BSCI 124 Lecture Notes

Undergraduate Program in Plant Biology, University of Maryland

LECTURE 2 - Introduction to Plants

  1. Plant Physiology
    1. Plants are photosynthetic -- they gather their food energy directly from sunlight
    2. To perform photosynthesis, plants need to have a supply of:
      1. Sunlight
      2. Carbon dioxide gas from the atmosphere
      3. Water
      4. Mineral nutrients
    3. During photosynthesis, plants release Oxygen, but they need to use oxygen at night and in parts of the plant (like the roots) that do not perform photosynthesis.
    4. The structure of a plant is adapted to gathering the things that the plant needs.
  2. Plant Structure - the external structure of a generalized plant: the plant body is divided into two basic parts, the shoot, which usually grows above ground, and the root, which usually grows below ground.
    1. Shoot
      1. Shoots are made of leaves attached to a stem.
      2. Leaf (singular; plural is leaves)
      3. Leaves are often the primary site of photosynthesis.
        1. Leaf Blade - large, flat part of leaf that collects sunlight.
        2. Petiole - narrow stick that holds leaf blade away from the stem.
        3. Leaf Base - slight broadening at base of petiole, protects bud when leaf is young, and may include stipules.
        4. Simple Leaves - leaf blade is composed of only one piece.
        5. Compound Leaves - leaf blade is divided into two or more leaflets. In some plants the leaflets of compound leaves look very similar to leaves, but only true leaves have buds in their axils (see below).
      4. Stem
        1. Holds leaves, transports and stores water and nutrients, and is sometimes photosynthetic.
      5. Nodes
        1. A node is the place on the stem where a leaf is attached
      6. Internode
        1. An internode is the piece of stem between two nodes
      7. Buds
        1. Buds are can develop into new shoots. Buds are named according to where they occur on the shoot. Each shoot has an apical bud at the tip of the shoot, as well as an axillary bud associated with each leaf.
        2. Apical Bud
          1. The apical bud is found at tip (or "apex") of the shoot.
          2. This is the point from which the shoot will grow.
        3. Axillary Buds
          1. The angle between the leaf and stem is called the axil.
          2. There is normally a bud in the axil of each leaf
          3. Axillary buds are of elongating into a new shoot (a branch). Therefore there will be a leaf (or a leaf scar) below each branch on a stem. In trees the leaf scars disappear over time, but on young branches of trees the leaf scars are easy to find.
          Dioscorea sp.; Puerto Rico Populus tremuloides; California Ulmus americana; Wisconsin
          The growing tip of a shoot; the shoot apical meristem, a developing leaf, and two mature leaves are visible. Note the leaf blade and petiole of the leaves. An aspen shoot, showing leaves with leaf blades and petioles. A shoot from an elm tree. Axillary buds are clearly visible. The structures to the left are elm fruits.
    2. Root
      1. The root anchors the plant in the soil, absorbs water and mineral nutrients from the soil, and often serves for storage.
      2. Roots are underground, so people don't think very much about them, but they are very important.
      3. Branch Roots
        1. Roots do not have leaves or axillary buds
        2. Branch roots emerge from the inside of the root
      4. Root Hairs
        1. Absorption
        2. Found just behind growing tip of root
      5. Root Cap
        1. Protects the delicate tip of the root as it grows through the soil.
        2. Found in front of the root apical meristem.
    3. Specialized Structures
      1. Some plant parts are greatly modified, and these may look very different from the "typical" plant part. For example:
        1. Cactus spines are leaves, modified to protect the plant.
        2. A potato is an underground stem, modified for storage.
        3. A radish is a root, modified for storage.
        4. Some orchids perform photosynthesis only with their (above-ground) roots.
      2. Structures that look the same to us may be made of different plant parts in different kinds of plants. For example, spines, thorns, and prickles are all sharp plant parts that protect the plant from herbivores. Depending upon the kind of plant, spines may be made from many different plant parts:
        1. Leaves - e.g., cacti
        2. Stipules - e.g., Euphorbia (African plant that looks very much like a cactus)
        3. Shoots - e.g., buckthorn
        4. Epidermal Hairs - e.g., roses
        5. Roots - e.g., some tropical trees
        6. And others...

  3. Plant growth is fundamentally different from animal growth.
    1. Animal growth is determinate; an animal embryo develops into a young animal, then an adult. In other words, the overall shape of the adult animal is genetically determined from its earliest developmental stages. Once an animal has become an adult, it may become heavier or fatter, but it will not become larger.
    2. Plant growth is often indeterminate; even an adult plant retains tiny regions of embryonic tissue called meristems that are capable of developing into new parts of the plant. Although the plant does grow according to a set of rules (similar to a fractal), the plant is growing new shoots and roots for as long as it is alive. Thus the overall shape of the plant is not determined in advance, and the growth is said to be indeterminate.
    3. Meristems are regions of embryonic tissue capable of growing into new plant parts. Meristems are found in both roots and shoots.
      1. Primary meristems make the shoot or root grow longer. This kind of growth is called primary growth.
        1. A shoot apical meristem is found within each bud.
        2. A root apical meristem is found at the tip of each root, and is protected by the root cap.
      2. Secondary meristems make the stem or root grow larger in diameter. This kind of growth is called secondary growth. Not all kinds of plants are capable of secondary growth. Secondary growth gives rise to wood, and plants that are not capable of secondary growth do not develop wood.

  4. Plant Reproduction
    1. Asexual Reproduction
      1. Because plant growth is indeterminate, each meristem can potentially develop into a complete plant. This means that it is very easy to clone plants, and many plants can grow from cuttings or broken plant parts. This is asexual reproduction (also called vegetative reproduction).
    2. Sexual Reproduction
      1. Alternation of Generations - plant sexual reproduction is unusual, and involves an alternation between two partially independent life stages. We will discuss this later in the course.
      2. Flowers are special reproductive structures found in the Flowering Plants (=Angiosperms)
      3. A flower is a specialized shoot, adapted for sexual reproduction.
      4. A fruit develops from a flower following fertilization.
      5. Other plants perform sexual reproduction, but do not use flowers, and do not form fruit.

    Return to main page

    January 27, 1998 Delwiche