Biology For Dummies
Part V It’s Not Easy Being Green: Plant Structure and Function
In this part . . .
Plants have many similarities to animals: They have tissues, they circulate materials, and they reproduce sexually. Yet plants are also remarkable in their own right. Consider, for example, how well you’d do if someone buried you in the ground up to your knees and then left you there. You wouldn’t fare so well, but a plant would be just fine because it can make its own food. Not only that but it may also have strategies to attract pollinators to help it reproduce and defenses to help protect it from predators. Not too shabby for an organism that can’t make a sound, huh?
In this part, you get to know all about the structure and function of the green things that call planet Earth home.
Living the Life of a Plant
In This Chapter
Looking at the structure of plants
Acquiring what a plant needs to keep growing
Exploring the differences between asexual and sexual reproduction
A plant’s structure suits its lifestyle. After all, it has flat leaves for gathering sunlight, roots for drawing water up from the soil, and flowers and fruits for reproduction. Plants begin their lives from seeds or spores, grow to maturity, and then reproduce asexually or sexually to create new generations.
In this chapter, we present the fundamental structures of plants, how they get the energy they need to grow, and their reproductive strategies.
Presenting Plant Structure
Like animals, plants are made of cells and tissues, and those tissues form organs, such as leaves and flowers, that are specialized for different functions. Plants have two basic organ systems: a root system (which exists underground) and a shoot system (found aboveground). The root system is responsible for anchoring the plant and also absorbing minerals and water from the soil. The shoot system ensures the plant gets enough sunlight to conduct photosynthesis; it also transports water upward from the roots and moves sugars throughout the plant.
Within their organ systems, plants have up to three types of tissues. Biologists look at the types of tissues a plant has to help them classify plants into four different groups. They also look at the different structures of plant stems.
All plants have tissues, but not all plants possess all three of the following types of tissues:
Dermal tissue: Consisting primarily of epidermal cells, dermal tissue covers the entire surface of a plant. Guard cells in a plant’s epidermis control the opening and closing of little holes called stomates that allow the plant to exchange gases with its environment (you can see a stomate in the leaf cross section in Figure 20-1).
Ground tissue: This tissue type makes up most of a plant’s body and contains three types of cells:
• Parenchyma cells are the most common ground tissue cells. They perform many basic plant cell functions, including storage, photosynthesis, and secretion.
• Collenchyma cells have thick cell walls in order to help support the plant.
• Sclerenchyma cells are similar to collenchyma cells, but their walls are even thicker — so thick, in fact, that mature schlerenchyma cells die because they can’t get food or water across their walls via osmosis (more about that in Chapter 4).
Vascular tissue: The system of tubules inside a plant that carries nutrients around is made up of vascular tissue. Vascular tissue consists of a water-transport system called xylem and a sugar-transport system called phloem. Vascular tissue also contains the vascular cambium, a tissue of cells that can divide to produce new cells for the xylem and phloem. (Vascular plants make up the majority of plants on Earth. You can see the basic cells and structures of a vascular plant in Figure 20-1.)
The types of plants
Based on the types of tissues they have and reproductive structures they make, plants can be organized into four major groups:
Bryophytes are plants such as mosses that don’t have a vascular system and don’t produce flowers or seeds.
Ferns have vascular tissue, but they don’t produce seeds.
Gymnosperms (also known as conifers) have vascular tissue and produce cones and seeds, but they don’t produce flowers.
Angiosperms (or flowering plants) have vascular tissue and produce both flowers and seeds. Two distinct groups exist among flowering plants:
• Monocots, like corn and lilies, have seeds that contain one cotyledon (tissues within the seed that supply nutrition to the embryo and then emerge as the first leaves after the seed begins to grow; they’re also referred to as seed leaves).
• Dicots, like beans, oak trees, and daisies, have seeds that contain two cotyledons.
Figure 20-1: The basic structures of a vascular plant.
Table 20-1 presents several of the key structural differences between monocots and dicots.
Table 20-1 Structural Differences between Monocots & Dicots
Cotyledons in seeds
Bundles of vascular tissue
Form definite ring pattern
Xylem and phloem
Found in stem
Found in stem
Form a net pattern
Are in threes and multiples of threes
Are in fours and fives and multiples of fours and fives
Herbaceous versus woody stems
Biologists use the appearance and feel of a plant’s stem to place it into one of two categories: herbaceous (where the stem remains somewhat soft and flexible) and woody (where the stem has developed wood). All plant cells have primary cell walls made of cellulose, but the cells of woody plants have extra reinforcement from a secondary cell wall that contains lots of a tough compound called lignin.
