OH SHOOT, continued...

Vascular differentiation in dicot shoot.

Apical meristem leaves behind bundles of cells that will differentiate into primary xylem and phloem.
Bundles may be arranged as a ring, or may be dispersed throughout stem.
            Fig. 6.5
    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.
    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???
 

Leaves
 The principal photosynthetic organs of plants.
 Great example of evolution working on structure vs. Function.

 Consider a leaf....

 Function: Light absorbtion, gas exchange for photosynthesis.
 Structure?
  Flat, high surface to volume ratio.
  Epidermis with cuticle and wax coating prevents undo water loss and protects against  pathogens.
  But controlled gas exchange occurs through STOMATES.    What are guard cells??   Fig. 7.8
   Carbon dioxide, oxygen, and gaseous water
          Water loss through stomates is critical.
 
    Leaves act as radiators. Cool the plant via EVAPORATION.
       A KIND OF AIR CONDITIONING

Leaf morphology.   Fig. 7.4
 Flat. Why is this important for what the leave DOES?
 Two parts of a typical leaf: Blade, petiole.
  Is the petiole a stem? HOW CAN YOU TELL????
       Bud in leaf axil.
 Many leaves do not have petioles.  e.g. Grasses.
 Leaves may be entire, or LOBED (pinnately lobed or palmately lobed). Name some examples...
 Leaves may be divided into leaflets. COMPOUND LEAVES
  Pinnately compound (pea, hickory) or Palmately compound (buckeye).

Leaf anatomy.  Figs. 7.6, 7.11
 Again, evolution of structure in hand with function.
  Bounded by epidermis on top and bottom.
   Epidermis covered by waxy cuticle.
   Epidermis a complex tissue.
    i.e.contains several cell types.
     e.g. Guard cells control gas exchange in and out of leaf.
  Internal tissues.
   Photosynthetic cells
    Palisade and spongy mesophyll. What are their 'secondary' functions???
    Venation.
       Leaves permeated by veins.   Fig. 7.9
           No photosynthesizing cell is more than  several cells away from a small vein.

Leaves are NOT permanent structures.
 They fall off.  Callled ABSCISSION.
  Special layers formed at base of petiole.   Fig. 7.25
   Cells weak, break easily.
   Other cells impregnate walls with sealants, sealing off wound.
       Leave leaf scars.

Abscission is stimulated by environmental signals.
 Shortening day length, cool nights.

 Leaves senesce, or die.

How do leaves change color?
  Two ways:
  1. Chlorophyll vs carotenoids.
   As chlorophyll breaks down, ‘reveals’ other   chloroplast pigments, which are mainly yellow, orange.
   E.g. Oaks, hickories, birches, tulip tree.  These pigments are hydrophobic, or water insoluble.
  2. In other species, new pigments are synthesized.
   In response to bright days, cool nights.
   Water soluble anthocyanins stored in vacuoles.
    E.g. Dogwoods.
    Bright sunny days and cool nights lead to production of excess sugars, which are used for anthocyanin synthesis

Determinants of fall color.
 Genes - species and variety differences.
 Latitude and climate - color strongest at higher latitudes. Again, species, but also cool nights.
 Seasonal fluctuations.
  Again, cool nights, but also soil moisture, pH.
  Drought a factor, as is too much rain.
At the same time this is going on, sugars and other  substances in the leaf are recovered into the rest  of the plant.
    Much of the leaf is broken down,  leaving just cellulose.