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Plants IV - Seed Plants (Flowering)

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Terms

You should have a working knowledge of the following terms:

  • angiosperm
  • anther
  • apical meristem
  • carpel
  • carpellate
  • cotyledon
  • determinate growth
  • dicotyledon (dicot)
  • embryo sac
  • endosperm
  • filament
  • herbaceous
  • imperfect flower
  • indeterminate growth
  • internode
  • lateral meristem
  • meristem
  • monocotyledon (monocot)
  • node
  • perfect flower
  • petal
  • pollination
  • primary growth
  • root
  • secondary growth
  • sepal
  • shoot
  • stamen
  • staminate
  • stigma
  • style
  • tendril
  • tuber
  • vegetative reproduction


Introduction and Goals

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. The various parts of these plants and their adaptations will also be discussed. By the end of this tutorial you should have a basic working understanding of:

  • The anatomy of a flower
  • The anatomy of a seed
  • The life cycle of flowering seed plants
  • Plant growth
  • Adaptations of roots, stems, and leaves

Flowering Seed Plants

Flowering seed plants are more prevalent than nonflowering seed plants. As depicted in the figure at the right a flower consists of four main parts: sepals, petals, stamens, and carpels.


Figure. 1 (Click image to enlarge)

Sepals are the outermost, leaf-like parts of the flower. 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. 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. 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 pollen grain lands on the stigma, where it germinates, then the pollen tube grows through the style to the ovule within an ovary. 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 in an ovary.

Flower Anatomy

All flowers have stamens and/or carpels. In addition to these basic parts, a flower can have petals and/or sepals. These parts are arranged in various patterns, however, there is an evolutionary trend toward bilateral symmetry. If a flower has both reproductive parts (stamens and carpels), it is a perfect flower. If a flower has only one of these reproductive parts, it is an imperfect flower. Imperfect flowers are further classified as staminate or carpellate, depending on which part is present.

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 the previous tutorial, these are 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.

The stamen, the male part of a flower, consists of a filament and an anther. The filament is just what its name implies; a long, thin structure that supports the anther. The anther is at the tip of the filament, and its function is to produce pollen grains. The carpel, the female part of a flower, consists of a stigma, a style and an ovary. The ovary is at the base of the carpel and it houses the ovule(s), which produces the egg cell. The style is the long portion of the carpel that supports the stigma. The stigma is the tip of the carpel. It functions to catch the pollen grains, hence it is often sticky.


Figure. 1 (Click image to enlarge)

Pollination

In order for a plant to reproduce sexually, the male pollen must reach the female stigma in a process called pollination. Some plants (e.g., corn) pollinate via the wind. 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 able to see white and yellow coloration. 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 eats 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 the bees (and ourselves) find desirable.


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 (shown here on the right). 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.


Figure. 2 (Click image to enlarge)


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.

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 cell begins to grow a pollen tube that extends down into the style. The other cell can then divide 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 the previous tutorial, 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. The zygote grows within the seed and eventually the seed germinates and grows into a mature, diploid, sporophytic plant.


Figure. (Click image to enlarge and play the animation)


Seed Anatomy

A seed is a genetically 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 F1 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. It has one nucleus formed from two nuclei from the female gametophyte, and one nucleus from the male gametophyte. This triploid nucleus divides to produce a multinucleate cell. 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.

Figure. 4(Click image to enlarge)

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.

Plant Growth

Figure. 5 (Click image to enlarge)


Figure. 6 (Click image to enlarge)

Plant growth is astonishing. Not only can plants live incredibly long (e.g., 4,600-year-old 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. There is a giant sequoia tree named General Sherman in California (shown above). 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.

This continuous growth can be attributed to meristems. Meristems are unique tissues that have the ability to divide. In many cases meristems contain cells with the potential to form any cell in the plant. Thus, they provide a reservoir of cells that can continuously become more specialized cells; this enables the plant to generate new tissues and organs throughout its life.

There are two main types of plant meristems: apical and lateral. Apical meristems are found at the apex, or tip, of a plant (i.e., there are root apical meristems and shoot apical meristems). The apical meristems are responsible for adding new plant parts (e.g., stems and leaves), and to extend roots. This is known as primary growth. All plants undergo primary growth. Herbaceous (nonwoody) plants, the monocots, and some dicots only exhibit primary growth.

The lateral meristem is responsible for lateral growth. In other words, a plant grows wider due to lateral meristematic action. This is known as secondary growth. For instance, consider a tree. It starts life as a short, thin seedling. The first, or primary, need for the plant is the height to access light for photosynthesis. The apical meristem provides for this height, and also for adding length to the roots. As the tree starts to get taller, there is the secondary need for a wider trunk that is stronger. The lateral meristems provide this growth in width. Specifically, lateral meristems are responsible for the tree rings that are added with each year of new growth. The primary growth at apical meristems happens in younger tissues, and the secondary growth at lateral meristems happens mostly in older tissues.

Roots

A plant can be divided into two main parts: the shoots (the aerial part of the plant) and the roots. The roots serve many purposes. First, they anchor the plant in the ground. Second, they take up water and mineral nutrients. Water is constantly being used and evaporated from the shoots, so it needs to be replaced.

In addition to these two primary functions of all roots, some roots are modified for special purposes. For example, consider a carrot. The part that we eat is modified for storage (it also anchors the plant). If we did not eat the carrot, the reserves it contains would be used for the next year's growth of new shoots in the spring. Likewise, deciduous trees (non-evergreens) have many carbohydrate reserves in their roots to help them grow new leaves each spring.


Some roots are modified to accommodate special living conditions. Epiphytic orchids reside up high in trees. They have aerial roots that attach the plant to the trunk or branch of a tree (image on right). Other plants, such as poison ivy, have typical roots in the soil, as well as aerial roots that the climbing vines use to attach to the trunk of a tree as it grows upward.


Figure. 7(Click image to enlarge)

Roots can also be modified to interact with other organisms. Many roots form symbiotic relationships with fungi (mycorrhizae) or bacteria (rhizobia). In tutorial 20, you learned about the nitrogen-fixing bacteria that form a mutualistic symbiosis, whereby the bacteria receive carbohydrates and shelter from the plant; the plant receives usable nitrogen from the bacteria, who fix atmospheric nitrogen into ammonia.

While we normally think of only leaves or stems as capable of photosynthesis, some roots also photosynthesize. Their cells contain active chloroplasts, therefore, the root tissue is green in appearance. Of course, most of these roots exist above ground.

Some roots are capable of vegetative reproduction. If one were to cut a carrot into pieces, they could grow a new carrot plant from each piece of the original root.


Stems

While roots are primarily found below the ground, stems are primarily found above the ground. Stems consist of nodes and internodes. Nodes are the location where leaves are attached to the stem, and internodes are the spaces between the nodes.

The stem has two main functions: support and transport. Recall, vascular tissue is composed of xylem and phloem, which act as vessels to move water and nutrients in the plant.

Stems, like roots, come in many shapes and sizes, with modifications for special functions. For example, consider a cactus. Some have stems that have been modified to store large amounts of water. This feature is adaptive for the desert climate in which they live. The plants take up large quantities of water during the few short periods of rain, and this allows them to live through periods of drought. Stems can also be modified to store carbohydrates. A potato is actually a special type of modified stem called a tuber. Technically, it is stem tissue that grows underground, but often it is mistaken for a root.

Another modification that stems share with roots is the ability to reproduce vegetatively. Remember, vegetative reproduction does not involve seeds. Many plants (e.g., geraniums and philodendrons) can be reproduced by stem cuttings. The piece that is cut off then forms new roots from its meristematic tissue.

Stems can also provide the plant with protection. They can have thick epidermal cells, or have thorns along their length. Anchorage is another function of some stems. The stems of the morning glory vine twine themselves around other plants or fences. This anchors them in the wind and rain.

Although we do not think of plants as being mobile, some stems can use growth to change their location. Plants can grow taller by adding length to their stems (e.g., competing trees in a forest). Vines can grow longer to get to different habitats for things such as light, water, or additional space.

Leaves

Leaves are located on stems at nodes. Unlike humans, who obtain their carbohydrates from the foods they eat, plants make their own carbohydrates. Most leaves contain more chloroplasts than other parts of the plant, and this is where most photosynthesis takes place.

Just as roots and stems can have specially modified structures for special functions, so can leaves. We already discussed how roots and stems can function in anchorage. Leaves can also be modified to hold a plant in place. The tendrils of cucumbers or pea plants are modified leaves. Therefore, pea plants are able to climb up a fence by sending out modified leaves.

Leaves also function in protection, both physically and chemically. The spines of a cactus are actually modified leaves. They strongly discourage herbivores from eating their protected fleshy stems. Other leaves, like those on nettle plants, have both physical and chemical modifications. Tiny needle-like structures on the leaves contain a toxic substance designed to discourage herbivores. If you have ever accidentally run into a nettle, you understand just how powerful this deterrent is!

Leaves can also have modifications to help collect and store water. Have you ever noticed the rosette formation of leaves on the top of a pineapple? Pineapples belong to a group of plants known as the bromeliads. The rosette pattern of their leaves is used to capture water until the plant can absorb it. A second family of plants, succulents, is well known for having leaves that store water. Are you familiar with the aloe plant? It has thick leaves that are very juicy inside. The leaves are designed to store water so that the plant can survive during periods of drought.

Leaves can function in much the same way as roots and stems for vegetative propagation. Two examples are Rex begonias and African violets. The leaves of these plants contain the meristematic tissue necessary to produce both stems and roots, under the appropriate conditions.

Probably the most intriguing modification of leaves occurs in carnivorous plants. The Venus flytrap has leaves that lay flat, waiting for an unsuspecting fly to land. When the fly lands, it triggers sensory hairs that cause the leaf to rapidly fold at its hinge, trapping the fly inside. Digestive juices are then secreted to kill and metabolize the fly. The resulting nutrients are then absorbed into the leaf. This process is similar to the digestive tract of an animal, with one notable exception. The fly primarily provides mineral nutrients (e.g., nitrogen), rather than carbohydrates

Summary

This concludes the tutorial on flowering seed plants, and also the four-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. When studying, be sure that you understand the basic similarities and differences in how this alternation is accomplished.

We also surveyed the major plant structures of flowering seed plants, and how they have evolved for a variety of purposes. Keep in mind, evolution works on existing structures, and one of the remarkable features of plant evolution is the functional diversity of these basic structures.

We also briefly reviewed the role of meristems, and their contributions to plant growth. Basically, all meristems supply the plant with cells that will go on to differentiate into various specialized tissues. The two major meristematic centers are the apical meristems and the lateral meristems. The apical meristems function to allow linear growth (they allow shoots to grow up and roots to grow down), whereas lateral meristems add girth, or sideways growth to organs.