Plant structure and function:
Basic plant anatomy [OVERHEAD, fig. 31.3, p. 624]:
Roots: - anchor plant
- absorbs minerals & water
- stores foods
- have root hairs that increase the surface area and allow water & mineral
absorption.
Shoots: - stems, leaves & reproductive parts
- stems: above ground, support leaves & flowers
- leaves: main site of photosynthesis
- associated structures include:
- nodes - point where leaves are attached to stem
- internodes - areas between nodes
- terminal bud - at tip of stem, has developing leaves, etc.
- axillary buds - between the leaf & stem. Usually dormant, but
can start to grow if influence of terminal bud wears off (terminal
bud prevents axillary buds from growing).
Roots & shoots can be highly modified by different plants [Fig. 31.4 p. 625]:
- Carrots & sugar beets have an enormous root that stores energy.
- strawberries have horizontal stems that run along the ground
- potatoes have tubers, enlarged areas at the ends of roots where food is
stored.
Leaves can be highly modified as well.
- have many different shapes
- some are even modified as tendrils (see fig. 31.4C)
Basic plant cells:
Basic structures are familiar: nucleus, chloroplasts, vacuoles [Fig. 31.6A, p. 628]
May have one or two cells walls (primary & secondary).
Plant cells come in three (five?) basic types [Figs. 31.6B - 31.6F, pp. 628 - 629]:
- parenchyma - abundant, perform many different function:
- food storage/photosynthesis/aerobic respiration & more
- can differentiate into other types of cells (e.g. the types
below).
- collenchyma - resemble parenchyma, but have thicker primary walls.
- sclerenchyma - very thick & rigid secondary walls filled with lignin.
This is the “woody” component of plants.
- some examples of sclerenchyma cells include:
- fibers - long slender supportive cells, often in stems.
- sclereid - shorter, irregular, shells of nuts, etc.
- water conducting cells (generally, “xylem”) - move water around. Both
have thick secondary cell walls (add structural support). Dead at maturity:
- tracheids (long porous cells with tapered ends)
- vessel elements (wider, shorter).
- chaining together tracheids or vessels can make long tubes to
move water long distances
- food conducting cells (generally, “phloem”) - move food around. Alive
at maturity:
- sieve tube members - only a primary cell wall.
- companion cells assist the sieve tubes.
Plant tissue systems [Fig. 31.5A, p. 627]:
- Epidermis - outside covering of the plant
- protects the plant, acts as a barrier
- vascular tissue - made up of xylem and phloem
- ground tissue - everything else. Photosynthesis, storage, support, etc.
Monocots vs. Dicots
Before continuing, we need to know the difference. Both have all the above
structures, but they arrange them in different ways [Fig. 31.2, p. 623].
- named for the number of seed leaves (one vs. two)
- in lab you’ll see that the arrangement of vascular tissue in the two plants
is different:
- leaves have different shapes (parallel veins vs. branched)
- # of flower parts are different (in multiples of 3 for monocots, 4 or 5 for
dicots).
- the overall structure of the root is different (tap root for dicots, fibrous
for monocots).
Leaves [Fig., not in text & 31.5, p. 627]:
petiole - the “stem” of the leaf.
blade - the leafy part of the leaf.
Leaves have many of the different tissue types in them:
- Epidermis -protects the leaf (covered by a cuticle)
- To allow air to move in or out, the epidermis is covered with stomata
- the size of this opening is controlled by guard cells that can close
or open the stomata.
- can help prevent water loss by closing off during dry
conditions.
- the ground tissue system is represented mostly by mesophyll.
- consists of parenchyma cells specialized for photosynthesis
- running through the leaf are veins. These are bundles of vascular tissue
that move fluids and nutrients around the leaf.
Plant growth:
- in general, plants grow throughout their lives (don’t stop growing).
- plants are either annuals, biennials, or perennials:
- annuals - live one year (wheat, corn, some wildflowers)
- biennial - live two years (beets & carrots)
- perennials - live more than two years (e.g. trees, etc.)
- plants can get old. Some giant sequoias are over 3000 years old. Some
Bristlecone pines over 4000 years [Fig., not in text]. Most don’t live that long.
- meristem are unspecialized cells that cause growth. When this makes new
branches, roots, or makes existing branches or roots longer, this is “primary
growth” [Fig. 31.7A, p. 630].
- “apical” meristem is usually found at the tips of branches & roots, or at
the axillary buds.
- Secondary growth - this is the widening of woody plants (e.g., how you get a
trunk from a twig) [Fig. 31.8A, p. 632].
Two layers of meristem:
- vascular cambium - makes secondary xylem (more xylem) on the
inside, and secondary phloem on the outside.
- the secondary xylem is composed of fibers, tracheids &
vessels, and so is very strong (woody).
- over the years, it’s the secondary xylem that makes the
“wood” in a tree.
- the secondary phloem never gets much thicker - it stays
on the outside, and excess cells are sloughed off.
- cork cambium - makes cork. A thick outside layer that protects
the tree (basically bark). As the tree grows, older cork is also
sloughed off like the secondary phloem.
- Tree trunks [Fig. 31.8B, p. 633]
- dark center is heartwood. Non functioning xylem, filled with stuff to
prevent rotting.
- lighter circle - sapwood. Functioning xylem
- Rays running through wood - parenchyma cells that move nutrients
around.
- then the outside layers discussed above:
- vascular cambium
- bark (secondary phloem, cork cambium, cork)
- Secondary xylem cells are much larger in the spring; this gives trees in
temperate climates rings (the cells put down during the rest of the year are
much smaller).