13 Shoot of Plant

The Shoot of a Plant

  • Definition: The shoot of a plant consists of the stem and leaves, forming the aboveground portion of the plant.

  • Composition in Early Growth:

    • Plumule: The shoot tip or the stem of the seedling from which the shoot grows.

    • Epicotyl: The part of the seedling stem above the cotyledons.

    • At least one young leaf: Initial leaves that contribute to photosynthesis.

    • Apical meristem: The region of actively dividing cells at the tip of the shoot.

    • Complexity: Shoots are generally considered more complex than roots due to the presence of nodes (locations of leaves' attachment) and internodes (segments between nodes).

Roles of the Shoot

  • Primary Functions:

    • Support: Keeps leaves elevated for photosynthesis.

    • Conduction: Movement of nutrients and water.

  • Transport Mechanisms:

    • Phloem: Transports substances manufactured in leaves down through the stem.

    • Xylem: Carries water and mineral nutrients from roots to shoots.

  • Photosynthesis Implication:

    • The longer the shoot, the closer it is to light, enhancing photosynthesis efficiency.

Primary Tissue Growth

  • Apical Meristem Functions:

    • Shoot Elongation: Enables growth in the shoot’s length.

    • Production of Leaf and Bud Primordia:

    • Leaf Primordia: Structures that will develop into leaves.

    • Bud Primordia: Structures that will develop into lateral shoots.

  • Phytomeres: Repetitive units of growth produced as the shoot elongates.

  • Protection: The shoot does not have a root cap like the roots; it is instead covered by young leaves for protection.

Important Distinction

  • Shoot Apex vs. Shoot Apical Meristem:

    • Shoot Apical Meristem: The meristematic (actively dividing) part of the apex above leaf primordia.

    • Shoot Apex: Comprises the shoot apical meristem and the subapical region containing leaf primordia.

Tunica-Corpus Organization

  • Characteristics:

    • Most flowering plants exhibit a structure called tunica (outer layer) and corpus (inner layer).

    • Division of Tunica: Anticlinal (perpendicular to surface), contributing to surface growth without increasing cell layers.

    • Division of Corpus: Occurs in multiple planes, forming the body of the developing shoot.

  • Species Variation:

    • The number of tunica layers varies by species.

    • The total number of meristem initials equals the number of tunica layers plus one (the corpus).

    • Most eudicots typically have two tunica layers and one corpus layer.

The Shoot Apex

  • Structure:

    • Central region comprises layers of tunica overlaying the corpus.

    • Promeristem: A term used to describe the shoot apical meristem as a whole.

  • Cell Division Dynamics:

    • Central zone experiences infrequent cell division, akin to the quiescent center of the root apical meristem.

    • Peripheral Zone: Highly mitotically active region flanking the central zone, responsible for rapid growth.

Growth of the Stem

  • Difference from Roots:

    • No distinct regions of cell division, elongation, and maturation observed in stems as seen in roots.

    • Leaf primordia form quickly, and nodes and internodes become distinguishable as growth advances.

  • Increasing Stem Length:

    • Mainly due to internodal elongation occurring simultaneously across multiple internodes.