PLANT ANATOMY – MODULE 113
I. PLANT ORGANS
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Like animals, plants are also made up of cells, tissues and organs. However, the structure and
function of these are somewhat different from animals’. This difference is mostly due to the
fact that plants are mostly unable to move away from their position so they have to be more
tolerant to environmental changes and to herbivores.
Review the differences between plant cells and animal cells.
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Most plants have three basic organs: roots, stems and leaves. All these organs are made up of
tissues.
A. Roots:
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Roots generally anchor the plant into the ground and absorb water and other nutrients from the
soil. Roots are generally located in the ground, but not always. They form a complex root
system.
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The root, like the rest of the plant grows by a set of undifferentiated but quickly reproducing
cells called apical meristem cells. These cells are found in the root tip of roots.
Check out the microscope slides on this. Draw what you see with magnification.
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There are two main types of root systems.
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Taproots consist of a main root or taproot while the lateral roots grow out of the taproot.
Taproots tend to be thick and sturdy, while the lateral roots are thinner. The taproot is also
frequently used for storage (like in carrots and sweet potatoes). Taproot system is found in
coniferous plants and dicot plants.
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In monocot plants, the dominant root system is a fibrous root system. These roots tend to grow
in many smaller branches from the same stem.
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Roots as a whole anchor plants but the absorption of water and minerals mostly occurs on the
surface of some thin hair-like structures called root hairs. These are just above the root tip.
Structure and function relationship: The root hairs have only a thin primary cell wall, one layer
or cells and large surface area. All of these are beneficial to make the absorption of materials
the most efficient possible.
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Root hairs absorb water by growing into the gaps between the soil particles and absorbing
water by osmosis from there. They absorb minerals by using a secondary active transport. They
release H+ ions into the soil, set up a concentration and charge gradient and absorb the ions
they need by cotransport (if the absorbed ions are negative) or by using a channel protein (if the
absorbed ions are positive).
B. Stems:
 The stem of a vascular plant supports the plant, stores nutrients, generates new tissues,
makes and supports leaves.
 The very tip of the stem is called the apical bud. This and the axillary buds contain meristem
cells that divide and lengthen the plant as well as form new side branches. In many plants,
the apical bud is dominant over the axillary buds, so as long as the apical bud is there, the
axillary buds are dormant, so the plant grows in length not width – apical dominance.
 Stems have a major role in transporting inorganic materials into the leaf and organic
materials back into the stem and roots for storage.
Look over the structure of the leaf from the previous chapter on photosynthesis.
II. PLANT TISSUES
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All plants contain three main types of tissues: dermal, vascular and ground.
A.
Dermal Tissue:
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Provides a protective covering for the roots, stems and leaves, so it is found on the outside of
the plant. Mostly made up of epithelial cells that are tightly packed together in a single layer.
These cells are usually impermeable to water because of a waxy covering called cuticle. This
covering also protects the plant against bacteria and viruses. During secondary growth this
epidermal layer is replaced by other layers of tissues.
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Root hairs are epidermal cell extensions that frequently live in a symbiotic relationship with
fungal fibers. These fungal fibers make the absorption of nutrients and water even more
efficient because they provide extra surface area.
B.
Vascular Tissue
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Serves to transport food, water, hormones and minerals, so it is generally found in the plant’s
interior. It also provides structural support and includes fibers. The fibers are formed from
specialized cells that have very thick cell wall for support.
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The shoot system (leaves and stems) need minerals and water from the root system to produce
sugars while the root system takes the sugars from the shoot system and uses or stores it.
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There is two separate transport system, one for water and minerals, the other is for organic
matter.
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Xylem tissue transports water and minerals from the roots to the shoots. There are two basic
types of cells that form this tissue: tracheids and vessel elements, these cells differ in thickness
and how they transport water. However, both of these cells become functional only when dead,
their cytoplasm disintegrates and they basically form a hollow tube system between the leaves
and the root. Water and nutrient transport is only one directional, from the root into the
leaves.
