Cracking the AP Biology Exam

10

Plants

Let’s begin our discussion of multicellular organisms with the plant kingdom. The plant kingdom includes thousands of species, which have established themselves in every possible habitat. With such a wide range of habitats, plants have naturally taken on a dazzling variety of forms: There are over 260,000 different species of flowering plants alone!

ETS expects you to be familiar with the general characteristics, life cycles, and classifications of plants. We’ve already covered some of these characteristics in earlier chapters. You should recall that plants:

  • Are multicellular, eukaryotic organisms
  • Have a cell wall made of cellulose
  • Are photosynthetic, meaning that they convert light energy to chemical energy by means of chloroplasts located primarily in their leaves
  • Take up water via capillary action

PLANT CLASSIFICATION

Because we’ve already discussed plant classification, we’re going to review the principal subdivisions. Classifying plants is also useful because it gives you an idea of their evolutionary history.

Let’s start with a simple flow chart of the different subdivisions of plants:

You’ll notice from our chart that all plants fall into three major groups: nonvascular, seedless vascular, and seeded vascular. Bryophytes are the more primitive plants: They lack true stems, roots, and leaves. Tracheophytes are more advanced plants that possess specialized conducting and vascular tissues. As we’ll see later on, these tissues are essential for the transport of material throughout the plant.

BRYOPHYTES VERSUS TRACHEOPHYTES

Members of the phylum bryophyta are the simplest plants, and are characterized by their lack of true stems and leaves. Common bryophytes include mosses and liverworts. They lack the vascular tissues—stems, roots, and leaves. The lack of specialized transport tissues is a major limitation for these simple plants.

The other phylum, tracheophyta, includes the most common and widespread of land plants. Plants in this phylum have true vascular tissues. These tissues enable them to thrive on land by facilitating the transport and storage of water and nutrients. Because vascular tissues are the deciding factor in splitting plants into the two major phyla, we should take a closer look at them.

Vascular Tissues

Tracheophytes contain two types of vascular tissues: xylem and phloem. Xylem is tissue that conducts water and minerals up a plant from its roots. There are two types of xylem cells: tracheids and vessel elements. The tracheids are long and thin; the vessel elements are short and thick.

Water enters the plant through the roots. Roots have special features in their outer layer called root hairs that increase the surface area for absorption of materials.

In addition to absorbing water, minerals, and nutrients, roots also anchor the plant in the soil.

Phloem vessels carry nutrients, such as glucose, throughout the plant. Phloem cells are made up of sieve tube elements and companion cells. Sieve tube elements are the cells that actually carry the nutrients in a plant. The companion cells hang around to lend “support” to sieve tube elements. Ferns, trees, and flowering plants are all examples of tracheophytes.

Let’s recap:

  • Xylem carries water and minerals.
  • Phloem carries nutrients.
  • Root hairs carry water and increase the surface area for absorption.
  • Roots also serve to anchor the plant.

Of all tracheophytes, ferns are the simplest and the most ancient. Enormous ferns covered much of the earth even before the dinosaurs appeared. Ferns are known as the “seedless plants.” They’re able to transport water, minerals, and nutrients throughout the plant because they have vascular tissue. However, because they’re seedless, they still need an abundant water supply for fertilization. That’s because water is needed in order for the sperm to make their way to the egg cells in reproduction.

GYMNOSPERMS AND ANGIOSPERMS: THE SEEDED PLANTS

Gymnosperms

Tracheophytes can be further subdivided into gymnosperms and angiosperms. These are the “seeded plants.”

Gymnosperms are among the woody plants and are, evolutionarily speaking, among the oldest plants around. They include such common evergreens as spruces, hemlocks, and firs and are perennial, meaning they live year after year. One way to determine the age of a gymnosperm is by counting the number of tree rings. These rings, which are composed of dead xylem, represent the tree’s annual growth. Gymnosperms contain unenclosed seeds which are often found naked on the scales of a cone or similar structure.

Angiosperms

Of all the plants on the planet, the angiosperms are the most varied and widespread. Angiosperms are also known as the “flowering plants.” They have enclosed seeds located within a fruit or nut. Some flowering plants are woody, such as oak, cherry, and walnut. Others are more supple. Angiosperms can be further divided into two classes: monocots and dicots. The monocots have a single cotyledon—the embryonic seed leaf. Monocots are known to have leaves with parallel veins and flower parts in multiples of three. Some examples of monocots are orchids and lilies. Dicots, on the other hand, have two cotyledons. They have broad leaves with netted veins, and flower parts in multiples of four or five. Some examples of dicots are geraniums and smapdragons. Take a look at the major differences between monocots and dicots.