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Prelab Reading
Plants and Their Structure
EVOLUTION OF PLANTS
Fossil and biochemical evidence indicates plants are descended from multicellular green algae.
Various green algal groups have been proposed for this ancestral type, with the charophytes often
being prominently mentioned. Cladistic studies support the inclusion of the charophytes
(including the taxonomic order Coleochaetales) as sister taxa to the land plants. Algae dominated
the oceans of the Precambrian time over 700 million years ago. Between 500 and 400 million
years ago, some algae made the transition to land, becoming plants by developing a series of
adaptations to help them survive out of the water.
Plant Adaptations To Life On Land
Organisms in water do not face many of the challenges that terrestrial creatures do. Water
supports the organism, the moist surface of the creature is a superb surface for gas exchange, etc.
For organisms to exist on land, a variety of challenges must be met.
1. Drying out. Once removed from water and exposed to air, organisms must deal with the
need to conserve water. A number of approaches have developed, such as living in very
moist environments (bryophytes), and production of a waterproof surface (the cuticle in
plants, cork layers and bark in woody trees).
2. Gas exchange. Organisms that live in water are often able to exchange carbon dioxide
and oxygen gases through their surfaces. ese exchange surfaces are moist, thin layers
across which diffusion can occur. Organismal response to the challenge of drying out
tends to make these surfaces thicker, waterproof, and to retard gas exchange.
Consequently, another method of gas exchange must be modified or developed. Plants are
thought to share common ancestry with algae. e plant solution to gas exchange is a new
structure, the guard cells that flank openings (stomata) in the above ground parts of the
plant. By opening these guard cells the plant is able to allow gas exchange by diffusion
through the open stomata.
3. Support. Organisms living in water are supported by the dense liquid they live in. Once
on land, the organisms had to deal with the less dense air, which could not support their
weight. Adaptations to this include specialized plant cells/tissues that support the plant.
4. Conduction. Single celled organisms only have to move materials in, out, and within
their cells. A multicellular creature must do this at each cell in the body, plus move
material in, out, and within the organism. Plants developed and refined the root-shootleaf axis with its specialized conducting cells of the xylem and phloem. is conducting
tissue often resembles tubes or “pipes” inside plants and is called vascular tissue. e
earliest vascular plants, such as Cooksonia and Rhynia, were little more than naked
(unleafed) photosynthetic stems. Some plants later developed secondary growth that
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produced wood. Some multicellular algae and bryophytes also have specialized
conducting cells.
5. Reproduction. Organisms in water can release their gametes into the water, where the
gametes will swim by flagella until they encounter each other and fertilization happens.
On land, such a scenario is not possible. Plants had to adapt, either by living in moist
environments like the ferns and bryophytes do, or by developing specialized delivery
systems like pollen and pollen tubes to get the sperm cells to the egg.
In addition, plants show a trend for reduction of the complexity, size, and dominance of
the gametophyte generation. In nonvascular plants the gametophyte is the conspicuous,
photosynthetic, free-living phase of the life cycle. Conversely, the angiosperm
gametophyte is reduced to between three and eight cells (hence it is very inconspicuous)
and is dependent on the free-living, photosynthetic sporophyte for its nutrition.
A third trend related to reproduction is the development of the seed to promote the
dormancy of the embryo. e seed allows the plant to wait out harsh environmental
conditions. With the development of the seed during the Paleozoic era plants became less
prone to mass extinctions.
e fourth trend related to reproduction is the encasing of a seed within a fruit. e only
plant group that produces true fruit is the flowering plants, the angiosperms. Fruits serve
to protect the seed, as well as aid in seed dispersal.
Plants divide into two groups: plants lacking lignin-impregnated conducting cells (the
nonvascular plants) and those containing lignin-impregnated conducting cells (the vascular
plants). Living groups of nonvascular plants include the bryophytes: liverworts, hornworts, and
mosses. Vascular plants are the more common plants like pines, ferns, corn, and oaks.
Bryophytes are small, nonvascular plants that first evolved approximately 500 million years ago.
e earliest land plants were most likely bryophytes. Bryophytes lack vascular tissue and have life
cycles dominated by the gametophyte phase. e lack of conducting cells limits the size of the
plants, generally keeping them under five inches high. Roots are absent in bryophytes, instead
they have simpler root-like structures called rhizoids. Bryophytes include the hornworts,
liverworts, and mosses.
e vascular plants have specialized transporting cells xylem (for transporting water and mineral
nutrients) and phloem (for transporting sugars from leaves to the rest of the plant). When we
think of plants, we generally picture vascular plants. Vascular plants tend to be larger and more
complex than bryophytes, and have a life cycle where the sporophyte is more prominent than the
gametophyte. Vascular plants also demonstrate increased levels of organization by having organs
and organ systems.
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GENERAL ORGANIZATION OF VASCULAR PLANTS
A plant has two organ systems: 1) the shoot system and 2) the root system. e shoot system is
above ground and includes the organs such as leaves, buds, stems, flowers (if present), and fruits
(if present). e root system includes those parts of the plant below ground, such as the roots,
tubers, and rhizomes.
Figure 1. Major organ systems of the plant body.
PLANT TISSUES
Plant cells are formed at mitotic zones called meristems, and then develop into cell types which
are grouped into tissues. e tissues of the higher plants may be categorized into four groups—
dermal, ground, vascular, and meristematic tissues. ese four tissue types form the basic
structural units of a plant’s roots, stems, leaves, and reproductive organs.
