Introduction and Learning Objectives
This tutorial will examine the second group of seed plants, the flowering seed plants (sometimes referred to as angiosperms). In particular, we will look at their life cycle and how it relates to their flowers and seeds. By the end of this tutorial you should have a basic working understanding of:
- The anatomy of a flower
- The life cycle of flowering seed plants
- The anatomy of a seed
- The role of pollination and seed dispersal in the angiosperm life cycle
- Identify the characteristics of flowering plants
- Describe the structure of a flower and the difference between perfect and imperfect flowers
- Summarize the life cycle of a flowering plant, identifying the sporophyte, the gametophytes, and when mitosis/meiosis/fertilization occur
- Diagram the process of double fertilization
- Explain the importance of pollen and the different types of pollination seen in this group
- Describe the role of the seed, its relationship to fruit, the different types of seeds, and seed dispersal mechanisms
Flowering Seed Plants
There are many more species of flowering seed plants than nonflowering seed plants; more than 250,000 species of angiosperms have been described. A flower consists of four main parts: sepals, petals, stamens, and carpels (Fig. 1).
Figure 1. Basic anatomy of a flower. (Click image to enlarge)
Sepals are the outermost, leaf-like parts of the flower. They are usually green but may be pigmented the same color as the petals. Petals are the next whorl of parts in from the sepals, and they are usually the structure that we think of when we talk about flowers. Petals are often brightly colored in plants that use pollinators to transfer pollen from one plant to another. However, plants can reproduce without petals or sepals because neither is a gamete-producing structure. The stamen is the male structure of a flower. It is made up of a slender filament and the pollen-producing anther carried on the filament. The carpel is the female structure. It is located in the center of the flower and consists of a stigma, a style, and an ovary. The term pistil describes the entire female reproductive structure and is composed of one or more carpels (in Figure 1, the pistil is composed of one carpel). A pollen grain lands on the stigma (which often has a sticky surface), where it germinates, then the pollen tube grows through the style to an ovule, which is contained within the ovary. The ovule produces the egg cell. This is in contrast to the female cone of a nonflowering seed plant. Recall, in the nonflowering seed plant the pollen grain enters the micropyle and the pollen tube grows through the ovule tissue, but the ovule is not contained within an ovary.
Types of Flowers
All flowers have stamens and/or carpels. In addition to these basic parts, a flower may have petals and/or sepals, or these structures can be absent, or fused. If a flower is bisexual, with both male and female reproductive parts (stamens and carpels), it is called a perfect flower (a plant that has perfect flowers can be called synoecious). If a flower is unisexual, having only one of these reproductive parts, either male or female, it is an imperfect flower. Imperfect flowers are further classified as staminate or carpellate (also pistillate), depending on the reproductive part that is present (Fig. 2).
Some plants (e.g., corn) have staminate flowers on one part of the plant (the tassels at the top) and carpellate flowers on another part (the ears on the stalk). Recall from tutorial 14, these are called monoecious species. The word monoecious comes from the Greek words meaning "one house." When the staminate and carpellate flowers are found on separate plants, they are a dioecious species ("two houses"). Kiwi plants are an example of a dioecious species. Kiwi growers need to grow many female plants for fruit and some male plants for pollination
Figure 2. Staminate flowers (left) and carpellate flowers (right) of Red Maple (Acer rubrum) a dioecioius flowering plant. (http://www.missouriplants.com/Redopp/Acer_rubrum_page.html).
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.
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)
The Life Cycle of Flowering Seed Plants
Like all plants, flowering seed plants exhibit alternation of generations. We will look at some of the important features in this life cycle (Fig. 4). The anthers are made up of diploid (2n) cells that undergo meiosis to form haploid (1n) cells (microspores). A microspore will undergo a series of changes (including the acquisition of a tough cell wall), leading to the formation of a pollen grain, the male gametophyte that contains two nuclei.
Meanwhile, in the female ovary each diploid ovule undergoes meiosis to produce a megaspore that is haploid. This megaspore divides by mitosis, three times, to produce eight haploid nuclei in seven cells. One of these cells is the egg, and one cell is larger than the others and contains two nuclei. Together, the seven cells constitute the embryo sac, which is the female gametophyte. This gametophyte is totally enclosed within the ovary of the parent plant.
Figure 4. Life cycle of a flowering plant. (Click image to enlarge)
At this point, the egg is ready to be fertilized and the pollen grain is ready for dispersal. Pollen grains need their tough outer wall to survive the process of pollination (the transfer of pollen to the female flower parts). Once the pollen grain lands on the stigma, it germinates. One nucleus begins to grow a pollen tube that extends down into the style. The other nucleus then divides by mitosis to form two haploid sperm cells. It is at this point (with two sperm cells and one pollen tube) that the male gametophyte is mature and contains three haploid cells. If successful, the pollen tube will grow into an ovule and discharge its sperm cells. There are mechanisms to prevent more than one pollen tube from entering a given ovule.
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.
Figure 5.Pollination, Germination and Double Fertilization. (Click image to enlarge and play the animation)
How can plants prevent self-fertilization?
Unlike most animals, many species of plants are self-fertile, which means that pollen from a plant can fertilize ovules of the same plant. While this can be very beneficial if a plant germinates in a place away from other members of the same species, so it has no other plant to exchange pollen with, the result is extreme inbreeding which can have harmful results. Dioecious plants must cross-pollinate. However, monoecious and synecious plants have the potential to self-pollinate.
Some plants have physical mechanisms that prevent or reduce the chance of self-pollination. For instance, the pollen may mature before the stigma is receptive (sunflowers), or the stigma may be receptive before the pollen matures (magnolias). In others, the placement of the stigmas relative to the anthers makes self-pollination difficult. For example, in this hibiscus (Fig. 6), the stigmas are positioned above the anthers so that pollen cannot drop onto them. What type of animal do you think pollinates this plant? How could the pollinator help to ensure cross-pollination?
Figure 6. A perfect flower on a hibiscus plant. (http://www.ck12.org/concept/Angiosperm-Life-Cycle/)
Some plants are unable to self-pollinate because they are genetically self-incompatible. This system was first discovered by Dr. The-hui Kao and his research team at Penn State.
In this system, if the pollen that lands on a stigma shares an allele at the S locus with the stigma, the pollen tube cannot grow to reach the ovule. However, if the pollen has a different allele than the stigma, then the pollen tube will be able to grow and fertilization can take place (Figure 7). This requires the interaction of a molecule produced in the carpal and a similar molecule produced in the pollen. Dr. Kao’s group discovered that the two genes producing these interacting molecules are located very close together on the same chromosome. Why is this important (hint: what could be the result of recombination if these genes were farther apart?).
Figure 7. The role of the S locus in flowering plant self-incompatability. (http://science.psu.edu/news-and-events/2004-news/Kao5-2004.htm)
If pollination is successful and fertilization takes place, a zygote is formed. It will then develop into an embryo that is contained within a seed. A seed is a complex structure. It contains diploid and triploid components. In addition, the outer layers of the seed are derived from maternal ovule tissue. Therefore, in addition to having two different ploidy states, the resulting seed is made up of parental and offspring sporophytic tissue. To examine this more closely, we will explore the development and anatomy of a seed.
The cell that results from the fusion of the egg and one sperm becomes the zygote. It is diploid and begins to form the basic structures of the embryo (i.e., the embryonic root and the cotyledons). The cotyledons are also called "seed leaves." Plants that belong to Class Monocotyledone contain only one cotyledon and are referred to as monocotyledons or monocots; some common examples are grass and corn. The other class of angiosperms is Dicotyledone. Plants in this class have two cotyledons and are referred to as dicotyledons or dicots; some common examples are beans and pumpkins.
The cell that results from the other fertilization event is 3n (triploid). It has a nucleus formed from the fusion of two nuclei from the female gametophyte, and one nucleus from the male gametophyte. The tissue that forms from the triploid cell is known as endosperm, which contains the nutrients needed by the developing embryo. As a dicot embryo matures, most, if not all, of the endosperm is absorbed by the embryo. However, a monocot embryo usually does not absorb all of the endosperm. When we eat corn, the majority of the nutrients we eat are endosperm.
Recall from the nonflowering seed plant tutorial, the seed coat is formed from tissues in the ovule termed integuments. These tissues are diploid and are part of the parent plant. Thus, a seed consists of tissues that are made up entirely of the parental type (maternal sporophytic) and tissues that result from the recent fertilization.
Figure 8. A comparison of monocot and dicot seeds. (Click image to enlarge)
Many seeds have shapes or structures that facilitate dispersal away from their parent plant and other seeds produced by their parent. This is important because once a seed germinates and takes root, it is unable to move, so if it takes root too close to another plant, they will compete for resources. Many seeds are dispersed by the wind (Fig . 9). These have wings or “parachutes” that help carry them away.
