PLANT ANATOMY and PHYSIOLOGY
Levels of Organization to produce a functioning plant
I.
Plant cells- the basic building blocks.
A.
B.
II.
III.
each cell is approximately 1/10- 1/100th of a millimeter long
cells can specialize in form and function to provide certain specialized
functions to the whole plant
C.
Each cell can live on its own under certain conditions- however, by
working together they provide a way to survive in more varied conditions
Plant tissues- collections of similar cells that serve a specific purpose by
functioning together
A. Unlike animals, the major organs of plants (roots, stems, and leaves) are
all composed of the same three tissues (epidermis, vascular tissues, and
ground tissues).
B.
Each tissue carries out the same fundamental activities throughout the
plant.
C.
Three types of tissues
1.
Epidermis - the exchange of matter between the plant and the
environment.
a.
the epidermis on aboveground organs (leaves and stems) is
involved with gas exchange
b.
the epidermis on belowground organs (roots) is involved
with water and ion uptake
2.
Vascular tissues - the transport of water and dissolved substances
inside the plant
a.
the xylem carries water and dissolved ions from the roots to
stems and leaves
b.
the phloem carries dissolved sugars from the leaves to all
other parts of the plant
3.
Ground tissues - metabolism, storage, and support activities
a.
the ground tissue of the leaf (called mesophyll) uses the
energy in sunlight to synthesize sugars in a process known
as photosynthesis
b.
the ground tissue of the stem (called pith and cortex)
develops support cells to hold the young plant upright
c.
the ground tissue of the root (also called cortex) often
stores energy- rich carbohydrates
. Plant organs- tissues that act together to serve specific functions for the whole
plant
A. Roots
1.
Functions a.
Anchorage
b.
Absorption of water and dissolved minerals
c.
Storage (surplus sugars transported from leaves)
d.
e.
2.
3.
4.
5.
Conduction
Cross section of herbaceous dicot root
See branch root of willow (Salix), x.s.
Epidermis
a.
Single layer of cells for protection (from disease
organisms) and absorption (water and dissolved minerals)
b.
Root hairs- tubular extensions of epidermal cells
i.
short lived
ii. greatly increase surface area of root, in contact with
soil
iii. confined largely to the region of maturation of the
root
Cortex
a.
Store starch and other substances
b.
Contain numerous intercellular spaces - air spaces essential
for aeration of the root cells (for cellular respiration)
c.
See monocot root of an orchid (Orchidaceae), x.s.
Xylem
a.
Conducts water and dissolved minerals
b.
composed of
i.
ii.
6.
See vessel elements of oak
(Quercus), x.s.
a) vessels: tube-like structures composed of hollow
elongate cells (vessel members) placed end-to-end
and connected by perforations
See tracheids from pine (Pinus), x.s.
a) tracheids: elongated conducting and supporting
cells with tapering and pitted walls without
perforations
1.
Upward movement caused by transpiration
from the leaves aided by the properties of
water: polarity of water molecules, cohesion
of water molecules to each other, adhesion
to xylem cell walls
2.
Very rapid- 2 feet/ minute
Phloem
a.
Conducts food (dissolved sugar)
b.
Phloem composed of sieve elements (sieve tube members,
companion cells)
i.
ii.
iii.
B.
Stems
1.
Functions of stems - an important site with a thorough review and
useful illustrations
a.
Support leaves and fruits
b.
Conduction of water and sugars throughout plant
2.
IV.
Sieve tube is a series of sieve tube members
arranged end-to-end and interconnected by sieve
plates
Movement of sugars up or down through
plasmodesmata of sieve elements
One inch/ minute
See cross-section of an herbaceous
stemCross section of herbaceous dicot stem
Tissues of stem
a.
Epidermis
i.
Protection
ii. Cuticle to conserve moisture
b.
Cortex
i.
Store food
ii. Photosynthesis (when stem is green)
iii. Some support cells
- e. pith to store food
c.