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Phloem tissue is living tissue that transports organic nutrients to all parts of the plant. The
phloem moves solutes from the sugar source (the producer of the sugar) to the sugar sink (the
user of the sugar). The movement of the sugary solution can occur up, down or sideways in the
plant. The main cells that form the phloem are sieve elements, companion cells and fibers.
The sieve elements are living cells that form a tube by joining the cells with plasmodesmata.
These cells are missing many basic cell components, so the companion cells perform
metabolism, reproduction and other functions for them.
Look at the various transport tissues in a cross section of plant stems, roots.
C.
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Ground Tissue
Forms the majority of a plant body. Includes a wide range of different, specialized set of cells.
This tissue type is responsible for most metabolic processes, such as photosynthesis, cellular
respiration, cell division, storage, structural support, protection, regeneration.
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Depending on their function, the specialized cells can be thin-walled, young cells, older live cells
with thicker cells walls or cells that are dead and have very thick cell walls.
PLANT STRUCTURE AND FUNCTION – MODULE 115
I. PLANT ADAPTATIONS ABOVE GROUND
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One of the major challenges for plants is the availability of water on land. There is a wide range
of adaptations that they developed through natural selection and other mechanisms of
evolution that helps them to save and absorb water the most efficient way.
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Plants lose most of their water by evaporation through stomata on the leaves. Depending on
the climate that the plant adapted to the leaves can have a wide range of shapes and other
properties. In general, plants in arid (dry) and cold climates grow smaller leaves with smaller
surface-area-to-volume ratio to decrease evaporation. While plants that live in tropical areas
can have larger, leaves to increase the surface area for photosynthesis.
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Leaf orientation toward the sun can also maximize photosynthesis.
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As the number of plants increased, the competition for sunlight also increased. Adaptations in
plants that enhance their ability to trap light are beneficial. For example growing larger, taller or
climbing on other plants help to maximize photosynthesis.
II. PLANT ADAPTATIONS BELOW GROUND
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Plants also need efficient and extensive root system to efficiently support the plant and absorb
nutrients.
Taproots usually have fibers and bulk to support the plant but the root hairs on their smaller
roots make absorption efficient.
Some plants also have symbiotic relationships with fungal fibers to improve surface area for
more efficient absorption. The fungal fibers in exchange can get organic nutrients.
III. THE DYNAMIC SYMPLAST
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Transport in plants takes place in both living and nonliving cells. The combined cytosol of all
living plant cells form a single entity by connecting each cell by plasmodesmata. This combined
system is called symplast. Water and small solutes easily move through plasmodesmata, but
even certain proteins and RNA can pass through it.
For this segment, you also must know the information and examples on the given handout on
hydrophytes and xerophytes.
PLANT TRANSPORT AND TRANSPIRATION – MODULE 116
I.
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Transport In Plants
Water and solutes can travel in and out of plant cells by passing through the cell wall and cell
membrane. However, not all solutes can pass freely. Solutes and water can freely move inside
of the cytoplasm and can easily move through plasmodesmata. These connections for the
symplast of the plant. Water and some polar substances can travel in the symplast entirely.
Apoplast includes everything outside of the plasma membrane of living cells, like the cell wall,
extracellular spaces and the interior of dead cells specialized for transport.
Transport can occur by the symplastic route entirely, the apoplastic route entirely or by the
transmembrane route:
II.
Short Distance Transport
 The basics of transport processes apply to moving substances across the membrane. The
concentration, polarity, size of substances determines the direction and the type of transport
that occurs.
 H+ ion pumps usually help to establish a potential energy that can be used to move substances
from the lower concentration area to the higher concentration area.
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Water movement is determined by the water potential between two areas. Remember: water
moves from the higher to the lower water potential area. Water movement is also supported
by aquaporins, channel protein that make water transport more efficient.
Review what happens with plant cells when they are placed in solutions with different tonicities.
III.
Movement From Sugar Sources to Sugar Sinks (Movement of Organic Substances)
 Organic materials are transported in the sieve-tube members of the phloem.
