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

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:
 

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

Plant and animals have a hierarchy of cellular architecture

• At the lowest level are cells
  • Example:  Parenchyma, Sclerenchyma, vessel elements
• Cells are organized together to form tissues
  • Example:  xylem, phloem
• Tissues are organized together to form organs (two or more tissues performing specific functions)
  • Example: Leaves, stamens
• Organs are organized together to form organ systems
  • Example: Flowers, shoots

Cell Types in the Plant Body

• Least specialized plant cells
• Thin and somewhat flexible cell walls
• Living at maturity
• Carry on most of the plant's metabolic functions
• Generally have a large central vacuole
• Most parenchyma cells have the ability to differentiate into other cell types under special conditions
  • During repair and replacement of organs after injury

• Thicker primary cells walls (usually with uneven thickness)
• Living at maturity
• Role in support of herbaceous plants
  • Example - the "strings" of celery

• Thick secondary cell walls
• Dead at functional maturity
• Cannot increase in length - occur in parts of the plant which have quit growing in length
• Two types - fibers and sclerids
  • Fibers - long, slender cells with a more or less regular secondary cell wall
    • Example - hemp fibers for making rope
  • Sclerids - shorter cells with an irregular shape
    • Example - stone cells in pears and hard nut and seed shells

Some Tissues in the Plant Body

Vascular Tissue - Xylem and Phloem

• Involved in conduct of water and ions in the plant
• Typically composed of non-living conductive cells and living parenchyma cells
  • Conductive cells have thick secondary cell walls, often deposited unevenly in a coil-like pattern so that they may stretch
  • Dead at functional maturity
  • Two types of conductive cells- tracheids and vessels
    • Tracheids - long, slender cells connected to each other by pits.  Found in all vascular plants
    • Vessels - shorter, larger diameter cells with completely perforated cell wall ends.  Found only in Angiosperms
• Xylem Parenchyma arranged in rays
• Xylem parenchyma are also distributed throughout the tracheids and vessel elements 

• Involved in transport of sucrose, other organic compounds, and some ions
• Conductive cells living at functional maturity
  • Protoplast may lack organelles and nucleus, though
• Endwalls connect to each other via sieve-plates
• Two types of conductive cells in the phloem - sieve-tube members and companion cells
  • Sieve-tube members - actual conduit for sucrose transport
  • Companion cells - has a nucleus that may also control the sieve-tube element and may aid in sucrose loading
• Sclerenchyma tissue is often associated with phloem

• Generally a single layer of cells
• The "skin" of the plant
• Primarily parenchyma cells 
• Main role is protection of the plant

• Makes up the bulk of the plant
• Predominately parenchyma, but collenchyma and sclerenchyma cells are found
• Diverse functions including photosynthesis, storage, and support
• Pretty much, if you look at something and it isn't dermal tissue nor is it vascular tissue, it is ground tissue

Plant Growth

• After fertilization, the zygote cells are rapidly dividing, undifferentiated cells
• However, after a certain critical stage, the cells differentiate and form tissues.
  • From this point onward, their developmental fate is sealed
  • There are exceptions to this (i.e. stem cells in bone marrow)
• Most animals have a pre-programmed body plan (i.e. barring mutation or accident, most humans have 10 fingers and toes, two eyes, a heart with four chambers, etc..)
• Most animals quit growing after a certain age

• The plant retains areas where rapidly dividing, undifferentiated cells remain all through the life of the plant
• These areas are called meristems
  • Meristematic tissue continues to rapidly divide producing undifferentiated cells which may eventually differentiate to form the tissue and cell types discussed above
  • They are similar in function to stem cells in animals
• Plants do not have a pre-programmed body plan
  • There are constants like leaf shape and branching patters (opposite, alternate, etc.) but you can never predict where a new branch will come about on a tree...
• Plants continue to grow throughout their life

The pattern of plant growth depends upon the location of meristems

Apical meristems

• located at the tips of roots and shoots
• supply cells for the plant to increase in length (grow up for shoots and down for roots)
  • growth in this direction is known as primary growth
  • primary growth found in herbaceous and woody plants
  • primary growth found in monocots and dicots

Lateral meristems

• located near the periphery of the plant, usually in a cylinder
• supply cells for the plant to increase in girth
  • growth in this direction is known as secondary growth
  • found in all woody and some herbaceous plants
  • lateral meristems and secondary growth found only in dicots