Xylem
i.
Conduction of water and minerals
ii. Second function - has strong supporting cells
(fibers)
d.
Phloem
i.
Conduction of food
ii. Second function - support
Leaves- organs of photosynthesis
See leaf of privet (Ligustrum),
x.s.
A. Relate anatomy of leaf to its primary function of photosynthesis
1.
carbon dioxide + water -------> sugar + oxygen
2. Cross section of mature leaf
B.
Major tissues of the leaf
1.
Epidermis
a.
Transparent- light goes right through
(a) Main function - protects against drying out (cuticle)
(b) Stomata with guard cells
b.
C.
Function- gas exchange, especially common on lower
epidermis
2.
Mesophyll
a.
Site of photosynthesis
b.
Air spaces between cells for gas exchange to each cell
3.
Veins
a.
Xylem- water conduction
b.
Phloem- food conduction
c.
Bundle sheath- one or more layers of fiber cells
surrounding a vein; strengthens vein to support leaf
d.
Branching extensive in veins- no mesophyll cell is far from
a vein
Transpiration- loss of water vapor
Abscission- leaf fall
Plant physiology - how all the tissues and organs work together
I.
Water and ion transport pathway (water is needed in leaves but available only in
soil)
A. Water and ion uptake occurs at the root hairs and the rest of the root
epidermis.
B.
Water and ions move in the cells and the intercellular spaces of the root
cortex.
C.
The Casparian bands in the endodermis (the innermost layer of the cortex)
function as an impermeable barrier, which allows the endodermis to
selectively absorb desirable ions (e.g., K, Ca, PO4 , NO3, Cl) and block
undesirable ions (Na, Al).
D. The water and absorbed ions diffuse into the hollow water-conducting
cells (tracheids and/or vessels) in the root xylem.
E.
The water and ions move up in the water-conducting cells in the xylem
which form many microscopic channels like straws connected end- to-end
that reach into all organs in the plant.
F.
The water and ions move from the xylem into the mesophyll of the leaf.
G. The water not needed for metabolism or growth evaporates from the
surface of small pores called stomates in the leaf epidermis via a process
called transpiration.
H. How water moves up the plant
1.
In essence, water moves via the same mechanism that we use to
suck soft drinks up a straw.
2.
In very thin channels like the water-conducting cells, the water
molecules are said to have great cohesive force, meaning that they
cling very tightly to each other.
3.
The evaporation of water molecules at the surface of the stomates
in the leaves generates the sucking force that pulls the adjacent
water molecules up to the leaf surface.
4.
II.
Like a lengthy chain extending all the way back to the roots, each
water molecule pulls up the molecule below it, and so the whole
water column moves up the plant.
5.
What is truly amazing about this process is that it does not involve
the input of biological energy. Water moves up the tallest tree
simply using the energy from sunlight necessary to evaporate the
water molecules at the stomatal surface.
6.
The rate of water movement up the tree must therefore depend on
the rate of water evaporation (transpiration ) at the stomates. The
plant regulates transpiration by opening and closing its stomates.
Dissolved sugar transport (sugars are made in leaves through photosynthesis, but
must be moved to other parts of the plant to power growth and life functions)
A. Sugar diffuses from the mesophyll in the leaf to the phloem cells in the
vascular bundles.
B.
Specialized cells called companion cells load the dissolved sugar into the
sugar-conducting cells (called sieve elements) of the phloem by using
cellular ATP as an energy source.
C.
Since the high concentration of dissolved sugar dilutes the water in the
conducting cells, more water molecules diffuse via osmosis from the
intercellular spaces (with high water concentration) around the vascular
bundles into the sugar conducting cells (with low water concentration).
D. This osmotic water flow generates a high hydraulic pressure that moves
the dissolved sugar solution through the phloem conducting cells from the
leaves to the rest of the plant where the sugar is unloaded by other
companion cells.