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Biology Basics: Interaction among Species

Types of Tissue in Plants

Plant organs are made of plant tissues, which are made of plant cells. All plants have tissues, but not all plants possess all three of the following types of tissues:

  • Dermal tissue: Consisting primarily of epidermal cells, dermal tissue covers the entire surface of a plant.

  • Ground tissue: This tissue type makes up most of a plant’s body and contains three types of cells:

    • Parenchyma cells are the most common ground tissue cells. They perform many basic plant cell functions, including storage, photosynthesis, and secretion.

    • Collenchyma cells thicken their cell walls with extra cellulose to help support the plant.

    • Sclerenchyma cells are similar to collenchyma cells, but their walls are even thicker and reinforced with lignin, a tough molecule found in wood. The cell walls of sclerenchyma cells are so thick, in fact, that mature sclerenchyma cells die because they can’t get food or water across their walls via osmosis.

  • Vascular tissue: You can think of vascular tissue as the plant’s plumbing. The cells within xylem and phloem link up with one another end-to-end to form long columns of cells that carry nutrients around the plant and water up from the roots.

    • Xylem contains specialized cells called vessels and tracheids. These cells die at maturity, but their cell walls remain intact so that water can continue to flow. Vessel cells are wide and barrel-shaped, while tracheids are slimmer and have pointed ends.

    • Phloem contains sieve cells for transporting sugars. Sieve cells remain alive but lose their nuclei at maturity as they become specialized for sugar transport. Nearby companion cells retain their nuclei and support the function of the sieve cells.

    • Vascular tissue also contains parenchyma cells in the vascular cambium, a tissue of cells that can divide to produce new cells for the xylem and phloem.

Biologists use the appearance and feel of a plant’s stem to place it into one of two categories: herbaceous (the stem remains somewhat soft and flexible) and woody (the stem has developed wood). All plant cells have primary cell walls made of cellulose, but the cells of woody plants have extra reinforcement from a secondary cell wall that contains lignin.

Plants that survive just one or two growing seasons — that is, annuals or biennials — are typically herbaceous plants. Plants that live year after year, called perennials, may become woody.

The stems of herbaceous and woody dicots (plants whose seeds contain two cotyledons) are organized differently. You can see these differences most clearly if you look at a cross section (a section cut at right angles to the long axis) of a stem.

Imagine taking a hot dog and slicing it into little circles and you have a pretty good picture of how biologists make stem cross sections. If you did the same thing with a young dicot stem, you would see that

  • The stem’s center consists of pith (a soft, spongy tissue), which has many thin-walled cells called parenchyma cells. The thin walls allow the diffusion of nutrients and water among the cells.

  • The vascular tissue is organized in vascular bundles that contain both xylem and phloem, as well as some vascular cambium. The vascular bundles are arranged in a ring around the pith.

  • Outside the vascular bundle ring is the stem’s cortex. It contains a layer of endodermis, additional parenchyma cells, and supporting tissue like collenchyma cells to help support the plant’s weight and hold its stem upright.

  • On the stem’s surface are the epidermis and the cuticle, which is often covered with wax.

Woody dicots start life with green herbaceous stems that have vascular bundles. As they grow, however, the bundles merge with one another to form rings of vascular tissue that circle the stem. If you were to examine a cross section of the stem of a woody dicot that was a couple of years old, you’d see that

  • The very center of the stem consists of a circle of pith.

  • The xylem tissue forms a ring around the pith.

    • As woody plants grow, they add new layers of xylem every year, forming rings inside the woody stem. As these rings of xylem accumulate year after year, the woody stem’s diameter increases.

    • During the spring, when lots of water is available, xylem vessels are larger, whereas during the drier summers, xylem vessels are smaller. The alternation of larger and smaller vessels gives wood a ringed appearance. You can count these rings in a tree’s stem to tell how old it was when it was cut.

  • Just outside the xylem rings is a thin ring of vascular cambium that’s only one cell thick. As the stem grows, the vascular cambium divides to produce new xylem cells toward the inside of the stem and new phloem cells toward the outside of the stem.

  • Outside the vascular cambium ring is a ring of phloem. The phloem of woody plants gets pushed farther and farther outward as the xylem tissue increases in size year after year. Phloem cells are fairly delicate, and the old phloem cells get crushed against the bark as the stem grows. The only phloem that serves to transport materials through the woody plant is the phloem that’s newly formed during the most recent growing season. Phloem tissue is surrounded by strong cells called fibers, which are a type of sclerenchyma, and parenchyma cells that form the cortex.

  • Outside the phloem ring is the bark, a ring of boxy, waterproof cells that help protect the stem. Bark includes the stem’s outermost cells and a layer of cork cells just beneath that outermost layer. The cork cambium is a layer of parenchyma cells that divides to produce new cork cells, increasing the woody stem’s diameter.

Use the terms that follow to identify which type of tissue would perform the function in each question.

a. Parenchyma

b. Sclerenchyma

c. Collenchyma

d. Xylem

e. Phloem

  1. A leaf cell does photosynthesis.

  2. The cells that make the strings in celery thicken their cell walls with extra cellulose.

  3. Sieve cells connect end to end to transport sugary sap through a tree trunk.

  4. The cells that make the gritty texture in pears thicken their cell walls with lignin.

  5. Hollow, open-ended cells called vessels conduct water through a flower stem.

The following are the answers to the practice questions.

  1. The answer is a. Parenchyma.

  2. The answer is c. Collenchyma.

  3. The answer is e. Phloem.

  4. The answer is b. Sclerenchyma.

  5. The answer is d. Xylem.

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