• Root Cap
  • Thimble-like covering which protects the delicate apical meristem
  • Produced from cells derived from the root apical meristem
  • Secretes polysaccharide slime that lubricates the soil
  • Constantly sloughed off and replaced
• Apical Meristem
  • Region of rapid cell division of undifferentiated cells
  • Most cell division is directed away from the root cap
• Quiescent Center
  • Populations of cells in apical meristem which reproduce much more slowly than other meristematic cells
  • Resistant to radiation and chemical damage
  • Possibly a reserve which can be called into action if the apical meristem becomes damaged
• The Zone of Cell Division - Primary Meristems
  • Three areas just above the apical meristem that continue to divide for some time
    • Basically, there is a slight specialization of the meristematic region
  • Protoderm - outermost primary meristem - produces cells which will become dermal tissue
  • Ground meristem - central primary meristem - produces cells which will become ground tissue
  • Procambium - innermost primary meristem - produces cells which will become vascular tissue
• The Zone of Elongation
  • Cells elongate up to ten times their original length
  • This growth pushes the root further downward into the soil
• The Zone of Maturation
  • Region of the root where completely functional cells are found

Epidermis

• Dermal tissue
• Protection of the root

• Ground tissue
• Storage of photosynthetic products
• Active in the uptake of water and minerals

• cylinder once cell thick that forms a boundary between the cortex and the stele
• contains the casparian strip, which will be explained later when we discuss water uptake

• found just inside of the endodermis
• may become meristematic
• responsible for the formation of lateral roots

• Xylem and Phloem
• Forms an X-shaped pattern in very center of root

Epidermis

• Dermal tissue
• Protection of the root

• Ground tissue
• Storage of photosynthetic products
• Active in the uptake of water and minerals

• cylinder once cell thick that forms a boundary between the cortex and the stele
• even more distinct than dicot counterpart
• contains the casparian strip, which will be explained later when we discuss water uptake

Pericycle

• monocot roots rarely branch, but can, and this branch will originate from the pericycle

• Xylem and Phloem
• Forms a ring near center of plant

• Center most region of root

Water flowing through the apoplast contains many minerals that the plant needs - it may also contains toxins and substances that the plant may not want.  However, since the water is flowing through the apoplast, there is no way to prevent the passive transport of these toxins, until the water hits the endodermis.

Endodermis

Cells of the endodermis possess cell walls that are ringed by the Casparian Strip, a waxy layer (composed of suberin).

• The Casparian Strip is a wax and therefore prevents the apoplastic flow of water
• Water must pass through the plasma membrane and enter the symplast
• The plasma membrane of the endodermal cells contain many transport proteins to actively transport some molecules in and others to pump other molecules out
• Once water passes under the Casparian Strip in the endodermal cells, it is free to enter the apoplast again on its way to the xylem.

Primary Growth of Shoots

Apical Meristem

• Dome-shaped mass of dividing cells at tip of terminal bud
• Gives rise to three primary mersitems: protoderm, ground meristem, and procambium just as root apical meristem
• Leaves arise as leaf primordia on the flanks of apical meristem

• Regions of meristematic tissue left behind from apical meristem
• Dormant, but have the ability to become activated and form a branch (i.e. becomes the branch's apical meristem)
  • Note difference between how shoots forms a branch versus how a root forms a branch
  • This is do to the position of the vascular tissue in a root vs. the vascular tissue in a shoot
• Subtended by a leaf

Lateral Meristems add girth by producing secondary vascular tissue and periderm

• Secondary Plant Body - tissue produced mersitems involved in secondary growth
• Vascular Cambium - secondary growth meristem which produces xylem and phloem
• Cork Cambium - secondary growth meristem which produces cork, a tough substance that replaces the epidermis

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.

• Both herbaceous and woody dicots exhibit secondary growth.
• In herbaceous dicots, secondary xylem and phloem are in a single ring of discrete bundles which form 
• In woody dicots, the secondary xylem forms a continuous cylinder

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

• A cylinder meristematic tissue that initially forms from the outer cortex of the stem
• Cork cambium produces cork cells, which form exterior to the cork cambium
• As cork cells mature, they secrete suberin (a waxy substance) in their cell walls and then die
• Cork cells function as a barrier to protect the stem from physical damage and from pathogens

• What would happen if you removed a large ring of bark from a tree?

• After a few weeks, the cork cambium loses meristematic ability
• Expansion splits the original periderm
• New cork cambium then forms deeper in the cortex of the stem
• Eventually no more cortex remains, so the cork cambium then forms from parenchyma cells of the secondary xylem

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

• No vascular cambium nor cork cambium
• Stems usually uniform in diameter
• Scattered vascular bundles (not in a ring like dicot stems)

Animation

• Plant Growth