Study Guide: Anatomy of Flowering Plants

ANATOMY OF FLOWERING PLANTS

TISSUE SYSTEMS OF PLANTS

  • Plant Organs and Tissue Types: Roots, stems, and leaves are considered the plant's organs, composed of three distinct tissue types:

    • Dermal:
    • Definition: A single layer of tightly packed cells covering the exterior of young plants.
    • Functions:
      • Protection of the plant.
      • Acts as an absorptive interface (as in roots).
      • Serves as a watertight barrier (as in stems and leaves).
    • Vascular:
    • Definition: A system of two distinct tissue types.
      • Xylem:
      • Description: A complex tissue made up of several cell types.
      • Function: Conducts water and dissolved solutes in a unidirectional flow from the roots to the leaves.
      • Phloem:
      • Description: A composite tissue.
      • Function: Transports the products of photosynthesis from the site of production (mainly in mature leaves) to roots and non-photosynthetic parts.
      • Note: Sieve elements of phloem are living cells that utilize cellular energy for bidirectional material movement.
    • Ground:
    • Definition: The remainder of the plant tissue that is neither dermal nor vascular.
    • Functions:
      • Supports the plant's structure.
      • Sites for photosynthesis and storage of products.

  • Differences in Vascular Tissue Distribution:

    • The distribution of vascular tissue in roots, stems, and leaves varies between monocots and dicots.
    • Knowledge of these differences is essential for practical examinations.

PLANT ANATOMY

A. ROOT ANATOMY

  • Examination of a cross section of the root of an herbaceous dicot, Ranunculus sp.:

    • Epidermis:
    • Description: Outermost protective layer; single cell layer of dermal tissue.
    • Function: Site of water and mineral absorption.
    • Cortex:
    • Description: Large region of ground tissue adjacent to the epidermis, composed of loosely packed parenchyma cells.
    • Features of Parenchyma Cells:
      • Relatively unspecialized with thin, flexible cell walls.
      • Major site of metabolic activity within the plant; contain starch-storing plastids.
    • Endodermis:
    • Description: A single layer of rectangular cells that define the innermost boundary of the cortex.
    • Function: Acts as a selective barrier regulating mineral passage from cortex into vascular tissue.
    • Stele:
    • Definition: The central core of the dicot root.
    • Pericycle:
      • Description: Layer of cells with potential meristematic activity underlying the endodermis.
      • Function: Can give rise to lateral roots, maintaining connection with vascular tissue for water and mineral conduction.
    • Xylem:
      • Description: Star-shaped bundle of large, hollow cells.
      • Function: Conducts water; consists of tracheids and vessel elements that die and lose contents after maturation, acting as conduits.
    • Phloem:
      • Description: Composed of sieve-tube members and companion cells.
      • Functions:
      • Sieve-tube members act as conduits for bidirectional movement of organic material.
      • Companion cells support sieve-tube member activities.

  • Examination of a cross section of the root of a monocot, Zea mays:

    • Structures and functions similar to those in the dicot root, but with differences:
    • Xylem not centrally located; forms a ring surrounding central parenchyma mass (pith).
    • Small phloem bundles form a separate ring outside xylem.

  • Examination of a longitudinal section of root tip of a monocot, Allium cepa:

    • Root Cap:
    • Description: Zone of loosely packed cells at the extreme tip of the root.
    • Apical Meristem:
    • Description: Tightly packed cells in the zone of cell division; numerous cells in various stages of mitosis.
    • Zone of Elongation:
    • Description: Region behind apical meristem; contains elongating cells that push root forward.
    • Root Hairs:
    • Description: Epidermal extensions increasing surface area for absorption.
    • Distinction: Be able to distinguish between stele and cortex.

B. STEM ANATOMY

  • Transition from Root to Shoot:

    • Vascular tissue in the root splays into vascular bundles in stems with different distributions in monocots and dicots.

  • Examination of cross section of a young dicot stem, Helianthus sp.:

    • Epidermis:
    • Description: Outermost layer.
    • Vascular Bundles:
    • Distribution: Form a ring within the ground tissue.
    • Ground Tissue Components:
      • Cortex:
      • Collenchyma and parenchyma cells located between the bundles and epidermis.
      • Pith:
      • Region of parenchyma cells at the center of the stem.
      • Functions of Parenchyma Cells:
      • Nutrient storage and maintaining turgor for stem support.
      • Cambium:
      • A thin layer of meristematic tissue between xylem (interior) and phloem (exterior).
      • Function: Active area of cell division and differentiation leading to new xylem and phloem formation.
    • Sclerenchyma Cells:
      • Location: Form a distinct fiber cap outside the vascular bundles.
      • Function: Provide support due to secondary cell walls embedded with lignin.

  • Examination of a monocot stem, Zea mays:

    • Epidermis surrounds uniform ground tissue with scattered vascular bundles.
    • Vascular tissue has xylem toward the interior and phloem toward the exterior, without pronounced pith and cortex distinction.
    • Most monocots show absent cambium.

  • Examination of woody dicot stem, Tilia sp.:

    • Secondary growth described:
    • Allows the plant to grow wider as well as taller.
    • Cambium and Secondary Growth:
    • Cambium produces new xylem and phloem, leading to growth rings seen in cross-sections.
    • Seasonal growth leads to different sizes of xylem cells resulting in annual rings.
    • Early wood (spring growth) has larger cells; late wood (summer/fall) has smaller cells.
    • Dormancy in winter leads to inactivity and formation of distinct rings.
    • Xylem and Phloem Rays:
    • Xylem rays radiate outward from the edge of the pith; some expand as phloem rays, acting as conduits for lateral flow of nutrients and water.

  • Formation of Cork Cambium for Secondary Growth:

    • Outer cortex parenchyma cells turn meristematic, forming cork cambium.
    • Results in cork cell production, leading to decreased water permeability and enhanced resistance to damage.
    • Periderm:
    • Combination of cork cells and cambium.
    • Bark:
    • Encompasses all living and dead cell layers, including secondary phloem and periderm.

C. LEAF ANATOMY

  • Examination of prepared leaf cross sections:
    • Dicot (Syringa sp.): and Monocot (Lilium sp.):
    • Distinct layers of cells:
    • Upper and Lower Epidermis:
      • Form of dermal tissue, resistant to damage and disease.
      • Cuticle covers outer walls to retard water loss.
    • Stomata and Guard Cells:
      • Stomata allow gas exchange; guard cells regulate opening based on turgor pressure.
    • Mesophyll Layers:
      • Palisade Parenchyma:
      • Upper portion; cylindrical cells, site of major photosynthesis.
      • Spongy Parenchyma:
      • Lower portion with irregularly spaced cells for gas exchange.
    • Vascular Bundles (Veins):
    • Continuous with those of the stem.
    • Function: Transport products of photosynthesis to the rest of the plant and facilitate ongoing processes.
    • Xylem brings water and minerals into the mesophyll, while phloem carries organic materials out.