Cracking the AP Biology Exam

10

Plants

SPECIAL STRUCTURES IN PLANTS

ROOTS

The growing root includes three regions: the root tip, the elongation region, and the maturation region. The root tip and elongation regions are the sites of ongoing primary growth. The root apical meristem includes tiny, undifferentiated cells that continually divide and form the zone of cell division. As cell division in the root apical meristem continues, the new cells left behind grow rapidly in length and push the root tip along. As the cells absorb water, elongation occurs. Tiny root hairs, extensions of the epidermal cells, form and provide an increased surface area through which water and dissolved minerals can move into the plant.

All roots have an epidermis (an outer protective covering), a cortex (a middle region which stores starch and other minerals), and a stele (the inner cylinder which contains xylems and phloems). How do water and minerals enter the root? They travel through the root cortex either by the apoplast (porous cell walls) or symplast (through plasmodesmata). Once water and minerals reach the inner layer of the cortex, called the endodermis, they must cross the endodermal cells, tightly-packed cells that regulate the selective passage of water and minerals into the vascular tissue, in order to reach the root’s interior. In the walls of endodermal cells is the Casparian strip, a belt made of a fatty tissue called suberin which blocks the movement of water and minerals between the endodermal cells.

LEAVES

We already know that leaves play an important role in photosynthesis. But did you know that leaves are sometimes modified for other purposes?

Here’s a list of some of the other functions of leaves:

  • Leaves can be modified to form spines, as in a cactus. This adaptation is great for protection.
  • Leaves can be adapted for water storage. Fleshy leaves allow plants to survive particularly harsh environments where the water supply is intermittent or undependable.
  • Leaves can also be modified to trap prey. Insectivorous plants have specialized leaves that digest insects. Because they grow in soils deficient of essential nutrients, especially nitrogen, these plants are forced to eat insects. There are basically two general forms of these adaptations:

1. Some leaves have tiny hairs that act like bear traps. For example, an insect brushing against the hairs in a Venus flytrap triggers the leaves to snap shut.

2. Other leaves are adapted to form a “slippery slope” that traps insects. In a Pitcher plant, for example, once an insect gets inside, it can’t get out. It slips down into the bell-shaped interior of the leaf, where it drowns in a mixture of water and enzymes. These enzymes then finish the job by digesting the insect.

FLOWERING PLANTS

When it comes to plant structures, one that you are sure to run into on the AP Biology Exam is the flower. You don’t need to know everything about flowers for the test. Let’s take a look at what you do need to know.

Flowering plants have several organs: the stamen, pistil, sepals, and petals.

The male parts are collectively called the stamen, and the female parts are called the pistil. The sepals are the green, leaf-like structures that cover and protect the flower, while the brightly colored petals attract potential pollinators. Let’s review each of these structures.

The Stamen

The stamen consists of the anther and the filament. The anther is the structure that produces pollen grains. These pollen grains, called microspores, are the plant’s male gametophytes, or sperm cells. Pollen grains are produced and released into the air. The filament is the thin stalk that holds up the anther.

The Pistil

The pistil includes three structures: the stigma, style, and ovary. The stigma is the “sticky” portion of the pistil that captures the pollen grains. The style is a tubelike structure that connects the stigma with the ovary.

The ovary is where fertilization occurs. Within the ovary are the ovules, which contain the plant’s equivalent of the female gametophytes. In a fertilized plant the ovary develops into the fruit. Apples, pears, and oranges are all fertilized ovaries of flowering plants. The female gametes of plants are known as megaspores. They undergo meiosis to produce eight female nuclei, including one egg nucleus and two polar nuclei.

DOUBLE FERTILIZATION

Now that we’ve seen both the male and the female organs in plants, let’s take a look at how they actually reproduce. Flowering plants carry out a process called double fertilization. When a pollen grain lands on the stigma, it germinates and grows a thin pollen tube down the style, which meets up with the ovary. The pollen grain then divides into two sperm nuclei. One sperm nucleus (n) fuses with an egg nucleus (n) to form a zygote (2n). This zygote will eventually form a plant. The other sperm nucleus (n) will fuse with two polar nuclei (2n) in the ovary to form theendosperm (3n). The endosperm will not develop into a plant. Rather, it will serve as food for the plant embryo. Double fertilization produces two things: a plant and food for the plant.

Let’s review the steps involved in double fertilization:

  • Grains of pollen fall onto the stigma. The pollen grains grow down the style into the ovary.
  • The pollen grains (microspores) meet up with megaspores in the ovule. Microspores fertilize the megaspores.
  • One microspore unites with an egg nucleus and eventually develops into a complete plant.
  • The other microspore unites with two polar nuclei and develops into food for the plant, often in the form of a fruit.

Early Seedling Development

As the embryo germinates, different parts of the plant begin to develop. The cotyledons are the first embryo leaves to appear. They temporarily store all the nutrients for the plant. The epicotyl is the part at the tip of the plant. This portion becomes the stems and leaves. The hypocotyl is the stem below the cotyledons. This portion becomes the roots of the plant. In some embryos, root development begins early, and the well-defined embryonic root is referred to as a radicle.

What triggers sexual reproduction in plants? Plants flower in response to changes in the amount of daylight and darkness. This is called photoperiodism. Plants fall into three main groups: short-day plants, long-day plants, and day-neutral plants. Although you’d think that plants bloom based on the amount of sunlight, they actually flower according to the amount of uninterrupted darkness.

Short-day plants require a long period of darkness, whereas long-day plants need short periods of darkness. Short-day plants usually bloom in late summer or fall when daylight is decreasing. Long-day plants, on the other hand, flower in late spring and summer when daylight is increasing. Day-neutral plants don’t flower in response to daylight changes at all. They use other cues such as water or temperature.

The light receptor involved in photoperiodism is a pigment called phytochrome. In short-day plants, it inhibits flowering, whereas in long-day plants it induces flowering.

VEGETATIVE PROPAGATION

Flowering plants don’t always reproduce via fertilization. In some cases, flowering plants can reproduce asexually. This process is known as vegetative propagation. That means parts of the parent plant—such as the roots, stems, or leaves—can produce another plant. Some examples of plant parts that can reproduce this way include tubers, runners, and bulbs. For instance, suppose you wanted to make white potatoes without fertilization. All you’d have to do is cut out the “eyes” of a potato, the tubers, and plant them. Each of the eyes will develop into a separate potato plant. Grafting is another way plants can be reproduced asexually.

Here’s a list of the different types of vegetative propagation: