Biology

Plant tissues & Organs

Big Picture

Plants have specialized cells not found in animal cells, such as cells that carry out photosynthetic functions. A group of the same type of cells doing the same job makes up a tissue. Vascular tissues, which transport fluids, are one type of tissues found in vascular plants. More than one type of tissue working together to do specific jobs form an organ. The parts of a plant that we can easily identify - roots, stems, and leaves - are examples of plant organs.

Key Terms

Parenchymal: Type of plant cell that carries out photosynthesis, cellular respiration, and/or storage functions.

Collenchymal: Type of plant cell that functions to support the plant.

Sclerenchymal: Type of plant cell that functions to strengthen and support the plant.

Xylem: Type of vascular tissue in a plant that transports water and dissolved nutrients from roots to stems and leaves.

Phloem: Type of vascular tissue in a plant that transports food from photosynthetic cells to other parts of the plant.

Meristem: Type of plant tissues consisting of undifferentiated cells that can continue to divide and differentiate and from which plants grow in length or width.

Primary Growth: Plant growth in length.

Secondary Growth: Plant growth in width.

Taproot: Single, thick primary root that characterizes the root system of some plants.

Fibrous Root: Threadlike root that makes up part of the fibrous root system of some plants.

Root System: All the roots of a plant, including primary roots and secondary roots.

Root Hair: Tiny hair-like structure that extends from a plant root.

Bark: Tissue that provides a rough, woody external covering on the stems of trees.

Stomata (singular, stoma): Tiny pore in the epidermis of a plant leaf that controls transpiration and gas exchange with the air.

Mesophyll: Specialized tissue inside plant leaves where photosynthesis takes place.

Cuticle: Waxy, waterproof substance produced by cells on the surface of leaves, shoots, and other above-ground parts of plants.

Deciduous Plant: Type of plant that seasonally loses its leaves.

Evergreen Plant: Type of plant that keeps its leaves and stays green year-round.

Plant Cells

Some structures found in plant cells but not in animal cells
are:

  • Large central vacuole: Contains water and dissolved substances. Helps give the cell shape and support the plant.
  • Cell wall: Located outside the cell membrane. Shapes, supports, and protects the cell.
  • Plastids: Organelles with their own DNA. Examples include chloroplasts, which contain the green pigment chlorophyll.

There are three different types of basic plant cells:

  • Parenchymal: Contains chloroplasts. Carries out photosynthesis and cellular respiration. Stores organic molecules.
  • Collenchymal: Elongated cells that provide support.
  • Sclerenchymal: Cells with very thick walls that also provide support. More rigid than collenchymal cells.

All three of these cell types are found in most plant tissues.

Plant Cells
Image credit: Jin Yu, CC-BY-NC-SA 3.

Biology

Plant tissues & Organs cont.

Plant Tissues

Three major plant tissues are:

  • Dermal: Dermal tissue is the layer of cells covering the external side of plants(called epidermis),and it controls the plant’s interactions with the environment.
  • Ground: Ground tissue makes up most of the interior of plants and carries out basic metabolic functions. Some ground tissues also act to store food and water(example: ground tissue in the roots).
  • Vascular: Vascular tissue, which is also an interior tissue of the plant, consists of xylem and phloem.
    Xylem and phloem, which are packaged together in bundles, transport fluids throughout the plant.
Plant Tissues
Image credit: Zephyris, CC-BY-SA 3.0

Plant Growth

Plant growth relies on a type of plant tissue called meristem tissue. Most plants continue to grow as long as they live, and they do so by growing and then dividing and differentiating the undifferentiated meristem tissue cells. Growth exists as either primary growth or secondary growth.

Plant Organs

The three major specialized organs of most vascular plants include roots, stems, and leaves

Roots

Most vascular plants have a root system where the primary roots grow downward and secondary roots branch out to the side. There are two basic types of root systems: taproots and fibrous roots. Roots generally function to absorb water and minerals, store food, and anchor the plant. Being such complex organs, roots have several different types of tissues that are organized according to their function.

  • Roots have both primary and secondary meristem tissues for growth
  • At the tip of the root is the root cap, which has gravity-detecting cells that allow the roots to always grow down toward the ground.
  • Root hairs on the surface of roots increase the surface area for absorption.
Plant Organs
Image credit: Cehagenmerak, GNU-FDL 1.2
Plant Organs
Image credit: JoJan, GNU-FDL 1.2
Figure: Diagram of a plant root.

Biology

Plant tissue cont.

Plant Organs cont.

Stems

In vascular plants, stems hold the plants up to get the air and sunlight needed for survival. Leaves, flowers, cones, and secondary stems can grow out of the stem at branching points called nodes. The stem also makes reproductive parts more accessible to pollinating agents. Stems transport water and minerals from the roots to the leaves of a plant (sites of photosynthesis). The products of photosynthesis travel down vascular tissue in the stem so that they can be used in certain parts of the plants or stored in the roots.

  • In trees, some of the tissue found near the surface of the stem is replaced with bark, which gives trees the tough, woody covering.
  • The outer part of bark provides strength, support, and protection. Only the outer part of the bark is dead. The inner part of bark is alive and growing
  • Stems have primary meristem at the tips and at nodes that allow them to grow longer. Secondary meristem are located in and around the vascular tis-sues and allow stems to grow thicker.

Secondary growth forms new vascular tissues and bark. This creates the rings seen in tree trunks.

Leaves

Leaves work primarily to collect sunlight and to carry out photosynthesis. The variety of leaves helps different plants thrive in a wide variety of environments. Like all other plant organs, leaves also have their own specialized tissues. Most of these special types of cells exist to maximize the photosynthetic functions of the leaves.

  • The stomata of leaves are important pores that, with the help of guard cells, maintain the transpiration of water as well as carbon and oxygen exchange.
  • Photosynthesis takes place in specialized mesophyll tissues.
  • The cells on the surface on leaves secrete cuticle to prevent damage and loss of water by evaporation.
  • Some plants lose their leaves, some don’t
  • The leaves on deciduous plants change color in autumn and fall off during winter to reduce water loss during the cold and dry season each year. New leaves grow when spring arrives.
  • Evergreen plants keep their leaves all year long. The leaves are usually needle-like with thick cuticles.

Notes