PLANTS AND THEIR STRUCTURE
A plant has two
organ systems: 1) the shoot system, and 2) the root system. The
shoot system is above ground and includes the organs such as leaves, buds,
stems, flowers (if the plant has any), and fruits (if the plant has any). The
root system includes those parts of the plant below ground, such as the roots, tubers,
and rhizomes.
Major organ
systems of the plant body. The above image (left) is from Purves et al., Life:
The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com), used with permission. The above
illustration (right) is from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Intr._Plant_Body_Spring_/Primary_130_Lab_Images/Bean_whole_morphology
Plant cells are
formed at meristems, and then develop into cell types which are grouped into tissues.
Plants have only three tissue types: 1) Dermal; 2) Ground; and 3)
Vascular. Dermal tissue covers the outer surface of herbaceous
plants. Dermal tissue is composed of epidermal cells, closely packed cells that
secrete a waxy cuticle that aids in the prevention of water loss. The ground
tissue comprises the bulk of the primary plant body. Parenchyma, collenchyma,
and sclerenchyma cells are common in the ground tissue. Vascular tissue transports
food, water, hormones and minerals within the plant. Vascular tissue includes
xylem, phloem, parenchyma, and cambium cells.
Two views of
the structure of the root and root meristem. Images from Purves et al., Life:
The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com), used with permission.
Plant cell
types rise by mitosis from a meristem. A meristem may be defined
as a region of localized mitosis. Meristems may be 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).
A generalized
plant cell type, parenchyma cells are alive at maturity. They function
in storage, photosynthesis, and as the bulk of ground and vascular
tissues. Palisade parenchyma cells are elogated 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 within a woody
stem. Parenchyma cells also occur within the xylem and phloem of vascular
bundles. The 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.
Diagram of leaf
structure. Note the arrangement of tissue layers within the leaf. Image from
Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer
Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
used with permission.
Cross-section
of a stained leaf of Syringia. The above images is modified from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Leaf/Syringa_leaf/Cross_Section/Syringa_Blade_composite_large.
Lily Parenchyma
Cell (cross-section) (TEM x7,210). Note the large nucleus and nucleolus in the
center of the cell, mitochondria and plastids in the cytoplasm. This image is
copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.
Collenchyma cells support the plant. These cells are
charcterized by thickenings of the wall, the are alive at maturity. They tend
to occur as part of vascular bundles or on the corners of angular stems. In
many prepared slides they stain red.
Collenchyma
cells. Note the thick walls on the collenchyma cells occurring at the edges of
the Medicago stem cross section. The above image is cropped from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Cells_and_Tissues/Medicago_Stem/Collenchyma.
Sclerenchyma cells support the plant. They often occur as
bundle cap fibers. Sclerenchyma cells are characterized by thickenings in their
secondary walls. They are dead at maturity. They, like collenchyma, stain red
in many commonly used prepared slides.
A common type
of schlerenchyma cell is the fiber.
Sclerenchyma
cells. The above (left) image is cropped from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Cells_and_Tissues/Scherenchyma/Fibers_-_Tilia_Phloem/Stem_cross_section_1000x.
Some
sclerenchyma cells occur in the fruits of Pear. These cells (sclereids
or stone cells) give pears their gritty texture. View stone cells by clicking here.
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 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 end-plates that connect
cell to cell. Vessel elements are shorter, much wider, and lack end plates.
They occur only in angiosperms, the most recently evolved large group of
plants.
Xylem cells.
The above image (left) is from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Stem/Zea_cross_section/Vascular_Bundle_labelled.
Tracheids,
longer, and narrower than most vessels, appear first in the fossil record.
Vessels occur later. Tracheids have obliquely-angled endwalls cut across by
bars. The evolutionary trend in vessels is for shorter cells, with no bars on
the endwalls.
Conducting
cells of the xylem; tracheids (left) are more primitive, while the various
types of vessels (the other three) are more advanced. Image from Purves et al.,
Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com), used with permission.
Conductive
Vessel Element in Mountain Mahogany Wood (SEM x750). This image is copyright
Dennis Kunkel at www.DennisKunkel.com, used with permission. Phloem | Phloem cells conduct food from leaves to rest
of the plant. They are alive at maturity and tend to stain green (with the
stain fast green). Phloem cells are usually located outside the xylem. The 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 food, as sucrose, flows through the sieve cells.
Phloem cells.
The above (left) image is cropped from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Cells_and_Tissues/Cucurbita_Stem/Cross_Section/Phloem/Sieve-plate.
Phloem cells as
seen in longitudinal section. Note the longitudinal view of the sieve plate
inside the large sieve tube cell. Right image is a diagram of the longitudinal
view of phloem cells. The above image(left) is cropped from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Cells_and_Tissues/Cucurbita_Stem/Longitudinal_Section/Sieve-plate_l.s.
Right image is from Purves et al., Life: The Science of Biology, 4th
Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
used with permission.
Epidermis
The epidermal
tissue functions in prevention of water loss and acts as a barrier to fungi
and other invaders. Thus, epidermal cells are closely packed, with little
intercellular space. To further cut down on water loss, many plants have a waxy
cuticle layer deposited on top of the epidermal cells.
Guard
Cells
To facilitate
gas exchange between the inner parts of leaves, stems, and fruits, plants have
a series of openings known as stomata (singular stoma). Obviously these
openings would allow gas exchange, but at a cost of water loss. Guard cells
are bean-shaped cells covering the stomata opening. They regulate exchange of
water vapor, oxygen and carbon dioxide through the stoma.
Scanning
electron micrograph of Equisetum (horsetail or scouring rush) epidermis.
Note the oval stomatal apparatuses in the center of the stem. The above image
is from http://www.mcs.csuhayward.edu/sem/images/horsel4.gif.
Epidermal
cells, including guard cells, of corn. The above image is from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Leaf/Corn_epidermal_peel.
Pea Leaf Stoma, Vicea
sp. (SEM x3,520). This image is copyright Dennis Kunkel at www.DennisKunkel.com,
used with permission.
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