Biology 102
Fall 2001
R. Brundage
Lecture 6: Part 2
Plant Tissues
I.Overview of the Plant Body
A.Although no one species of the 260,000 species of plants can be considered
typical, the focus here is on angiosperms.
B.Shoots and Roots
1.Shoots consist of stems, leaves, and flowers (reproductive structures).
a.Water, minerals, and organic substances are transported.
b.Stems are frameworks for upright growth and to display flowers.
c.Parts of the system store food.
2.Roots usually grow below ground.
a.A root system absorbs water and minerals from soil and conducts
them upward.
b.Roots store food; they also anchor and support the plant.
C.Three Plant Tissue Systems
1.The ground tissue system makes up the bulk of the plant body.
2.The vascular tissue system contains two kinds of conducting tissues that
distribute water and solutes through the plant body.
3.The dermal tissue system covers and protects the plant’s surfaces.
D.Meristems?Where Tissues Originate
1.Meristems are localized regions of self-perpetuating, embryonic cells.
2.There are two kinds of meristems:
a.Apical meristem at the tips of roots and stems is responsible for
growth and elongation.
1.Descendants of some of these cells will develop into the
specialized tissues of the elongating root and stem.
2.Growth originating at root and shoot tips is labeled
primary growth.
3.Lateral meristem tissues are responsible for the increase in diameter of
older roots and stems.
E, Vascular Meristems
1.Vascular cambium and cork cambium are the two kinds
of lateral meristems.
2.These are responsible for secondary growth which adds
to woody parts of trees for example.
II.Types of Plant Tissues
A.Simple Tissues
1.Parenchyma makes up most of the soft, moist primary growth of plants.
a.Its thin-walled, pliable cells stay alive and retain the capacity to
divide.
b.Various types participate in photosynthesis (mesophyll), storage,
secretion, and other tasks.
2.Collenchyma cells are thickened and help strengthen the plant (for
example, "strings" in celery).
a.It is commonly arranged at strands or cylinders beneath the
dermal tissue of stems and stalks.
b.The primary cell walls of collenchyma become thickened with
cellulose and pectin at maturity, often at their corners.
3.Sclerenchyma cells provide mechanical support and protection in mature
plants.
a.The secondary walls are thick and often impregnated with lignin,
which strengthens and waterproofs cell walls.
b.Sclerenchyma cells form fibers such as in hemp and flax; others
called sclereids form strong coats around seeds as in a peach
pit.
B.Complex Tissues
1.Vascular tissues function in the distribution of substances throughout the
plant.
a.Xylem uses two kinds of cells (dead at maturity) to conduct
water and minerals absorbed from the soil:
2.Vessel members are shorter cells joined end to end to
form a vessel with perforation plates at the ends of each
member.
3.Tracheids are long cells with tapered, overlapping
ends.
4.Phloem transports sugars and other solutes throughout the plant body.
5.Phloem contains living conducting cells called sieve
tube members which bear clusters of pores in the walls
through which the cytoplasm of adjacent cells is
connected.
6.Companion cells, adjacent to the sieve tube members,
help to load sugars produced in leaves and unload them
in storage and growth regions.
C.A dermal tissue system called epidermis covers all primary plant parts.
a.A waxy cuticle covers the external surfaces of the plant to restrict water
loss and resist microbial attack.
b.Stomata openings between pairs of guard cells permit water and gaseous
exchange with the air.
c.The periderm replaces the epidermis when roots and stems increase in
diameter and become woody.
III.Dicots and Monocots?Same Tissues, Different Features
1.Dicots include common trees and shrubs (other than conifers).
2.Monocots include grasses, lilies, irises, and palms.
3.Monocot seeds have one cotyledon ("seed leaf"), and dicot seeds have two.
IV.Primary Structure of Shoots
A.How Stems and Leaves Form
1.Leaves develop from leaf primordia along the apical meristems of stems.
a.A node is the point where a leaf or leaves attach to the stem.
b.An internode is the region on the stem between two nodes.
2.Buds develop in the leaf axils (the upper angle where leaves attach to the
stem).
a.A bud is an undeveloped shoot of mostly meristematic tissue
covered by modified leaves (bud scales).
b.Buds give rise to stems, leaves, flowers.
B.Internal Structure of Stems
1.A vascular bundle is a multistranded cord of primary xylem and phloem
running lengthwise through the ground tissue of shoots.
2.The arrangement of vascular bundles is genetically different in dicots and
monocots:
a.The stems of most dicots have vascular bundles arranged as a
ring that divides the ground tissue into the outer cortex and inner
pith.
b.In most monocots, the vascular bundles are scattered throughout
the ground tissue.
V.A Closer Look at Leaves
A.Similarities and Differences Among Leaves
1.Leaves are metabolic factories equipped with photosynthetic cells.
2.Deciduous trees drop their leaves as winter approaches, evergreens retain
theirs.
