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Botany11
Chp 28. Stems and Roots
STEMS
Usually located above ground, provide the structural support for leaves
and flowers. Internal transport of water, nutrition via the xylem and phloem.
New tissues produced at the apical (primary growth) and lateral (secondary
growth) meristems.
As always, form is related to function…
In DICOTS the epidermis on the outside, protects the stem from damage
and predation, epidermis is covered by the cuticle. Under the epidermis is the
cortex. Vascular bundles are arranged in a circle, xylem to the inside. Between
the xylem and phloem is the vascular cambium. In the center of the stem is the
pith composed of parenchyma. Pith rays are the areas between the vascular
bundles. (see page 543)
In MONOCOTS stems are arranged the same way except vascular
bundles are randomly scattered and the pith and cortex are not differentiated.
Monocots lack vascular cambium.
ROOTS
Most obvious function is to anchor the plant. They transport water and
dissolved minerals to the upper part of the plant. Many have other functions
such as storage, some plants have aerial photosynthetic roots.
Structurally similar to stems (epidermis, cortex, vascular bundles, pith…)
Structure unique to roots are the root cap (protects the root apical meristem),
Root hairs (extensions of epidermis that increase the root surface area), root
epidermis does not secrete cuticle.
Cortex has loosely arranged parenchyma. Have unique structures such
as Casparian Strip on radial and transverse walls of root. Contains suberin
which is hydrophobic, waterproof.
Roots absorb water (no cuticle) via root hairs. Cell walls are made of
cellulose which absorbs water (cotton towels are made of the cellulose produced
by cotton plants). Cellulose is an interesting molecule, absorbs water like a
wick, but unlike other things that absorb water it doesn’t dissolve in water!
Water moves along the cells until it reaches the xylem at which point it is
transported upwards. Phloem carries dissolved sugars to the roots or from the
roots to other parts of the plant.. oxygen diffuses from the surrounding soil gas to
the root.
Tension-cohesion mechanism (pg 550) – water is drawn up a plant by the
pressure produced at the top of the plant by transpiration. Less important
mechanism is the root pressure mechanism where internal pressure pushes
water up the plant by water absorbed by the roots.
CHP 29 Flowers
Rhizomes, tubers, bulbs, corms and stolons are modified stems.
PARTS OF THE FLOWER –
See page 563, Figure 29-6
A perfect flower has both stamens (male parts) and carpels (female parts). A
flower with one or the other, but not both is said to be imperfect. Each stamen
composed of a thin stalk, called a filament upon which is the anther (the site of
meiosis) where pollen grains are formed. Each grain of pollen contains two male
gametes (sperm). Each carpel contains a stigma (where pollen lands), a style
(or a neck) and an ovary (which contains one or more ovules) Each ovule
contains an egg. The carpel or a group of carpels is sometimes called the pistil.
Sepals – lower and outermost whorl on a flower shoot. Sepals protect the flower
while it is a bud. Together, all of the sepals of a bud are called a calyx.
Petals – leaf like often colored parts of a flower…together all of the petals are
called a corolla.
Double Fertilization – the two sperm in the pollen grain perform a unique and
important function – one fertilizes the egg while the other fuses with the polar
nuclei in the embryo sac to form the endosperm. The endosperm serves the
embryo as a source of nutrition until it germinates.
Fruits are mature, ripened ovaries that contain seeds. A simple fruit is one
which develops from a single ovary of a single flower. A berry is a fleshy fruit
with soft tissues throughout (grapes, bananas), a drupe is a fleshy fruit that has
hard pit surrounding the seed (peaches, plums, avocados). Aggregate fruits
are formed from a single flower that contains many carpels. After fertilization the
ovaries enlarge and fuse to form a single fruit (raspberries). Multiple Fruits
similar to the aggregate fruit except here the ovaries from different flowers
enlarge and fuse to form a single fruit (pineapples). Accessory Fruits are fruits
that they include tissues from other plant parts in addition to the ovary to make
up the fruit. Parts such as the floral tube are incorporated into the fruit
development. (Strawberries, apples, pears).
Seed Dispersal.