Tissue Type
Functional Roles
Dermal
Protection and sometimes nutrient absorption
Ground
Bulk tissue; storage, processing, physical support
Vascular
Movement of fluids/food and physical support
Meristematic
Division of new cells for new growth or repair
Figure 2. Four Main Plant Tissue Types
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TISSUES THAT COVER AND PROTECT
Plants have special tissues that function to cover and protect the outside of their structures. e
epidermal tissue functions in prevention of water loss and acts as a barrier to fungi and other
invaders. us, epidermal cells are closely packed, with little intercellular space. Epidermal cells
lack chlorophyll and have thick cell walls, To further cut down on water loss, many plants have a
waxy cuticle layer deposited on top of the epidermal cells.
Figure 3. Plant Tissue Coverings. Cross Section of a stem of Zea mays (Corn).
GUARD CELLS
To facilitate gas exchange between the inner parts of leaves, stems, and fruits, plants have a series
of openings known as stomata (stoma, sing.). Obviously these openings would allow gas
exchange, but at a cost of water loss. Guard cells are bean-shaped cells covering the stoma
opening. ey regulate exchange of water vapor, oxygen and carbon dioxide through the stoma.
Figure 4. Scanning electron micrograph of Equisetum (horsetail or scouring rush) epidermis. Note the
oval stomatal apparatuses in the center of the stem.
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TISSUES PROVIDING STRUCTURAL SUPPORT
Tissues specialized to provide structural support are located throughout the bodies of
multicellular plants. Larger bodies require support to maintain shape and structural integrity. In
plants, these are referred to as ground tissues. Cells comprising this tissue type may have very
thick cells walls. ese cell walls may be impregnated with lignin, which is a structural polymer
and cement. Parenchyma, collenchyma and sclerenchyma tissues provide much of the structural
support for larger plants.
Parenchyma
A generalized or “generic” plant cell type, parenchyma cells function in storage, photosynthesis,
and as the bulk of ground and vascular tissues. Palisade parenchyma cells are elongated cells
located in many leaves just below the epidermal tissue. Spongy mesophyll cells occur below the
one or two layers of palisade cells. Ray parenchyma cells occur in wood rays, the structures that
transport materials laterally (from the center outwards) within a woody stem. Parenchyma cells
also occur within the xylem and phloem of vascular bundles. e largest parenchyma cells occur
in the pith region, often, as in corn (Zea) stems, being larger than the vascular bundles. In many
prepared slides they stain green.
Figure 5. Privet Leaf Section
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Collenchyma
Collenchyma cells support the plant. ese cells are characterized by thickenings of the wall.
ey tend to occur as part of vascular bundles or on the corners of angular stems. In many
prepared slides they stain red.
Figure 6. Collenchyma cells. Note the thick walls on the collenchyma cells occurring at the
edges of the Medicago stem cross section.
Sclerenchyma
Sclerenchyma cells also support the plant. ey often occur as bundle cap fibers. Sclerenchyma
cells are characterized by thickenings in their secondary walls. By the time the plant is mature,
these cells are dead. ey, like collenchyma, stain red in many commonly used prepared slides.
Some sclerenchyma cells occur in the fruits of Pear. ese cells give pears their gritty texture.
Figure 7. Sclerenchyma cells.
TISSUES INVOLVED IN TRANSPORT
Plants must transport water, nutrients, dissolved gases, and wastes to and from various parts of
their structures. is transport of materials occurs within specialized vascular tissues. In plants,
the two major types of vascular tissues are xylem and phloem.
Xylem
Xylem is a term applied to woody (lignin-impregnated) walls of certain cells of plants. Xylem
cells tend to conduct water and minerals from roots to leaves. While parenchyma cells do occur
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within what is commonly termed the "xylem," the more identifiable cells—tracheids and vessel
elements—tend to stain red with Safranin-O. Tracheids are the more primitive of the two cell
types, occurring in the earliest vascular plants. Tracheids are long and tapered, with angled endplates that connect cell to cell. Vessel elements are shorter, much wider, and lack end plates. ey
occur only in angiosperms, the most recently evolved large group of plants.
Figure 8. Xylem cells.
Phloem
Phloem cells conduct carbohydrates produced in photosynthesis from leaves to rest of the plant.
ey tend to stain green (with the stain fast green). Phloem cells are usually located outside the
xylem. e two most common cells in the phloem are the companion cells and sieve cells.
Companion cells retain their nucleus and control the adjacent sieve cells. Dissolved nutrients, as
sucrose, flows through the sieve cells.
Figure 9. Phloem cells.
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SPECIAL PLANT TISSUES—TISSUES RESPONSIBLE FOR GROWTH
Plant cell types rise by mitosis from a meristem. A meristem is defined as a region of localized
mitosis at the tip of the shoot or root (a type known as the apical meristem) or lateral, occurring
in cylinders extending nearly the length of the plant. A cambium is a lateral meristem that
produces (usually) secondary growth. Secondary growth produces both wood and cork (although
from separate secondary meristems). Meristematic regions generally are composed of cells with
thin walls, small vacuoles and no apparent specialization. ese cells have the capacity to produce
cells that can differentiate.
Figure 10. Two views of the structure of the root and root meristem
.
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