CHAPTER SUMMARY
31.1 IN PLANTS, UPWARD GROWTH BY STEMS OCCURS AT SHOOT APICAL MERISTEMS, POPULATIONS OF TOTIPOTENT CELLS THAT PRODUCE NEW CELLS FOR THE LIFETIME OF THE PLANT.
- Stems are built from repeating modules consisting of nodes, where one or more leaves are attached, and internodes, which are the regions of stem between each node.
- The shoot apical meristem, located at the tip of each stem, is the source of new cells from which the stem and the leaves are formed.
- Stems contain zones of cell division, elongation, and maturation. Most of the increase in size occurs in the zone of cell elongation.
- Leaves develop from leaf primordia produced by the shoot apical meristem.
- Leaves differ from stems in that they differentiate fully and thus have no capacity for continued growth.
- In seed plants, branching occurs at axillary buds at the base of each leaf.
- Shoot apical meristems and axillary buds can be transformed into floral meristems, at which point they lose their capacity for continued growth.
31.2 HORMONES ARE CHEMICAL SIGNALS THAT INFLUENCE THE GROWTH AND DIFFERENTIATION OF PLANT CELLS.
- Hormones are chemical signals that affect both physiology and development and are effective in extremely low concentrations. Traditionally five plant hormones have been identified: auxin, gibberellic acid, cytokinins, ethylene, and abscisic acid.
- The polar transport of auxin through PIN proteins guides the development of vascular channels (xylem and phloem) that connect leaves and stems.
- Xylem and phloem differentiate from procambial cells.
- In most seed plants, the stem’s vascular bundles are organized as a ring.
- Gibberellic acid causes internodal elongation of shoots.
- Apical dominance is the suppression of the outgrowth of axillary buds caused by hormones produced by roots (strigolactone) and shoots (auxin).
31.3 LATERAL MERISTEMS ALLOW PLANTS TO GROW IN DIAMETER, INCREASING THEIR MECHANICAL STABILITY AND THE TRANSPORT CAPACITY OF THEIR VASCULAR SYSTEM.
- The formation of two lateral meristems—the vascular cambium and the cork cambium—allow plants to grow in diameter.
- The vascular cambium produces secondary xylem toward the center and phloem toward the outside.
- The cork cambium produces a protective outer layer.
- Gymnosperm tracheids function in both mechanical support and water transport. Angiosperm xylem produces fibers that influence the strength of wood and vessel elements that function in water transport.
31.4 THE ROOT APICAL MERISTEM PRODUCES NEW CELLS THAT ALLOW ROOTS TO GROW DOWNWARD INTO THE SOIL, ENABLING PLANTS TO OBTAIN WATER AND NUTRIENTS.
- The root apical meristem is the source of new cells that allow roots to elongate.
- The root apical meristem has a cap that protects it from damage as the root grows through the soil.
- The root’s vascular tissues are located in the center of the root, surrounded by the pericycle, the endodermis, the cortex, and the epidermis.
- Lateral roots develop from new root apical meristems that form in the pericycle.
- Roots alter their development in response to conditions in the soil.
31.5 PLANTS RESPOND TO LIGHT, GRAVITY, AND WIND THROUGH CHANGES IN INTERNODE ELONGATION AND THE DEVELOPMENT OF LEAVES AND BRANCHES.
- Plant stems grow toward the light and away from gravity as a result of redistribution of auxin.
- Starch-filled statoliths allow roots to sense and respond to gravity. The weight of statoliths alters the lateral distribution of auxin, causing roots to bend downward.
- Seedlings germinating in the dark produce elongated internodes and suppress leaf expansion to increase their ability to reach the soil surface before their stored resources are depleted.
- Some plants respond to the presence of neighboring plants by elongating more rapidly and suppressing branching, which allow them to maximize height growth.
- Mechanical stress, such as the bending of stems in the wind, results in stems that are short and wide.
- Decreases in photoperiod trigger plants to prepare their meristems for winter by producing bud scales and reducing internode elongation.
Self-Assessment Question 1
Diagram the zones of cell division, elongation, and maturation, and explain why this organization allows stems to grow without a predetermined limit to their length.
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Model Answer:
Stems can grow without a predetermined limit to their length because the apical meristem is continually making new cells while the older cells are still elongating, pushing the apical meristem up.
Self-Assessment Question 2
Give two examples of how changes in internode elongation allow plants to alter the development of their stems.
