Plant Structure and Function


 
The "Typical" Plant Body

The Root System

  • Underground (usually)
  • Anchor the plant in the soil
  • Absorb water and nutrients
  • Conduct water and nutrients
  • Food Storage
The Shoot System
  • Above ground (usually)
  • Elevates the plant above the soil
  • Many functions including:
    • photosynthesis
    • reproduction & dispersal
    • food and water conduction
  • Note: the shoot system includes the leaves and the reproductive organs, although these will be covered in more detail separately


Before we look at plant anatomy in detail, I want to caution you that we will be looking almost exclusively at Angiosperms, also know as flowering plants.  Angiosperms are by far the most diverse group of plants known (over 275,000 named species and thought to be at least that many more unknown to science).

Within the Angiosperms, there are two plant groups, the Monocots and the Dicots.  The distinction between these two groups is not always clear, but some general trends are outlined below:
 

  Monocots Dicots
Floral Arrangement 3's 4's and 5's
Leaf Venation Parallel Net
Vascular bundles Scattered Ring
Habit Herbaceous Herbaceous + Woody
Roots Fibrous Taproot
Growth Primary only Primary and Secondary
Examples: Grass, Palm, Orchid Oaks, Roses, Sunflowers

Organization of Plants and Animals - Cells, Tissues, Organs, Organ Systems

Plant and animals have a hierarchy of cellular architecture

Cell Types in the Plant Body

Parenchyma Cells Collenchyma Cells Sclerenchyma Cells

Some Tissues in the Plant Body

Xylem Phloem Dermal Tissue Ground Tissue

Plant Growth

Plant growth is a phenomenon different from animal growth.
Animas exhibit a growth pattern called determinate growth. Plants, however, exhibit a growth pattern called indeterminate growth Meristems

The pattern of plant growth depends upon the location of meristems

Apical meristems

Lateral meristems

Primary Growth in the Root

Root Anatomy - Dicot Roots

Epidermis

Cortex Endodermis Pericycle Vascular Tissue Root Anatomy - Monocot Roots

Epidermis

Cortex Endodermis

Pericycle

Vascular Tissue Pith

Primary Growth of Shoots

Apical Meristem

Axillary Meristems

Secondary Growth

Lateral Meristems add girth by producing secondary vascular tissue and periderm

Vascular Cambium

Secondary growth begins with the initiation of the vascular cambium, a cylinder of meristematic tissue that produces additional xylic and phloic tissues. The cells that eventually form the vascular cambium come from two sources, the procambium in the vascular bundles and the interfascicular parenchyma cells between vascular bundles. The diagram below shows the positions of these two populations of cells in a stem with only primary growth.

The two populations of dividing cells unite to form a continuous ring of dividing cells, the vascular cambium.

If we look closely at the cells of the vascular cambium we see two patterns of division. Initial cells can undergo multiplicative divisions (red line in the following diagram) or they can undergo additive divisions (blue line). Multiplicative divisions produce more initial cells and result in the increased circumference of the vascular cambium. Of the two cells produced from an additive division one is retained as an initial cell that will divide again, and the other will become a phloem mother cell or a xylem mother cell. These mother cells will differentiate into their respective cell types.



Secondary Growth in Dicots - Herbaceous and Woody
 

The Cork Cambium and the Production of Periderm

During secondary growth, the epidermis produced by primary growth splits and falls off the stem
It is replaced by a new protective tissues produced by the cork cambium

The cork cambium + the cork are known as the periderm
The "bark" of the tree consists of the periderm + the phloem Unlike the vascular cambium which can grow in diameter via multiplicative growth, the cork cambium is fixed in size.

The Monocot Stem - A Stem Lacking Secondary Growth

Monocot stems differ from dicot stems in that they lack secondary growth