Mastering Concepts

22.1

1. How do stems, leaves, and roots support one another?

Stems hold leaves up to light, where they can produce carbohydrates by photosynthesis. Sugars move via the stem from leaves to roots, flowers, fruits, and other nonphotosynthetic plant parts. The roots anchor the plant and absorb water and minerals. These substances move through the stem to the leaves, where they are used in photosynthesis and other processes.

2. What is the relationship between the node and the internode of a stem?

The node of a stem is a place where leaves are attached. An internode is a length of stem located between nodes.

3. Give examples of stems, leaves, and roots with specialized functions.

Specialized stems include tendrils (support of climbing stems), stolons and rhizomes (horizontally oriented stems that produce new stems and roots), tubers (food storage), succulent stems (water storage), and thorny stems (protection). Specialized leaves include onion bulbs (food storage), cactus spines (protection), brightly colored leaves (pollinator attraction), and carnivorous leaves (prey capture and digestion). Specialized roots include beets and carrots (food storage), desert plant roots (water storage), aerial roots (gas exchange or photosynthesis), and buttress or prop roots (support).

4. Categorize each structure in figure 22.1 as vegetative or reproductive.

The flowers and fruits are reproductive structures; all others are vegetative.

22.2

1. Compare and contrast tracheids, vessel elements, and sieve tube elements.

Tracheids, vessel elements, and sieve tube elements are similar in that they are cells that transport materials within a plant. Tracheids and vessel elements are part of the xylem; they transport water and dissolved minerals. Sieve tube elements are part of the phloem; they transport sugars and other dissolved organic substances.  Tracheids and vessel elements are dead at functional maturity; sieve tube elements are alive at functional maturity.  Vessel elements lack end walls; tracheids have pits; and sieve tube elements have sieve plates through which strands of cytoplasm pass from cell to cell.

2. Where in the plant does ground tissue occur?

Ground tissue fills the spaces between more specialized cell types inside stems, leaves, fruits, and seeds. Most of the body of a plant is made of ground tissue.

3. What are the functions of vascular tissue?

The functions of vascular tissues are to move water, minerals, carbohydrates, and other dissolved substances within the plant.

4. How does the structure of dermal tissue contribute to its functions?

Dermal tissue consists of a single layer of flat, transparent, tightly packed cells that cover the leaf, stem, and roots of the plant.  Dermal tissue secretes the cuticle, which retards loss of water from deeper plant tissues. Stomata, which are surrounded by guard cells, are pores that allow gases to move into and out of a leaf. When stomata are closed, they prevent water loss from leaves.

22.3

1. Name the cell layers in the stem of a monocot and a eudicot, moving from the epidermis to the innermost tissues.

Moving from the epidermis of a eudicot stem toward its center, the tissues are: epidermis; cortex; vascular bundles composed of fibers, phloem, and xylem; and pith. Moving from the epidermis of a monocot stem toward its center, the tissues are epidermis; vascular bundles made of xylem, phloem, and fibers; and ground tissue. Vascular bundles are arranged in a ring in a eudicot stem but scattered throughout a monocot stem.

2. List the parts of a simple and a compound leaf.

Both types of leaves have a blade, petiole, and axillary bud. Simple leaves have a single blade while compound leaves have a divided blade.

3. Describe the internal anatomy of a leaf.

Inside a leaf, mesophyll cells are surrounded by air spaces that promote gas exchange as the cells carry out photosynthesis. Veins contain xylem and phloem, which deliver water and minerals and carry off sugars produced in photosynthesis. A waxy cuticle covers the epidermis, and stomata are pores through which gases enter and leave the leaf.

4. Corn is a monocot and sunflower is a eudicot. Make a chart that compares the stems, leaves, and roots of these plants.

Stem structure:  Monocots have vascular bundles that are scattered throughout the ground tissue of the stem; eudicots have a single ring of vascular bundles in the stem. Leaf venation:  Monocot leaves have parallel veins; eudicot leaves have netted veins.  Root organization:  Monocots have a ring of vascular tissue surrounding a central core (pith) of parenchyma cells; eudicots have a vascular cylinder consisting of a solid core of xylem, with ridges that project into the pericycle. The phloem strands are generally located between the “arms’ of the xylem core.

22.4

1. What is the difference between determinate and indeterminate growth?

A plant with determinate growth stops growing after it reaches its mature size. In contrast, a plant with indeterminate growth can keep growing as long as the environment can support it.