Plants that survive just one or two growing seasons — that is, annuals or biennials — are typically herbaceous plants. Plants that live year after year, called perennials, may become woody.
The stems of herbaceous and woody dicots (see the preceding section) are organized differently. You can see these differences most clearly if you look at a cross section (a section cut at right angles to the long axis) of a stem. Imagine taking a hot dog and slicing it into little circles, and you’ve got a pretty good picture of how biologists make stem cross sections.
When you look at a cross section of the stem of an herbaceous dicot, you see that
The center of the stem consists of pith (a soft, spongy tissue), which has many thin-walled cells called parenchymal cells. The thin walls allow the diffusion of nutrients and water between the cells.
The vascular tissue is organized in vascular bundles that contain both xylem and phloem, as well as some vascular cambium (all of which are described in the earlier “Plant tissues” section). The vascular bundles are arranged in a ring around the pith.
Outside the vascular bundle ring is the stem’s cortex. It contains a layer of endodermis, additional parenchymal cells, and mechanical tissue, which supports the weight of the plant and holds the stem upright.
On the surface of the stem are the epidermis and the cuticle.
Woody dicots start life with green herbaceous stems that have vascular bundles. As they grow, however, the bundles merge with each other to form rings of vascular tissue that circle the stem. If you were to examine a cross section of the stem of a woody dicot that was a couple years old (like the one in Figure 20-2), you’d see that
The center of the stem consists of a circle of pith.
The xylem tissue forms a ring around the pith. As woody plants grow, new layers of xylem are added every year, forming rings inside the woody stem. (You can count these rings in the stem of a tree to tell how old it was when it was cut.) As these rings of xylem accumulate year after year, the diameter of the woody stem increases.
The inner part of a woody stem is called heartwood. It consists of older xylem tissue that’s filled with material such as gums and resins and no longer conducts water. Outside the heartwood is the sapwood, newer layers of xylem tissue that transport water and minerals up through the stem.
Just outside the xylem rings is a ring of vascular cambium. As the stem grows, the vascular cambium divides to produce new xylem cells toward the inside of the stem and new phloem cells toward the outside of the stem.
Outside the vascular cambium ring is a ring of phloem. The phloem of woody plants gets pushed farther and farther outward as the xylem tissue increases in size year after year. Phloem cells are fairly delicate, and the old phloem cells get crushed against the bark as the stem grows. The only phloem that serves to transport materials through the woody plant is the phloem that’s newly formed during the most recent growing season.
Outside the phloem is the bark, a ring of boxy, waterproof cells that help protect the stem. Bark includes the outermost cells of the stem and a layer of cork cells just beneath that outermost layer.
Figure 20-2: A cross section of a woody stem.
Obtaining Matter and Energy for Growth
The biggest difference between plants and animals is how they get the matter and energy they need for growth. Animals have to eat other living things to get their food, but plants can produce their own food. Plants absorb sunlight and use that energy to make glucose from carbon dioxide and water during the process of photosynthesis (we describe photosynthesis in detail in Chapter 5); glucose is the food plants can use as a source of energy or matter for growth.
As you can see from the following list, plant structures are specialized to help plants get what they need for photosynthesis:
The shoot system helps plants capture energy from the Sun. Shoots grow upward, bringing leaves toward the Sun. Branches spread leaves out so they can absorb light over a wider area, and many leaves are flat so they have the most surface area possible for light absorption.
The root system absorbs water and minerals from the soil. Water is needed for photosynthesis and basic plant functioning. Minerals perform the same function for plants as they do for you — they improve general metabolism by helping enzymes function properly. Also, plants absorb nitrogen-containing compounds from the soil and use them, along with the carbohydrates made during photosynthesis, to construct plant proteins (flip to Chapter 11 for the scoop on how plants get nitrogen).
Stomates in the leaves allow plants to take carbon dioxide from the atmosphere and return oxygen to it. The carbon dioxide provides the carbon and oxygen atoms plants need to build carbohydrates. Also, photosynthesis produces oxygen when the hydrogen and oxygen atoms in water are separated. Oxygen gas leaves plants through their stomates.
