Transcript Botany Chapter 4 - Merrillville Community School

Plant Structure
Roots, Stems and Leaves
1. Describe the main functions of roots
2. Compare & contrast tap roots and fibrous roots
3. Describe the pattern of tissue development &
growth in roots
4. Explain how root structure contributes to its function
of absorbing water & minerals
5. Compare & contrast the organization of tissues in
monocot roots with those of dicots and
gymnosperms
6. Discuss some variations in root structure that
correspond to specialized functions
7. Discuss the benefits to plants gained by mycorhizal
associations and nitrogen fixing bacteria
 Anchor
the plant in the soil
 Absorb and transport water and minerals
 Receive and store organic materials from the
stems and leaves
 Tap
root systems
have a main root,
usually vertical, with
lateral roots and
smaller branching
roots
 Fibrous roots have
no main root. All
roots are
approximately equal
in size
 The
tap root is
derived from the
embryonic radicle
 Lateral roots grow
from the tap root, and
branch roots grow
from the lateral roots
 Tap root systems are
characteristic of
larger, longer living
plants
In plants with fibrous
root systems, the
embryonic radicle
dies, and roots are
derived from the lower
stem (“adventitious”
roots)
 Lateral roots branch off
from the adventitious
roots and extend
horizontally

 In
deep soil?
 In shallow soil?
 For rapid growth?
 For larger, taller plants?
 Initially after a light rain?
 In periods of drought?
 In porous soil?
 In storing food, water and minerals?
Growth initiates near
the root tip (“apical”
meristem)
 Meristematic initials
form a small
“quiescent center” just
inside the root cap
 Derivative meristem
differentiates into
procambium, ground
meristem, and
protoderm

 The
quiescent center and
fresh derivative cells form
the Zone of Cell Division
 Older derivative cells that
are growing rapidly form
the Zone of Elongation
 Cells that are fully grown
and differentiated form
the Zone of Maturation
 Root hairs are derived
from the zone of
maturation
 Water
uptake occurs mostly in the root
hairs, which are each only one cell thick.
The fine, threadlike nature of the root
hairs maximizes the surface area of
contact with the soil and its moisture
 Some of the water that is absorbed will
enter the cytoplasm by osmosis
 Some of the water will remain outside of
the cell, associated with the cellulose of
the cell wall
Water traveling
between cells
(“intercellular”),
conducted by the
cell walls is
“Apoplastic”
 Water traveling
through the
cytoplasm and
across cell
membranes is
“Symplastic”

 In
roots, the vascular tissue is at the core
of the root, in the Stele
 The endodermis forms a boundary
between the Cortex and the stele
 The intercellular spaces in the
endodermis are sealed off by the
Casparian Strip. The casparian strip is
impermeable to water flow, preventing
Apoplastic flow into the stele
 It
matters because apoplastic transport is
nonspecific. The cell walls and
intercellular spaces are not selectively
permeable. Only the cell membranes
are.
 By forcing only symplastic transport
(through the cells, not around them) into
the stele, the plant can control the
materials that enter the vascular tissue
 https://www.youtube.com/watch?v=o32j
qyIpoHg
 https://www.youtube.com/watch?v=TWN
tXw-MWtE
The arrangement of
vascular tissue varies
depending on the type
of plant
 In Monocots, the xylem
and phloem alternate
with each other in a
ring surrounding a core
of parenchyma
 In Dicots, the xylem
forms a lobed core with
phloem between the
lobes

 Aerial
Roots
• Epiphytes – Absorb water from humid air
• Climbing plants – Anchor the plant to a host
• As prop roots – Adventitious roots to stabilize and
anchor (usually in monocots)
 Buttress
roots – Lateral roots for support in
thin soil (usually in dicots)
 Pneumatophores – air roots to provide
oxygen (if roots are submerged in water)
 Storage roots – like carrots, radishes, beets
A
symbiotic association is a close
ecological relationship between two
different species, where one depends
upon the other. If both benefit, it is called
Mutualism.
 Plants live in close association with a
variety of microorganisms (fungi and
bacteria). Some are parasitic, but some
benefit the plant in specific ways
 Myco
= fungus, rhizal = root
 Fungi are heterotrophs, usually
decomposers, but mycorhizal fungi
obtain food from the roots of a host plant
 The host plant loses some food to the
fungus, but benefits because the fungus
provides water and minerals in greater
amounts than the plant could obtain on its
own – so both benefit
 Plants
are photoautotrophs. They produce
sugars by photosynthesis
 Producing amino acids and nucleotides is a
more complex issue because it requires
nitrogen, which must be obtained from the
soil and in a particular form
 Soil nitrogen originates from the activity of
nitrogen fixing bacteria
 Legumes (peas, beans, alfalfa) have nodules
full of nitrogen fixing bacteria in their roots
 Mycorhizal
 Legumes, with
Associations
 https://www.youtube.
com/watch?v=JFi6cJecm0
 https://www.youtube.
com/watch?v=DrsNu
wOnoEM
nodules full of
nitrogen fixing
bacteria
 https://www.youtube.
com/watch?v=4NKGS
4bj7cc
 https://www.youtube.
com/watch?v=9j6Jk8
YLgfY
1.
2.
3.
4.
5.
6.
How do taproot and fibrous root systems
differ?
Describe cell development and maturation
through the zones near a root tip
What are the functions of the root cap,
mucigel, and root hairs?
What are the roles of the pericycle and
endodermis?
What are some examples of specialized
adaptions in roots?
What are mycorhizae, and how are they
beneficial?
8.
9.
Discuss models of shoot primary growth


