Ch 35 Plant Structure & Growth
I. The Plant Body
A. Both Genes & Environment affect the plant body
1. Plant structure shows interaction w/ environment on two scales
a. evolutionary adaptations- let plants live in variety of environments
b. morphological adaptations- changes that depend both on genes & environmental influences
2. Ex. Evolutionary adaptations
a. cacti- leaves are needles, photosynthesis occurs along stem- reduces surface area & water loss
b. pine needles- shed snow & reduce water loss
3. Ex Environmentally caused adaptations
a. C4 & CAM plants- drought adaptation – not always in use
b. closing stomata – drought adaptation
B. The 3 basic organs of plants- roots, stems, & leaves
1. All plants have two basic ”zones”- roots & shoots
a. plants adapted to collect raw materials from both air & water
b. Flowering plants have two major groups-monocots & dicots
2. Root system
a. Function- anchoring, absorption of minerals & water, storing food
b. fibrous root system- found in monocots; mat of thin roots spread below the soil surface;
increases absorption area, anchors surrounding area
c. tap root- found in dicots, one long thick root that may or may not form branches’ usually used
for food storage or to go deep to get water
d. root hairs- extensions of epidermal cells on the root surface to increase surface area as an aid
to adsorption of water & minerals
e. adventitious roots- any plant root growing from an atypical location, ex from a leaf or node
3. Shoot system
a. Function- leaves absorb light & carbon dioxide for photosynthesis, stems support leaves &
flowers
b. shoot system -consists of leaves & stems (flowers are specialized leaves & stems)
c. Two categories:
1) vegetative- leaf bearing
2) reproductive- flower bearing
d. Stems are made of alternating nodes & internodes
1) nodes- points where leaves attach
2) internodes- stem segments between nodes
3) axillary bud (lateral meristem)- stem bud that could form a vegetative branch; found
in angle formed where leaf connects to stem
4) apical bud (apical meristem)- at tip of growing stem; has developing leaves & compact
series of nodes & internodes
5) apical dominance- apical bud inhibits growth of axillary buds
d. Leaves- main photosynthetic region of most plants
1) blade- flattened part of the leaf; may have a variety of shapes, vein patterns, & etc
2) petiole-stem that attaches leaf to plant
3) leaf shapes- simple, compound & doubly compound
4) monocot leaves- lance shaped, may be in leaflets, parallel venation
5) dicot leaves- many shapes including those with leaflets; netted venation
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Monocot vs Dicot (Use p. 721 to fill this in.)
Embryo
Leaf
Stems
Roots
Flowers
venation
Monocot 1 seed leaf
Parallel
Only soft stems scattered
Fibrous roots
Flower parts in 3’s
vascular bundles
Dicot
2 seed
netted
Soft & woody stems
Tap root
Flower parts in 4’s
leaves
Vascular bundles in a ring
& 5’s
C. Plant organs are composed of three tissues
1. Dermal tissue- (epidermis) skin of the plant; made of single layer of tightly packed cells
a. cuticle- waxy layer on surface of leaves & most stems; helps plant retain water
2. Vascular tissue- fxn is transport of materials between roots & shoots
a. xylem-carries H2O & dissolved minerals up to shoots & leaves through vessel elements
1) vessel elements –elongated cells, dead at functional maturity; interior substance is
gone; nonliving cell walls form tubes through which water can pass
2) tracheids- like vessel elements but narrower, spindle shaped with pits through
which water flows between cells; also fxn as support
Pits- thinner regions of the cell wall- only the primary cell wall is there
3) xylem vessels- vessel elements aligned end to end to from long micropipes – have
perforated ends
b. phloem- carries food made in leaves to the roots & the non-photosynthetic parts of the stem
1) sieve tube members-chains of cells that form larger tubes through which sucrose,
organic compounds & mineral ions are transported
Alive at maturity but lack nucleus, vacuole, ribosomes & most organelles
2) sieve plates- on the ends of sieve tubes, perforated (in angiosperms)
3) companion cells- attached to sive tube member cell through plasmodesmata and other
channels; fxn is to run & support the sieve tube member cell& load sugars into it for
transport
3. Ground tissue- is not dermal or vascular- fxn varies- photosynthesis, storage, support
a. pith- internal to the vascular tissue
b. cortex- external to the vascular tissue, below epidermis
D. Plant tissues are composed of three basic cell types
Protoplast- the cell contents other than the cell wall
1. Parenchyma- thin flexible primary cell walls, almost no secondary cell walls; large central vac. –least
specialized of all plant cells
> can perform most of the fxns of other plant cells
> all developing cells begin as parenchyma
> can be used to generate the entire plant
a. primary cell walls
b. secondary cell walls
2. Collenchyma cells- Unevenly thickened cell walls; grouped in strands or cylinders, help support the
young shoot
No secondary cell walls
Living, flexible, able to elongate with other growing cells so they don’t limit shot growth
3. Sclerenchyma cells-much more rigid, found in mature non elongating parts of plant; often dead at
functional lmaturity, primary fxn is support
a. fibers- long, slender, tapered & occur in groups- make useable fibers like rope