Plants extract energy from food molecules the same way animals do — by cellular respiration (see Chapter 5 for more on cellular respiration). When plants do cellular respiration, they produce carbon dioxide and use oxygen just like animals do. During the day, however, photosynthesis absorbs so much carbon dioxide and releases so much oxygen that plant respiration isn’t detectable. If you were to measure gas exchange around a plant in the dark, the plant would be exchanging gases just like you.
Going It Alone: Asexual Reproduction
Plants that reproduce asexually make copies of themselves in order to produce offspring that are genetically identical to them. The advantage to asexual reproduction is that it allows successful organisms to reproduce quickly. The disadvantage is that all the offspring are genetically identical, which decreases the ability of the population to survive changes in the environment.
The most common method of asexual reproduction is mitosis, which we describe in Chapter 6. Other ways a plant can reproduce asexually include fragmentation, a form of asexual reproduction that involves pieces of an individual growing into new individuals. If you break off a piece of many houseplants (the technical term is “take a cutting”) and stick it in water, new roots and shoots may grow, creating a whole new plant from a piece of the parent plant. Likewise, if you cut a potato into pieces, each piece that has an “eye” can produce a new potato plant. In essence, the new plant is a clone of the parent plant: It has all the same genetic information because the cells are identical.
Other plants, such as strawberry plants, produce special structures that help them spread asexually. In addition to producing stems, the strawberry plant produces a stolon, also referred to as a runner, that spreads across the ground. Wherever that stolon starts to put roots down is where a new strawberry plant grows. Similarly, many ferns reproduce asexually by underground stems called rhizomes.
Mixing Sperm and Eggs: Sexual Reproduction
Plants do have sex, believe it or not. First, they produce eggs and sperm through meiosis just like animals do (see Chapter 6 for the full scoop on meiosis). Then a sperm and an egg meet, creating offspring that have different combinations of genetic material than the parent plants.
The following sections get you familiar with the life cycle of plants that reproduce sexually along with all the little details and processes that involves, from flower structure and pollination to fertilization and the development (and protection) of plant embryos.
The life of a plant
The life cycles of plants are a bit more complicated than those of animals. In animals, gametes (sperm and eggs) are usually small and inconspicuous. In plants, however, gametes can almost have a life of their own.
Plant life cycles involve the alternation of generations between two stages called sporophytes and gametophytes (see Figure 20-3). Here’s a breakdown of the cycle:
1. Meiosis in a sporophyte (a parent plant) results in the production of spores that are haploid (meaning they have half the genetic information of the parent plant).
2. The spores develop by mitosis into multicellular haploid organisms called gametophytes.
The gametophyte step of the plant life cycle is a fundamental difference between plants and animals. In animals, no development occurs until a sperm and an egg combine to produce a new organism. In plants, there’s a little break between meiosis and the production of sperm and eggs. During that break, a separate little haploid plant grows.
3. Gametophytes produce gametes by mitosis.
In animals, sperm and egg are produced by meiosis, but in plants, meiosis occurs to produce the gametophyte.
4. The gametes merge, producing cells called zygotes that contain the same number of chromosomes as the parent plant (so the zygotes are diploid).
5. Zygotes divide by mitosis and develop into sporophytes so the life cycle can begin again.
Figure 20-3:Alternation of generations in plants.
The plants you see when you go for a walk in the woods may be sporophytes or gametophytes; it all depends on the type of plant you’re looking at.
The mosses you see growing on trees and on the forest floor are gametophytes. If you see little structures like flagpoles sticking off the moss, then you’re looking at a sporophyte. The little sporophytes grow like flags off the tops of the gametophytes. Inside the little flags, calledcapsules, meiosis is occurring to produce spores.
Ferns you can see are sporophytes. If you look on the back of a fern’s leaves, you can find little brown structures that seem dusty to the touch. These structures are where spores are being made, and the dust that comes off are the spores. Fern gametophytes are teeny — about as big as the fingernail on your pinky — making them very tough to find in the wild.
The conifers you see in a forest are sporophytes. The gametophyte generation in conifers is very small and contained within their cones.
Flowering plants that are visible to the eye are also sporophytes. In flowering plants, the gametophyte generation is very small and contained within the flowers.
