The Root This organ is the first to emerge from a seed.

Its functions are

Origin of the Root The primary root is the first root that forms in and emerges from the seed.

In most other plants, the primary root develops into a taproot, a large, central root from which lateral roots emerge.

  • Root depth depends on environment
  • Porous soil - deep roots; rocky soil - shallower roots
  • Temperature and soil water content also affect root depth.
  • And of course, different species have different root characteristics
  • The main "feeder roots" spread in the topmost soil layers, usually no deeper than about a meter.
  • Roots tremendously increase plant surface area where the plant needs it: for water absorption.
  • Root surface area is much greater than shoot surface area.
  • Root and shoot are balanced: If the roots are damaged, the shoot may die back.
    If the shoot is damaged, there's less photosynthate available for new root growth. In the most derived plants (monocots), the taproot is replaced by a system of fibrous roots that all emerge in a bunch at the base of the stem.

    Root Growth and Development Let's have a look at where it all begins: The Root Tip.

    Again, note the relative locations of the apical and three primary meristems.

    Can you see the quiescent center? Labeled with a radioactive nucleoide, the tip shows up on an autoradiograph with actively dividing nuclei showing dark (taking up the marker), and relatively inactive cells with invisible nuclei (not taking up marker).

  • The root cap is a protective cap of live parenchyma cells

    Root Meristems The apical meristem (from the Greek merismos, which means "division") is found at the very tip of the root, just behind the root cap.
    As in all meristems, the apical meristem contains some cells that will always remain meristematic: one daughter cell remains in the meristem (the initial) and continues to divide, whereas its sister cell (the derivative) stays behind as the meristem grows out. The derivative differentiates into some type of cell, depending on its gene expression. This is known as primary growth.

    The very end of the root tip contains the initials and the immediate derivatives, and is known as the promeristem. Note the relative locations of the apical and three primary meristems.

    (What's different about this quiescent center? Why?)

  • Region of Cell Division - the apical meristem
  • Region of Elongation - growing cells elongating (primary meristems)
  • Region of Maturation - cells mature, no longer elongating (mature tissues)

    Root Primary Structure Recall that any plant organ has three main layers:

  • epidermis (and derivatives)
  • vascular tissue
  • ground tissue

    Root Anatomy: Cross Sectional View

    From outermost layer to innermost:

    Root Epidermis Root epidermis is the surface that meets the environment, and it is the first selectively permeable membrane the plant uses to filter uptake.

    Surface area is increased by trichomes that form root hairs:

    These are found primarily in the Region of Maturation, and die off once the cells age. Although the cell walls contain suberin, water and minerals can pass easily between the cells of the epidermis, so further filtration is needed down the line.

    Mycorrhizae This is a symbiotic relationship between a fungus and a plant root. (What does each partner get out of the relationship?)

    Some of the most valuable edible organisms in the world are TRUFFLES, various species of mycorrhizal (ascomycete and basidiomycete) fungi that partner with plants.

    In mycorrhizal plants, root hair surface area is negligible compared to that provided by the interface of mycorrhiza, plant and fungus. Most absorption is done via the mycorrhizal hyphae.

    Recent research suggests that mycorhizzal associations may be a symbiotic partnership between not two, but three species, including special bacteria that live inside the gungus and are essential to the establishment of the symbiosis.

    The Cortex Just internal to the epidermis lies the cortex, composed primarily of parenchyma.

    Cortex plastids are primarily for storage (fats, carbs). Only in some species with photosynthetic roots (which types of plants would you expect these to be?) are there chloroplasts in these cells.

    In woody plants, the cortex is shed off once woody growth begins. In herbaceous plants, the cortex is maintained throughout the life of the plant.

    Most of the cortex is airy, with a lot of space (filled with fluid and or air) between the cells.

    Fluids travel via:

    (recall: the tonoplast is the continuous fluid pathway formed by the plasma membrane of the vacuoles)

    The innermost layer of the cortex is the endodermis, the main "filtration" surface of the root.

    The Casparian strips banding each endodermal cell are made of suberin (sometimes lignin, as well), and prevent interstitial entry of water into the stele (central core of vascular tissue). Thus, water cannot travel via the apoplast, and must pass through the selectively permeable plasma membrane of the endodermal cells before it reaches the vascular system.

    Pericycle This is a layer of pluripotent parenchyma cells located just inside the endodermis. Pericycle gives rise to side branch roots.

    The Stele Root morphology is fairly well conserved across plant taxa. Therefore, differences in the morphology of the stele can be an important tool for classifying plants and determining evolutionary relatedness.

    The stele consists (from outermost to innermost layers):

    The Variety of Roots

    Roots of various plant species have evolved various specializations.