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Model Answer:
As autumn approaches, the internodes of temperate plants cease elongating. As a result, bud scales become tightly packed together. (2) A plant adapted to full sun that is growing in the shade will elongate its internodes, maximizing its chance of reaching sunlight.
Self-Assessment Question 3
Name one role of the plant hormone auxin and describe how auxin is transported within a plant.
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Model Answer:
Auxin is responsible for stimulating the formation of root meristems, as well as guiding the development of vascular connections between leaves and stem, and between leaves and roots. It also causes shoots to elongate, causes branches to be suppressed, and helps orient stems toward the light.
Auxin is transported through the phloem by polar transport, which is the coordinated movement across many cells in a particular direction. Auxin can only exit cells through PIN proteins, so the placement of PIN proteins on the basal side of each cell will cause auxin to flow from the tip to the base of a plant.
Self-Assessment Question 4
Explain why a plant that has a vascular cambium also has a cork cambium.
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Model Answer:
Cork cambium protects the stem of the plant against herbivores, mechanical damage, desiccation, and, in some cases, fire. The plant needs this protection because as the activities of the vascular cambium increase the plant’s diameter, the primary epidermis eventually ruptures.
Self-Assessment Question 5
Explain why the vascular cambium forms a continuous sheath that runs from near the tips of the branches to near the tips of the roots, whereas the cork cambium is discontinuous in both space and time.
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Model Answer:
The vascular cambium forms a continuous sheath from root to tip; as a result it produces xylem and phloem along the length of the plant. Cork cambium is discontinuous in both space and time because it is relatively short lived. The cork cambium eventually becomes cut off from its source of carbohydrates—the phloem. A new cork cambium is formed when the distance between the original cork cambium and the functional phloem grows too large. In many cases, cork cambium forms in patches rather than as a continuous layer.
Self-Assessment Question 6
List three structural differences between roots and shoots that allow roots to grow through the soil. Explain how you would tell whether an isolated piece of a plant came from the shoot or from the root.
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Model Answer:
Roots are typically much thinner than stems, allowing them to grow around particles in densely packed soil. The root apical meristem is also protected by a root cap as the root grows through the soil. Finally, the root apical meristem does not produce any lateral organs such as leaves.
You could identify a root in several ways. First, does it have a root cap surrounding the apical meristem? Second, are there thin projections, root hairs, growing from the zone of maturation? Third, how big is the tissue in relation to the rest of the plant (as roots are typically thinner)? Fourth, is the tissue able to photosynthesize and is thus green (the color green is not usually indicative of root tissue)?
Self-Assessment Question 7
Describe how lateral roots form and how lateral roots enhance the ability of plants to obtain nutrients and water from the soil.
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Model Answer:
Lateral roots develop from a single layer of cells inside the endodermis called the pericycle. A new root apical meristem develops from the pericycle and grows out through the endodermis, cortex, and epidermis to the soil. New roots can form anywhere along the original root and, in some species, even along the stem of the plant. Lateral roots increase the surface area of the root system, allowing it to take up more water and nutrients from the soil.
Self-Assessment Question 8
Discuss the similarities and differences in the ways stems and roots respond to gravity and light.
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Model Answer:
Stems and roots use the same signals to respond to gravity and light but their responses are opposite. In the stem, light detected by photoreceptors signals the stem to preferentially transport auxin to the shaded side, causing the cells on that side to expand and the stem to bend toward the light. In roots, auxin is again transported to the shaded side of the root, but in this case the rate of cell elongation decreases on that side and the root ultimately bends away from the light. When stems or roots respond to gravity, auxin accumulates on the lower surface (in response to the movement of heavy starch-filled organelles), but the stem bends up whereas the roots turn down.
Self-Assessment Question 9
Give an example of how a plant’s ability to sense its environment improves the plant’s chances for survival and reproduction.
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Model Answer:
There are many examples of how a plant’s ability to sense its environment improves the plant’s chances for survival and reproduction. (1) Plants suited to growing in direct sunlight grow taller and branch less when light levels are low. This strategy allows the plant to put energy into reaching the light. (2) Roots elongate more and branch less when water is scarce. The root system is thus able to explore the soil, increasing the plant’s chance of encountering water reserves. (3) Exposure to wind results in shorter and stronger stems that help the plant withstand the mechanical stress of high winds. (4) Plants use day length as a cue to prepare for winter. For example, as a plant senses winter approaching because of the shortened length of daylight, it creates storage organs in the roots that will provide nutrients for the growth of new leaves and stems the following spring.