3.Leaves vary enormously in shape, size, texture, and surface features.
a.Monocot leaves tend to have a flat surface, like a knife blade, the
base of which encircles and sheaths the stem.
b.Dicot leaves have a broad blade attached by a petiole to the stem;
the blade may be lobed or composed of leaflets.
4.Leaves represent a large surface area that is exposed to sunlight and
carbon dioxide.
B.The Fine Structure of Leaves
1.Epidermis covers every leaf surface exposed to air.
a.A cuticle layer minimizes water loss.
b.Stomata are located mostly on the lower epidermis.
2.Mesophyll, consisting of photosynthetic parenchyma cells, extends
throughout the interior of the leaf.
a.Air spaces, which connect to the stomata, participate in gaseous
exchange.
b.Palisade mesophyll cells lie closer to the epidermis and are
columnar in shape compared to the spongy mesophyll below
them.
3.The leaf’s veins are vascular bundles of xylem and phloem that form a
network for movement of water, solutes, and photosynthetic products.
VI.Primary Structure of Roots
A.Taproot and Fibrous Root Systems
1.In most dicots, the primary root emerges from the seedling, increases in
diameter, and grows downward.
a.Lateral roots emerge sideways along its length.
b.The primary root plus lateral roots form the taproot system.
2.In monocots, the taproot is replaced by adventitious roots that arise from
the stem.
a.These roots and their branchings form a fibrous root system.
b.Fibrous roots do not penetrate as deeply as taproots.
B.Internal Structure of Roots
1.Cells in the apical meristem divide and then differentiate into root
epidermis, ground tissues, and vascular tissues behind the meristematic
region.
a.The root cap protects the apical meristem and pushes through the
soil.
b.Cells are torn loose as the root grows.
2.Protoderm gives rise to the root epidermis with its extensions called root
hairs for the outer absorptive interface with the environment.
3.Vascular tissues form a vascular cylinder arranged as a central column.
a.The column is surrounded by root cortex (ground tissue), which
has abundant air spaces within it.
b.The endodermis–the innermost layer of the cortex–surrounds the
vascular cylinder and helps control water movement into it.
c.Just inside the endodermis is the pericycle–it is meristematic and
can give rise to lateral roots.
C.Regarding the Sidewalk-Buckling, Record-Breaking Root Systems
1.Most roots extend to a depth of 2 to 5 meters, but desert species can
extend to 50 meters.
2.Roots can radiate outward for over 15 meters.
3.Total surface area of a single rye plant can equal 600 square meters.
VII.Accumulated Secondary Growth?The Woody Plants
A.Woody and Nonwoody Plants Compared
1.Seasonal growth cycles proceed from germination, to seed formation, to
death.
a.Annuals complete their life cycle in one season; they are
nonwoody, or herbaceous plants such as corn.
b.Biennials, such as carrots, live two seasons: vegetative growth
the first, flower and seed formation the second.
c.Perennials live many years and have secondary growth
(examples: roses, grape vines, apple trees).
2.Woody plants such as dicots and gymnosperms show secondary growth
by producing large amounts of secondary xylem.
B.Activity at the Vascular Cambium
1.Vascular cambium is a cylinderlike lateral meristem.
a.It produces secondary xylem on its inner face and secondary
phloem on its outer.
b.The secondary growth displaces the cells of the vascular
cambium toward the stem surface.
2.Secondary xylem and phloem form at the vascular cambium of the roots
also.
VIII.A Look at Wood and Bark
A.Formation of Bark
1.In response to rupture of the outer cortex (by girth expansion), cork
cambium produces the periderm–a corky replacement of the epidermis.
a.Periderm and secondary phloem constitute bark.
b.Periderm consists of cork, secondary cortex, and the cork
cambium.
c.Cork contain suberin; it protects, insulates, and waterproofs the
stem or root surface.
2.Lenticels are small channels through the corky surface of bark that allow
for exchange of gases.
B.Heartwood and Sapwood
1.Heartwood lies at the center of older stems and roots.
a.It is a depository for resins, oils, gums, and tannins.
b.It makes the tree strong and able to defy gravity.
2.Sapwood is secondary growth located between heartwood and the
vascular cambium.
a.It is wet, pale in color, and not as strong.
b.It is rich in the sugar-rich fluid of the phloem (for example,
maple trees).
C.Early Wood, Late Wood, and Tree Rings
1.In regions with cool winters or dry spells, the vascular cambium is
inactive during part of the year.
a.Early wood (start of growing season) contains xylem with large
diameters and thin walls.
b.Late wood contains xylem with small diameters and thick walls.
c.Growth rings appear as alternating light bands of early wood and
dark bands of late wood.
2.Hardwood (such as oak) has vessels, tracheids, and fibers in its xylem.
3.Softwood (such as conifers) have no vessels or fibers.
D.Limits to Secondary Growth
1.Some trees live in habitats too harsh and remote for most invaders.
2.Most trees use a strategy of compartmentalization to wall off invaders,
building a fortress of thickened cell walls around wounds, or deploying
toxic compounds.