Biology Semester II

Sections:

Introduction |  Section 1 | Section 2 | Section 3

Section One:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8

Biology: Plant Organs, Tissues, and Cells: Part Eight Secondary Growth Some plants germinate from seeds, grow, reproduce, and die within a single growing season. (Each winter in the Arizona desert, the normally brown desert mountains turn green and yellow from rapid plant growth.) Other plants persist in the environment from year-to-year. Often, these persistent plants produce a type of xylem commonly called wood. Wood consists of secondary xylem cells produced from a cambium. You may recall that a cambium is a lateral (as opposed to apical) meristem. As cambium cells divide and differentiate into rows of cork cells, secondary xylem cells, and secondary phloem cells, the plant increases its girth (not its height). Secondary xylem cells form the majority of what we call wood, a valuable construction tool and fuel resource. The increased strength of wood allows the plant to grow taller and compete for sunlight. The cork cells (produced from a cork cambium) become part of the epidermis and make up the bark of woody stems. How does the process of secondary growth occur in dicots? Recall that the vascular bundles are arranged in a ring around the dicot’s stem. Each bundle is made up of xylem cells and phloem cells, with a thin layer of living cambium cells in between. The vascular cambium grows outward from the vascular bundles until it creates a thin ring of cambium around the stem. Secondary xylem is produced to the inside of the cambium, and secondary phloem is produced to the outside. Most of the cambium cell divisions produce secondary xylem and secondary phloem. Occasionally, as the plant stem increases in girth, the cambium will divide to produce new cambium cells. Secondary xylem cells, like primary xylem cells, are dead by the time they become functionally mature and begin to transport water and minerals upwards from the roots. The living parts of the woody plant are next to the vascular cambium. The formation of the vascular cambium cylinder by growth between the separate vascular bundles of the primary dicot stem to produce a mature vascular cambium. Note the primary growth in cross section of a young Tilia (basswood) stem. Action of the vascular cambium producing new xylem cells to the inside and new phloem cells to the outside of the stem. Many woody plants live in areas that have definite growing seasons separated by periods of no growth or even dormancy. These seasonal differences in climate create visible differences in the wood produced during those different growing seasons. The most obvious effect is in what is called the grain of the wood. When viewed in cross section, the stem of a woody plant shows a definite series of growth rings. These alternating patterns of light and dark wood are created as plant growth slows and stops at the end of one growing season and resumes (at a quicker pace) at the beginning of the next season. Toward the end of the growing season, the vascular cambium stops growing. The secondary xylem cells that are produced are relatively small and have thick cell walls. These smaller cells are called summer or late wood. At the beginning of the next growing season, the new secondary xylem cells produced are larger so they can transport a great deal more water. These larger cells are called spring or early wood. Late wood appears as a darker ring and early wood appears as a lighter ring in the stem cross-section. One-, two-, and three-year-old twigs of Tilia. Note the annual growth rings and the complete vascular cylinder producing secondary xylem (a light violet) to the inside of the stem. Secondary phloem (and fibers, shown in this figure as a darker purple without evident growth rings) is produced to the outside of the stem. The approximate age of a tree can be determined by counting the growth rings outward from the pith to where the cambium was when the tree was cut. Often in the classroom laboratory, students will view slides containing one-, two-, and three-year-old sections of a woody twig. In contrast, the oldest trees are over 4500 years old. Imagine the task of counting all of those rings! The science of dendrochronology uses tree rings to date archaeological sites. Paleobotanists (scientists who study fossil plants) use growth rings to infer past climates and the existence of defined growing seasons. A growth ring in Tilia. The growth ring consists of a ring of small cells formed at the end of one growing season and larger cells produced at the beginning of the next growing season. Few monocots produce secondary growth. Bamboo and palm trees are monocots that do have a special type of secondary growth. In these monocots, new vascular bundles are formed at the edge of the stem. These new bundles are close together, providing support for the stem. In the case of palm trees, this process produces a wood that is softer than dicot wood. Bamboo, however, is a surprisingly strong wood that is used for construction scaffolding in China and other countries. Practice what you've learned. Plant Organs, Tissues, and Cells Quiz 25 points Congratulations on completing this section! In this section, you learned about: Roots and stems Parenchyma, Collenchyma, and Sclerenchyma Xylem and Phloem Epidermal Cells Monocot and Dicot Structures Secondary Growth Now it’s time to take the section quiz. Please make sure to check your understanding of the topics above before proceeding to the quiz. After you have completed the quiz, continue with the unit. Now go on to the next section.

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