2. What are the locations and functions of meristems?

Apical meristems are located at the tips of roots and shoots; they allow these organs to grow in length. Lateral meristems are form an internal cylinder of cells that extends along most of the length of the plant. Lateral meristems allow an increase in girth of roots and stems. Intercalary meristems occur in grasses and other monocots between the nodes of a mature stem, often at the base of an internode. These meristems allow a plant to tolerate repeated grazing and mowing because they regrow leaves from the base when the top of the leaf is clipped off.

3. What are the two lateral meristems in a woody stem or root, and which tissues does each meristem produce?

The two lateral meristems are the vascular cambium and the cork cambium.  Vascular cambium produces secondary xylem to the inside and secondary phloem to the outside.  The cork cambium produces parenchyma cells to the inside and cork to the outside.

4. What are the functions of wood and bark?

Wood, or secondary xylem, makes up most of the volume of a woody root or shoot.  Overall, wood is tough and provides support for the plant. In addition, the sapwood conducts water and minerals, although the heartwood is nonfunctional. The main function of bark is to protect the plant; in addition, secondary phloem in the bark conducts carbohydrates within the plant.

5. How do softwoods differ from hardwoods?

Softwoods are composed mainly of tracheids, whereas hardwoods contain both tracheids and vessels.

6. Explain the origin of tree rings.

Vascular cambium cells, dormant through the winter, divide and produce wood during the spring and summer.  During the wet spring months, the water conducting cells are larger and the wood is lighter.  During the drier summer, the cells are smaller and form denser rings.

21.6

1. What is the overall question the researchers were asking in this study? Summarize the data they used to arrive at their conclusion.

The overall question asked by the researchers was, “Why would natural selection select for plants that expend energy on homes for ants (domatia), and do domatia increase the reproductive success of the plant?”  The data came from several related experiments.  The researchers used H. brunonis as a model organism because the same species provides control groups without domatia and experimental groups with domatia. The researchers used the number of fruits as a measure of reproductive fitness. Their first experiment revealed that trees with domatia produce more fruit. Additional experiments revealed that tramp ants, living in the domatia, quickly recognize herbivores and actively attack them, thereby increasing the number of leaves available for photosynthesis. From these data the researchers were able to conclude that the domatia indirectly improved reproductive success in H. brunonis.

2. Propose an explanation for the observation that the trees secrete nectar only on young leaves and not on tougher, more mature leaves.

Answers will vary, but one possible explanation is that herbivorous caterpillars prefer young leaves. If so, then the plant would waste energy by attracting ants to mature leaves.

Write It Out

1. List the main vegetative organs of a plant, and explain how each relies on the others.

The vegetative (nonreproductive) organs of a typical plant include the roots, stems, and leaves. The stem and leaves constitute the shoot. The stem supports the leaves, the main sites of photosynthesis. The products of photosynthesis, in turn, nourish the nongreen plant parts, including the roots. Most roots exist below ground, anchoring the plant and absorbing water and minerals from the soil.  These resources move via stems to the leaves and other aboveground plant parts.

2. What selective pressures may have led to the thorny stems of cactus plants and the aboveground roots of mangrove trees?

Cactus plants live in dry habitats, where water is scarce. Plants that can stockpile water and defend it from thirsty animals should have greater reproductive success than those that cannot. Mangrove trees grow in waterlogged soils, where oxygen is scarce. Roots require oxygen to carry out their functions, so mangrove trees that can expose their roots to the atmosphere should have greater reproductive success than those that cannot.

3.  Imagine you are conducting an experiment on plant growth. You buy two seedlings, plant them in identical soil, and water them the same amount each week. Each week you also measure the plants’ heights. For the first two months, the plants grow at the same rate. Then, one plant continues growing while the growth of the other plant slows and eventually stops. Propose a possible cause for this observation.

One possibility is that the plant that kept growing was a variety with indeterminate growth, whereas the other plant exhibited determinate growth.

4. Many biology labs use preserved slides of root tips to demonstrate the stages of mitosis. Why is this a better choice than using a slide of a mature leaf?

The tips of roots consist of rapidly dividing meristematic cells that show all the stages of mitosis; the leaf does not contain meristematic tissue.

5.  Write nonbiological analogies for vessel elements, phloem, stomata, and lignin.

Many answers are possible; these are examples. Vessel elements are like Lifesavers candies stacked on top of one another. Phloem is like a conveyer belt that delivers baggage (carbohydrates) from an airplane (leaf) to the baggage claim (root or other “sink”). Stomata are like a home’s doors, which can open or close according to the occupants’ needs. Lignin adds strength to xylem cell walls, like the metal bars in reinforced concrete.

6.  Predict why it might be more adaptive for a tree to produce thousands of small leaves rather than one huge leaf.

One possible explanation is that multiple small leaves are more versatile than one large one. For example, branches can produce new leaves where the light is brightest and invest less energy in leaves in shaded areas. In addition, if a plant’s single huge leaf is cut off or damaged, the plant has no “backup” leaves to carry out photosynthesis.

7.  Search the Internet for an angiosperm evolutionary tree. Analyze the tree and then use the principles of shared ancestry to explain why monocots have embryos with one cotyledon, eudicots have embryos with two cotyledons, and both have seeds contained within fruits.

Monocots, eudicots, and all other angiosperms share a common ancestor that had seeds and fruits. The descendants of that common ancestor branched into multiple groups, some of which had one cotyledon and gave rise to the common ancestor of all monocots, and some of which had two cotyledons and gave rise to the common ancestor of all eudicots.

8.  List the structures that help a plant maintain water homeostasis.

Structures that help a plant maintain water homeostasis include the stomata, cuticle, xylem, endodermis, and root hairs.

9. Explain how conditions in the terrestrial environment selected for each of the following adaptations: cuticle, stomata, vascular tissue, roots, stems, and leaves.

Water evaporates quickly from a land plant’s tissues; the cuticle decreases water loss, and the stomata allow gas exchange in the presence of the cuticle. Air provides little physical support; vascular tissue provides not only support but also a transportation system in large plants. On land, water and minerals are in soil whereas CO2 and sunlight are aboveground. This division of resources selects for a division of labor in a plant: the roots both anchor the plant and maximize the surface area for water and nutrient absorption, whereas stems support the leaves. The leaves, in turn, maximize the surface area for photosynthesis.

10.  Consider the picture in the Burning Questions box on page 464. Which of the plant parts shown in the picture fit the biological definition of a fruit?

The cucumber and bell peppers are fruits.

11.  The Cork Forest Conservation Alliance is a nonprofit environmental organization dedicated to preserving the cork forests of the Mediterranean. Visit their website and then explain to a friend why buying wine closed with natural cork is more environmentally friendly than buying wine closed with a screw cap.

One argument that natural cork is more environmentally friendly than a metal screw cap is that cork is a renewable resource, whereas metal is nonrenewable.

12. Suppose you drive a metal spike from the outermost bark layer to the center of a tree’s trunk. Which tissues does your spike encounter as it moves through the stem, and what type of meristem produced each type?

The spike will strike the cork and a thin layer of parenchyma, both produced by the cork cambium. It will then strike the secondary phloem and enter the secondary xylem, both produced by the vascular cambium.

13. List a function of each organ, tissue, and cell type described in this chapter, and then list at least one feature that facilitates that function.

Vegetative organs include stems, leaves, and roots. Stems provide support; tough fibers help keep a plant upright. Leaves carry out photosynthesis; mesophyll cells contain chloroplasts with photosynthetic pigments. Roots absorb water and minerals; root hairs boost the surface area for absorption.

Tissues include ground tissue, dermal tissue, and vascular tissue. Ground tissue provides storage; some tissues are rich in starch-storing plastids. Dermal tissue protects a plant; a tough, waterproof periderm physically protects woody roots and stems. Vascular tissue transports materials; xylem and phloem are adjacent to each other, allowing xylem to contribute water to phloem sap

Cell types include parenchyma, collenchyma, sclerenchyma, vessel elements and tracheids, and sieve tube elements. Parenchyma cells provide storage; these cells contain vacuoles that store acids and other substances. Collenchyma cells provide support; these cells have thick, elastic cell walls. Sclerenchyma cells also provide support; they have thick, tough cell walls strengthened with lignin. Vessel elements and tracheids transport water within a plant; these cells lack cytoplasm that would otherwise interfere with water movement. Sieve tube elements transport dissolved organic substances; these cells have sieve plates that allow the movement of materials from cell to cell.

Pull It Together

1. Add the terms cortex, pith, mesophyll, endodermis, and Casparian strip to this concept map.

“Ground tissue” connects with the phrase “filling the interior of a stem or root is” to “pith” and “cortex.”  “Ground tissue” connects with the phrase “inside a leaf is” to “mesophyll.” “Casparian strip” connects with the phrase “is a waxy coating on the” to “endodermis,” which connects with the phrase “is a layer of cells inside a” to “root.”

2. Connect monocots and eudicots to vascular tissue. Your connecting phrases should indicate a difference between the two plant groups.

“Monocots” connects with the phrase “have stems with scattered bundles of” to “vascular tissue.” “Eudicots” connects with the phrase “have stems containing a ring of bundles of” to “vascular tissue.”