The parts of a flower
Whether a flowering plant’s flowers are large and showy or small and dainty, sexual reproduction occurs inside. Flowers form on specialized shoots of the plant, and they have specific parts. Following are descriptions of some of these parts (you can see several of them in Figure 20-4):
The receptacle is the base of the flower.
Sepals are the lowest layer of leaves on the flower. They’re usually green.
Petals are modified leaves that are often brightly colored to attract pollinators (see the next section for more on pollinators).
Stamens are the male parts of a flower. Each stamen consists of a threadlike filament and a little sac called the anther. Inside the anther, meiosis and mitosis occur to produce the male gametophyte, pollen.
Pistils are the female parts of a flower. The ovary is located at the swollen base of the pistil. Inside the ovary, meiosis and mitosis produce the female gametophyte and the egg, which are housed inside an ovule.
The style and stigma grow up out of the ovary. Pollen lands on the stigma and then travels down through the style to deliver sperm to the eggs inside the ovary.
How pollination and fertilization occur
Pollination, the delivery of pollen to a flower’s stigma, and fertilization, when a sperm joins with an egg, are two separate yet important events that occur within flowering plants.
Some flowering plants, like grasses, are wind pollinated, which means they make lots and lots of pollen and trust the wind to blow it to the right place. Other flowering plants rely on animals such as bees, wasps, birds, and even flies and bats to transfer pollen from one flower to another.
Here’s how different plants attract their animal pollinators:
Bird- and bee-pollinated plants are usually brightly colored to attract the animals; they may also provide nectar to encourage visits. Bee-pollinated flowers are often marked with lines just like little runways to direct the bees to the right place in the flower. These runways are invisible to human eyes but visible to the bees, who can see ultraviolet light.
Some plants, like certain wasp-pollinated orchids, trick the animals into thinking that the flowers are members of the opposite sex. In this case, the wasps attempt reproduction with the flowers and get covered with pollen before moving on to the next flower to try again.
Fly-pollinated plants smell like toilets or rotten meat in order to attract mama flies in search of a good place to lay their eggs.
Bat- and moth-pollinated flowers are usually white so they’ll be more visible at night, which is when they open.
When pollen arrives at the stigma of a flower, each pollen grain grows a long tube called a pollen tube. The pollen tube grows down through the flower’s style so the sperm cells inside the pollen can be delivered right to the ovules inside the flower’s ovary.
Figure 20-4:Parts of a flower and sexual reproduction in angiosperms.
After pollination has occurred, two sperm nuclei enter the ovule (the part of the ovary that contains the egg and develops into the seed after fertilization), and one fuses with an egg to form the zygote. The other sperm joins with two haploid cells inside the ovule, called the polar nuclei, to form a tissue called endosperm that helps support the developing embryo. In flowering plants, the fusion of two sperm cells, one with an egg and one with the polar nuclei, is called double fertilization.
From zygote to embryo
After fertilization, the zygote divides by mitosis to produce an embryo. The first cell division produces two cells: one large, one small. Several more cell divisions occur after that, producing a line of cells called a suspensor. Additional cell divisions form the embryo in such a way that the cells at the bottom grow downward to become roots, and the cells at the top grow upward to become shoots.
The embryo’s hypocotyl, which is attached to the bottom end of the suspensor, becomes the lower part of the stem and the roots. The cotyledons, or seed leaves, develop at the top end of the embryo; they’re temporary structures that serve as nutrient storage sites for the developing plant. After the plant is growing aboveground and can start producing nutrients on its own through photosynthesis, the cotyledons shrink away.
A little protection for the embryo: Seeds
Seeds are protective structures that contain plant embryos and nutritive tissue to support the embryo until it can survive on its own. The endosperm produced during double fertilization provides nutrient material to the developing embryo, tissues from the ovule harden to become the seed coat that protects the embryo, and the ovary of the flower forms a fruit around the seed. After a seed develops, it usually dries out, and its water content drops very low. This low water level keeps the embryo’s metabolism at a minimal level so it can survive on stored food for a long time. So the seeds that you buy in a little packet at the local nursery are very much alive, but they’re in a state similar to that of a hibernating bear.
When seeds are planted in an environment with water, the dry seeds take up water and swell. As water becomes available, the embryo’s metabolism speeds up, and it begins to grow by using the stored food inside the seed. Germination occurs when the seedlings emerge from the seed and begin to grow into a diploid plant.