Figure 9. Wind dispersed seeds. The winged seeds of red maples (left), tulip trees (center), and the parachute seeds of common milkweed (right). (http://www.cas.vanderbilt.edu/bioimages/pages/fruit-seed-dispersal.htm)
Some flowering plants have seeds that rely on animals for dispersal (Fig 10). In some cases, the animals eat the fruit and the seeds pass through their digestive system and are deposited away from where they were first eating. In other cases, animals will gather seeds (nuts) and hide or store them. While some will be eaten, others will “escape” and go on to germinate. These relationships are a type of mutualism; be able to describe how each species in this relationship benefits. There are also seeds that will “hitch a ride” on an animal’s fur or a bird’s feathers, and won’t be removed until the animal grooms itself, again depositing them some distance away from where they were first picked up. Read this web page ( http://www.americanforests.org/magazine/article/trees-that-miss-the-mammoths/ ) What animals are believed to have been responsible for dispersing the seeds within the osage orange fruit? How can these plants continue to survive today?
*Figure 10. Plants with fruits that rely on animals for dispersal. *Mulberry (left), oak (center), cocklebur (right). (http://www.cas.vanderbilt.edu/bioimages/pages/fruit-seed-dispersal.htm)
Figure 11. A Sequoia Tree. (Click image to enlarge)
Figure 12. The base of a Giant Sequoia Tree. (Click image to enlarge)
Plant growth is astonishing. Not only can plants live incredibly long (e.g., bristlecone pines), but they can also become quite large in a short lifetime. The tallest tree ever measured was a 435 foot eucalyptus in Australia. The current tallest tree is a coastal redwood that is over 379 feet tall. There is a giant sequoia tree (Figs. 11 and 12) named General Sherman in California. It is not the tallest, nor the widest, nor the oldest plant on Earth, but it does occupy more space than any other single organism. Its volume is estimated to be 52,508 cubic feet. That is equivalent to thirty, double-occupancy dorm rooms!
Plant growth differs from animal growth. Most plants follow a pattern of indeterminate growth, whereas most animals have determinate growth. Animals generally form all of their organs and then grow until they reach a certain size. Plants develop organs as they grow. They do not have a predetermined size, and they continue to grow as long as they live, although the rate of growth will change during their life time.
This concludes the tutorial on flowering seed plants, and also the three-part tutorial series on plants. The life cycle of flowering seed plants is closely tied to their flower and seed structures. In all cases, plants show a pattern of alternation of generations. In addition, all flowering plants show double fertilization, which produces a diploid zygote and triploid endosperm to feed the developing plant embryo. Pollination biology is an important part of the flowering plant life cycle. Pollination can be accomplished by the wind, or by animals pollinators that carry pollen from one flower to another. Plants have evolved a number of different strategies to reduce self-pollination, an extreme form of inbreeding. Seeds protect the developing plant embryo and are found within a fruit. The fruit can help to disperse the seed away from the parent plant to reduce competition. Plants show indeterminate growth and will grow throughout their life time, although the rate of growth will vary at different stages.
You should have a working knowledge of the following terms:
A Real Life Example: Stinky plants and their pollinators
Amorphophallus titanium is a large flowering plant with a strange odor. Its nickname is “the corpse plant” and it has been described as smelling like “a rotting elephant corpse”. Its flower is 5-12 feet tall and it has been described as the most amazing flower on earth. The bloom only lasts 2-3 days and the strong odor is only produced during the first 8 hours of blooming. Male and female flowers are separate but are found on the same plant; they bloom at separate times to avoid self-fertilization.
It was once rumored that the large flower was pollinated by elephants; however, this is not true. When the flower is pollinated, it provides its pollinator with food. It was also rumored that this plant could eat humans – this is also not true. In late 2005, this plant bloomed at the U.S. Botanic Garden and thousands of people went to view it. You can see a time lapse video of that bloom at http://www.usbg.gov/return-titan
- Using what you know about flower pollination, what is the likely pollinator of this flower?
- Is Amorphophallus titanium monoecious or dioecious?
- What type of relationship exists between Amorphophallus titanium and its pollinator?
Amorphophallus means "shapeless phallus."
(Click to enlarge)
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