Zone model
Cell layer model



Alternate
Opposite
Whorled
Discuss the arrangement of vascular bundles in
stems
10. Compare and contrast the structure of stems in
monocots, dicots, and ferns
11. Describe patterns of leaf arrangement on stems
12. Discuss specialized functions of modified stems
(see page 86 in textbook)
Primary growth results
from apical meristem
activity
 Primary growth in
stems follows a similar
pattern to root growth:

• The apical meristem
initials form a small ball
of slowly dividing cells
• The surrounding
meristematic derivatives
differentiate into the
three primary tissues:
protoderm, procambium,
and ground meristem
 There
are two models
of stem growth (the
Zone model and the
Cell Layer model)
 There are variations
between the two, but
they share some
fundamentals
 For diagrams of the
growth models see
page 82 in the text
 Apical
meristem will give rise to 3 primary
meristems: protoderm, ground meristem,
and procambium
 The apical meristem is divided into 3 zones:
• The central mother zone gives rise to the other 2
zones (peripheral and pith zones)
• The peripheral zone surrounds the central mother
zone, divides rapidly, and can derive all 3 primary
meristems laterally and distally to the central mother
zone
• The pith zone derives ground meristem only, on the
proximal side of the central mother zone
 The
outer 2 layers of initials in the apical
meristem form the Tunica. These cells
divide perpendicular to the surface
(“anticlinal), and give rise to the protoderm
 The 3rd layer of meristematic initials and
underlying derivatives forms the Corpus.
These cells divide in both planes
(“anticlinal” and “periclinal”) and give rise
to the procambium and ground meristem
 The
zone model emphasizes the regions
of cell division, while the cell layer model
emphasizes the cell divisions themselves
 Both models share the same basic
pattern
• Derivative cells at the top and sides of the ball of
initials grow laterally to form the Leaf Primordia
and Vascular tissue
• Derivatives at the base of the initials grow to
form the pith parenchyma
 Vascular
tissue in the
shoot system forms a
consistent pattern of
bundles
 Generally, xylem
cells are located
toward the center of
the stem, while
phloem is more
toward the surface
As in the roots,
vascular bundles
form distinct
organizational
patterns that vary
depending on the
type of plant
 The trend from
root to stem is a
periferal
migration of
vascular bundles
(from middle
towards surface)

 See
diagrams in textbook, page 83
 Bryophytes
• No vascular tissue
 Seedless Vascular Plants
• Vascular tissue in stems is the same as in roots
 Monocots
• Vascular bundles dispersed
 Dicots & Gymnosperms
• Vascular bundles in a ring (eustele) outside the
pith
 Rhizomes
• Underground, horizontal stems - Storage
 Corms
• Resembles a bulb – stores food in an enlarged stem
surrounded by thin leaves
 Bulbs
• Stores food in fleshy leaves on a short stem
 Tubers
• Underground storage stem
 Stolons
• Above ground horizontal stems for asexual
reproduction
1.
2.
3.
4.
5.
How does stem growth differ from root
growth?
Describe the basic types of steles in
stems
What is phyllotaxy?
What do the differences between palm
trees and wheat plants reveal about
stem growth?
What are some similarities between
modified stems and modified roots?
13. Describe the process of leaf formation
14. Identify characteristic parts of a leaf and describe the structure
and function of each
• External
 Blade, Petiole, Stipules
• Internal
 Epidermis, Cuticle, Guard Cells, Stomata
 Mesophyll: Palisade and Spongy
 Vascular Bundle: Xylem, Phloem, Bundle Sheath
15. Explain variations in leaf structure
• Simple vs. compound leaves
• Shape variations
• Venation pattern variations
16. Distinguish between deciduous and nondeciduous (evergreen)
trees
17. Explain the role of abcission zones in seasonal leaf loss (“fall”)
18. Discuss specialized functions of modified leaves (see page in
textbook)

Leaf Buttress
• Bulges that are the first indications of leaf growth

Leaf Primordium
• Elongate from the leaf buttress and form the petiole and
blade

Petiole and Blade
• Petiole
 May range from stem-like to leaf-like
 May be absent (“sessile” leaves – usually monocots)
 May have 2 “stipules” at the node
• Blade
 Usually flattened
 Growth rates of ridges near the edge of the leaf blade
produce variations in leaf margins
Leaf primordia form
from the leaf buttress
 Primordia are
associated with the
apical meristem
 Note also the axillary
buds at the next more
mature node. The
growth rate of these
will depend upon the
level of auxin hormone
 Note “apical
dominance”

 Spongy
mesophyll
 Guard
Cells
 Palisade
mesophyll
 Upper
epidermis
 Stomate
 Lower
epidermis



Compound leaves have
more than one blade
sharing the same petiole
A node bearing a simple
leaf will have an axillary
bud at the juncture of
petiole to node
A blade on a compound
leaf will not have axillary
buds where the blades
attach to the petiole
 Deciduous
trees lose their leaves due to
seasonal changes (autumn). Evergreens
keep their leaves year round
 Some evergreens are well adapted to
seasonal change, but others simply live in
tropical regions where seasonal change
is minimal
 Most deciduous trees are angiosperms
while gymnosperms are often evergreen
Pine leaves are
shaped to
minimize
surface area in
proportion to
volume
 A thick
epidermis and
heavy waxy
cuticle prevent
water loss

Deciduous trees drop their leaves as a result of
changes at the point of attachment of the petiole to
the stem
 The abcission layer forms after the plant has
reabsorbed water and other materials from the leaf
 The abcission layer is basically a protective scar

 Tendrils
• Coiling structures used for attachment by
climbing plants
 Spines
• Note: Thorns are modified stems (arise from
axillary buds), spines are modified leaves
 Bracts
• Modified leaves superficially similar to petals,
found at the base of flowers