 Sucrose is the most important organic material that is transported in the phloem but amino
acids, minerals and hormones are also transported in the phloem sap.
 The direction in which the phloem sap travels varies depending on the location of the organic
material. Sugar source – is the plant organ that is a net producer of sugar (leaf, storage root
etc.) A sugar sink – is a consumer of the sugar that needs to receive sugar from the source
(growing buds, stems, fruits). The sugar sink usually receives sugar from the nearest source.
 Sugar must be loaded into the sieve tube members before it can be transported. It usually
moves through the plasmodesmata of neighboring cells via the symplast, but in some plants
they also use the apoplast route.
 Cotransport with H ions move the sugar into the sieve tube members of the phloem. The sugar
concentration is always higher at the sugar source than at the sink so it will move in the phloem
from the high to the lower sugar concentration area. Water follows by osmosis. The movement
is sped up by positive pressure that is created by water from the xylem. The high pressure will
press the water with the sugar toward the sugar sink where the water exits back to the xylem.
This process that enhances sugar flow is called pressure flow.
IV.
Long-distance Transport of Water and Minerals
 Live cells with cell membranes and cytoplasms are not efficient enough to supply plants with
enough water. As a result, dead cells form a continuous tube with the cell walls existing only in
the xylem. These are formed by the tracheid and vessel element cells.
A. In the Root:
 Water enters the roots via a combination of pathways (symplastic or apoplastic routes). Both
routes are efficient enough until the endodermis (the innermost layer of the root cortex is
reached). Here a layer of cells, the Casparian strip is highly impermeable to water so to get
across this area, the water and minerals must be moving in the plasmodesmata with the
sympastic route.
 At the end, water and minerals end up in the xylem cells.
B. In the Stem:
 Once the water is in the xylem cells, the plant use various forces to move the water up:
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Root pressure –This water flow is due to the accumulation of minerals in the xylem that
lowers water potential there and pulls water in. Than the increased water concentration
forces pressure on the cell and forces water into the cells above. In smaller plants this
pushing force of water results in guttation – the collection of water droplets on the leaves
early in the morning. But this force is too small to push water up higher than a few meters.
Transpiration-Cohesion-Tension Mechanism -- a pulling force that is created by the loss of
water from the leaves and other aerial parts of the plant. Water molecules leave the plant
through the stoma of the leaf by diffusion because when the stoma is open, water
concentration is higher in the plant cells than in the surrounding atmosphere. Because
water molecules are polar, they are attracted to each other by hydrogen bond (cohesion).
These water molecules form continuous strings of water from the root to the leaves. In the
meantime, adhesion sticks molecules and ions of different kinds together. This force
adheres water molecules to cellulose molecules in the xylem cell wall. As water molecules
transpire away from one end of the string of water molecules because of the concentration
gradient, cohesion and adhesion pulls the remaining water molecules upwards from the
root. These processes do not require energy from the plant, they are all extended by
physical properties of water and the surrounding molecules.
Plant transpiration site (great) : http://croptechnology.unl.edu/animation/transpiration.swf
C. Stomata Opening and Closing:
 Stomata are pores distributed in the lower (most dry land plants) or upper (many water plants)
epidermis of plants. They are the structures of taking in CO2 for photosynthesis, releasing O2
from photosynthesis and releasing water by transpiration.
 Stomata are usually open during the day to take in CO2 for photosynthesis. There is an
exception however, some desert plants open their stomata only at night, store CO2 and use it
during the day. Stomata opening and closing is regulated by hormones that are controlled by
circadian rhythms, however, it can also be closed if the plant is not getting enough water, or
because of too much sunlight or too much wind.
 Closed stomata decrease the capacity of the plant to perform photosynthesis because of the
decreased CO2 concentration.
