SECONDARY GROWTH, conclusion
Sometimes, if a wet period occurs after a dry spell, a new spurt of
wide elements might form.
Thus, two 'annual' rings would appear in one
season.
Of what use are annual rings to scientists?
What is WOOD, botanically?
What is heartwood and sapwood? Fig. 6.9
As circumference of stem increases during secondary growth, and as
secondary
phloem pushes outward,
Epidermis cracks and sloughs off.
Why is this a potential problem
for the plant?
A new dermal layer forms as cortical cells resume division: PERIDERM
THE PATCHES OF DIVIDING CELLS ARE CALLED CORK CAMBIUM.
The products of these divisions specialize as cork
cells, which protect the plant in place of the epidermis.
Cork pattern on various trees is species specific.
Allows us to identify trees even in winter.
Figs. 6.6, 6.7
As stem expands, old cork also peels off.
Live cells in secondary
phloem resume division, giving rise to new cork cambia and therefore
new cork cells.
What is meant by bark, in the botanical sense and
the popular sense?
LOOK IT UP.
Rays: what are they? What is their function? Fig. 6.8
How does cross section surface area of a tree trunk
or stem impact ray numbers?
More about the vascular cambium (Fig. 6.4)
What are initials? Fusiform initials? Ray initials?
What are periclinal divisions and what do they
produce?
HOW DO THE INITIALS PRODUCE XYLEM INWARDS AND PHLOEM OUTWARDS?
Anticlinal divisions? What is their function?
HOW DOES THE CIRCUMFERENCE OF THE VASCULAR CAMBIUM INCREASE
WHAT ABOUT transverse divisions?
Where do new ray initials come from?
================
ENERGY TRANSFORMATIONS; PHOTOSYNTHESIS
Review:
Biological systems are very organized and complex.
I.e. Non-random.
Organelles, cells, large organisms,
ecosystems.
They become increasingly more complex with time.
That takes an input of energy to fight the tendency towards
disorder.
In other words, biological systems must do WORK to create and
maintain order
ORGANISMS DO NOT DEFY THE SECOND LAW OF THERMODYNAMICS.
ECOSYSTEMS
"
"
EVOLUTION
DOESN'T
"
"
All biological systems are fully consistent with thermodynamics
Energy transformations are very important to cells, organisms
and ecosystems.
An ecosystem is “a fountain of energy flowing through a
circuit
of soils, plants and animals”. Aldo Leopold
Or, everything in ecosystems is recycled EXCEPT energy
Where does the energy for the work required to create order come
from? (I.e., in most terrestrial systems)
PHOTOSYNTHESIS
“This process -- photosynthesis -- is the route by which virtually
all
energy enters our biosphere. Each year, more than 250
billion tons of sugar are produced worldwide by photosynthetic
organisms.
The importance of photosynthesis, however, extends
far beyond the sheer weight of this product. Without this flow of
energy
from the sun, channeled largely through the chloroplasts
of eukaryotic cells, the pace of life on this planet would swiftly
dininish and then would virtually cease altogether, as dictated by
the inexorable second law of thermodynamics.” Raven et
al., 6th ed.