormone ethylene plays a role in signaling these flower petals to separate and drop, a process known as abscission.
Gibberellins are growth-promoting hormones. When gibberellins are applied artificially to plants, the stems grow longer. Therefore, gibberellins can be used in horticulture to increase the growth of ornamental plants, whereas dwarf plants have low concentrations of gibberellins (Figure below). Another function of gibberellins is to break the dormancy of seeds and buds. Gibberellins signal that it’s time for a seed to germinate or for a bud to open.
Figure 10.29
Dwarf plants like this bonsai tree often have unusually low concentrations of gibberellins.
Cytokinins are hormones that promote cell division. Cytokinins were discovered from attempts to grow plant tissue in artificial media (Figure below). Cytokinins
also prevent senescence, the programmed aging process. As a result, florists sometimes apply cytokinins to cut flowers.
Figure 10.30
Cytokinins promote cell division and are necessary for growing plants in tissue culture; a small piece of a plant is placed in sterile conditions to regenerate a new plant.
Abscisic Acid is misnamed because it was once believed to play a role in abscission (the dropping of leaves, fruits and flowers), but we now know abscission is regulated by ethylene. The actual role of abscisic acid is to close the stomata and maintain dormancy. When a plant is stressed due to lack of water, abscisic acid signals the stomata to close. This prevents excess water loss through the stomata. When conditions are not ideal for a seed to germinate or for a winter bud to put out its leaves, abscisic acid signals for dormancy to continue. When conditions improve, the levels of abscisic acid drop and the levels of gibberellins increase, signaling that is time to break dormancy (Figure below).
CK-12 Life Science Page 24
