REVIEW:  Plants -- What kind of organisms are they?
 A matter of ‘life styles’
  Plants are sessile, terrestrial, photoautotrophs
Plants emerged on land around 450 million years ago
 from aquatic, green algal ancestors.
Those ancestors had a relatively simple morphology and anatomy.

Plants then rapidly diversified. The most successful land plants evolved  according to a standard body plan.

Since they are multicellular, parts of plants free to specialize.
 Autotrophy concentrated in specific organs, leaves.
  Maximizes surface relative to volume -- they are FLAT.
   Optimizes light reception, gas exchange.

 Roots specialized for anchorage and absorbtion of water and minerals.
  Why anchorage? Absorbtion?

 Stems connect roots and leaves -- function in transport, support.

So, we are both eukaryotes, but...
 Plants took a different path than animals, in many ways.
 (During second half of course, this will be a recurring theme.)

Plants exhibit INDETERMINATE growth.
 What is that?
 How is that adaptive from an evolutionary standpoint?

The seat of much of indeterminate growth is the MERISTEM.   Figs. 5.2, 6.2
 “Perpetually young tissues or populations of cells that retain
     the potential to divide.”
 “Populations of cells composed of initials and their immediate derivatives.”

What are initials? Cells that divide in such a way that one of the sister cells remains in the meristem as an initial
while the other becomes a new body cell or derivative. Initials are the cells that stay in the meristem.
But,  derivatives can still divide.

 Initials are the plant equivalent of animal stem cells.

Different kinds of meristems.
 We’ll focus on 2 of them:
  apical meristems.
  lateral or secondary meristems.

Apical meristems
 At the tips of roots and shoots.
 Increase the LENGTH of roots and shoots.
  Contribute to PRIMARY GROWTH.
   The herbaceous, soft, unhardened, non-woody parts of the plant.
 Produce cells that form the mature tissues of the primary plant body.

Lateral meristems
 Increase the DIAMETER, or width of the plant, via SECONDARY GROWTH.
 Main lateral meristem: vascular cambium

In one sense, meristematic cells are very unspecialized. They have to be to  develop into all the other specialized cells of the plant.
On the other hand, we can think of a meristem as a very specialized group of  cells whose function is potentiating growth.

Meristems are responsible for development.
What is development? The sum total of events that progressively form an  organism’s body.
Development is a result of an organism’s genetic program, AND its interactions with the environment.

It includes growth, morphogenesis and differentiation.
 What do these terms mean?

What is meant by ‘the shoot’?

Produced by shoot apical meristem.

How does this meristem compare to the root apical meristem?
 Architectural factors.
  Cell organization in shoot apex.   Fig. 6.2
     Simpler than apex in roots
      Has ‘Extra’ organs
           leaves, lateral buds

Pattern of divisions more complicated in shoot apex.
Enhanced cell division occurs on flanks of meristem.

Creates a bulge, then a peg- or ridge-like outgrowth.
            This is called a LEAF PRIMORDIUM
Primordia lengthen, then expand at margins to create characteristic leaf shape.
Still more divisions create lateral branch buds.

Study the patterns in which leaves occur on various plants, and think about it.
    What does the pattern have to do with the shoot apical meristem?????

The pattern of formation of leaf primordia and buds left behind by shoot apical meristem is genetically
    hardwired. The patterns are species, genus or family specific.
        Called PHYLLOTAXY.
Leaves arranged in various patterns.
e.g., alternate, opposite.
    arrangement can switch with age, environmental factors.
            e.g. English ivy

Location of leaves on stem called a NODE. Portion of stem between leaves, called INTERNODE.
Most stem elongation occurs at internodes.

Apical meristem leaves behind bundles of cells that will differentiate into xylem and phloem.
Bundles may be arranged as a ring, or may be dispersed throughout stem.
            Figs. 6.5, 6.12
    Bundles contain primary xylem and phloem.
        Xylem to inside of bundle, phloem to outside.

Some cells in a layer between xylem and phloem remain unspecialized and capable of division.
'Open' vascular bundles. Fig. 6.5
    Important for secondary growth.
In monocots, bundle cells fully differentiate.
    E.g. maize. Called 'Closed' vascular bundles.
Figs. 6.12, 6.13

How is the arrangement and type of vascular bundle important for secondary growth???