 Opening stomata: Guard cells pump H+ ions out by using active transport. They build up an
electrochemical gradient (- charge) inside of the cells. K+ ions move into the cell through K+
channels by passive transport, while Cl- ions use active, cotransport to move into the cells. Once
the concentration of ions is high inside, that decreases the water potential, so water moves in
passively. As water moves in, it increases the turgor pressure in the cells so the open up.
 Closing stomata: Roots and leaves produce a hormone called ABA (abscisic acid), this hormone
binds to receptors on the guard cells and stop the pumping of H+. Cl ions flow out and the
electrochemical gradient decreases between the two sides of the guard cells. K+ ions move out
of the cells as well and the water potential increases. Water moves out of the cells as well and
the cells become flaccid and collapse. The stomata close.
PLANTS AND LIGHT -- MODULE 123
I. Roles of Light in Plant Function
Review how human circadian rhythm is set
 Plants time many daily metabolic tasks, as well as life events, such as reproduction, to the cycles
of day and night. Changes in light intensity also signal information about seasons, which is
critical for germination, growth and flowering.
 Plants respond not just to the presence or absence of light but also to the specific characteristics
of light such as wavelength, intensity and direction.
II. Detecting and Responding to Light Quality and Quantity
 Plants primarily detect and interpret wavelengths of light using molecules called
photoreceptors. These trigger signal transduction pathways (will learn later) within the plant to
cause various physiological changes.
 The three major classes of photoreceptors are:
o phytochromes -- absorbs mostly red and far-red wavelengths
o phototropins -- absorbs mostly blue wavelengths
o cryptochromes -- absorbs mostly blue wavelengths
A. Phytochromes
 These photoreceptors mediate several different plant responses to red light, such as flowering,
stimulation of germination, shade-avoidance response, setting off the circadian clock, initiation of
dormancy in woody plants.
 From experiments, scientists also found out that germination for example is stimulated by red light,
but far-red light inhibits it. If seeds are exposed to alternating red and far-red light, the last light
exposure will determine if the germination is inhibited or activated.
 The explanation is that plants have multiple phytochrome types. Each phytochrome molecule is a
complex with a protein component and a light absorbing complex.
 When the phytochrome absorbs red light, this causes a change in the shape of the light absorbing
complex and that causes a shape change in the protein component (Pfr form) . Once this shape
change occurs, the light absorbing complex cannot absorb red light any more only far-red light. The
far-red light causes the molecules to get back to their original shape (Pr form). Extended period of
darkness also converts the Pfr form back to the Pr form. It is likely that most biological responses are
triggered by the presence of the Pfr form.
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The interconversion of phytochrome also allows plants to detect and respond to changes in light
quality. Under direct sunlight, plants have about equal exposure to red and far-red lights. In this
case the two forms, Pr and Pfr are found in equal quantities. Once the plant starts to be shaded by
other plants, they receive more far-red light that red light. This change in the light intensity can
trigger a shade-avoidance response in some plants, which will stimulate elongation of the stems,
increased apical dominance, and upward movement of the leaves.
B. Phototropins and Cryptochromes
 These photoreceptors are sensitive to blue light and can trigger phototropism in plants.
Phototropism is the directional growth toward (positive phototropism) or away (negative
phototropism) from light.
 Plant cryptochromes are also responsible for setting up circadian rhythms of plants and inducing
flowering.
III. Plants and Circadian Rhythms
 Circadian rhythms are processes that are set to a 24-hour cycle.
 Opening and closing stomata, for example is one of these processes. Stomata in most plants is
open during the day and close at night to preserve water.
 These cycles are controlled by the length of light and darkness, but continue even in continuous
light or darkness.
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Photoperiodism: Plants can be divided into three categories according to the triggering of
flowering.
o Neutral plants -- don't use night length to determine flowering
o Short-day (long night) plants only flower if they get a critical length or longer night time.
These are usually plants that flower in the spring or fall
o Long-day (short night) plants only flower if they get less than the critical length of night
time. These plants usually flower during the summer.
http://www.youtube.com/watch?v=POfyyQHx7iY -- excellent link that explains all you need to know for
this module.