b. scelreids- shorter, irregular shape, give hardness to nut shells
II. Plant Growth & Development
A. Meristems generate cells for new organs through our the plant’s life time.
1. Growth- to increase in mass due to cell division & cell expansion
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2. Development- all the changes that progressively change an organism from an immature form to a
more complex adult form
a. Indeterminate growth- growth through out the plant’s lifetime- there is no “stopping point”
b. determinant growth- seen in fruits, leaves, & flowers; when an organism or structure stops
growing after it reaches a certain size
3. Plant life cycles- most plants do die, they have a life span
Germination æ flowering æ seed production æ death
a. annuals- life span is complete in one year
b. biennials - life span is complete in two years
c. perennials- life span is many years long
4. Meristems- perpetually embryonic tissues in the growth region which divide to produce new cells
a. initials- new cells that remain on the meristem to continue to produce more cells
b. derivatives- new cells that are pushed out of the meristem region; undergo a rapid cell division
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phase followed by development into specialized tissues
c. meristems make indeterminate growth possible
5. Types of meristems
a. apical meristems- located at root tip & in bud at shoot tip; allow plants to increase in length
1) primary growth- lets roots push through soil & shoots increase exposure to light &
CO2 by increasing height
b. lateral meristems- cylinders of dividing cells extending along the length of the roots & shoots,
responsible for the production of bark, the formation of wood (largely xylem fibers) the
increase in diameter of a tree
2) secondary growth- progressive thickening of roots & shoots
6. Looking at a Twig (see p.730)
Draw & label the dormant twig with the following labels:
Terminal bud
Bud scale
axillary buds
bud scale scars
leaf scar
node
internode
lenticle
B Primary growth: Apical meristems extend roots & shoots by making the primary plant body.
1. Primary growth of roots
a. primary plant body- laid down
b. root cap- cap of dead, thickened cells that protects the root tip & secretes slime
c. zone of cell division-apical and primary meristems; produces cells for the growing root &
replaces root cap cells lost due to abrasion
d. quiescent center- in center of apical meristem, slowly dividing cells, can replace meristem it it
gets damaged
e. protoderm
three dividing primary meristems above the apical meristem
f. procambium
form the dermal, valcular and ground tissues, respectively of the
g. ground meristem
root.
h. zone of elongation- new cells get longer; provide most growth in length, push root tip through
i. zone of maturation- cells begin to differentiate into their fimal mature form
2. Primary root tissues
a. epidermis- outer surface, will develop root hairs to increase absorption
b. stele- also called the central cylinder; from the procambium, made of xylem & phloem
1) monocots-pith (parenchyma tissue surrounded by successive rings of xylem & phloem
2) dicots- two or more arms of xylem extend from center, phloem lies between the arms
c. cortex- from ground meristem, parenchyma cells; storage
d. endodermis- innermost layer of cortex, up against the stele, barrier to passage into the stele
e. pericycle- lies just inside the endodermis, part of the stele, functions in forming lateral roots
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f. lateral roots- elongates through the cortex to exit the main root; its stele remains connected to
the main root’s stele
3. Primary growth of shoots
a. apical meristem- produces protoderm, procambium, & ground meristem layers of cells
b. leaf primordial are laid down to the sides of the apical meristem
c. axillary buds form just above the leaf primordial- also left behind by the apical meristem- a
knot of meristematic cells that can form twigs
d. Elongation comes from elongation of cells left in internode areas
e. New twigs have vascular tissue continuous with the vascular tissue of stem- develop from
edges of twig instead of deep with in it.
C. Secondary growth: Lateral meristems add girth by producing secondary vascular tissue and periderm
1. Secondary plant body comes from Vascular cambium
2. Parenchyma cells differentiate into vascular cambium cells
a. Forms a continuous cylinder of vascular cambium- in a ring from bundle to bundle & fron=m
root tip to shoot tip
b. Xylem forms xylem rays to inside of the vascular bundles
c. Phloem forms phloem rays to the outside of the vascular bundles
d. Thin layer of vascular cambium cells forms in between xylem & phloem
e. Ray initials, specialized cells in the cambium, produce rays of parenchyma tissue in the
xylem & phloem
1)
Fxn of rays is storage & lateral transport
f. Fusiform initials- specialized cells in the cambium, produce secondary xylem & phloem
3. Wood- accumulation of secondary xylem cells w/ heavy walls thickened by lignin
a. Annual ring- comes from seasonal cycle of cambium dormancy, spring wood production (larger
vessels) & summer wood production (smaller, more densely packed vessels)
4. Bark- correct name is cork; consists of phloem & periderm
a. First epidermis splits off
b. Cork cambium- meristematic tissue that forms in the cortex
1)
Produces cork cells filled with suberin- waterproof, discourages insects
2)
Cork cells form outer covering now
c. Periderm –layer formed by cork & cork cambium
1) Lenticles- small splits in the periderm for gas exchange
5. Older trees center is supportive column of older, resin filled xylem & sapwood is functioning
secondary xylem- dead
 Living tissue- is only the bark area where the cambium & phloem cells are
6. Secondary growth in roots- also has two lateral meristems
a. vascular cambium- produces xylem internal to phloem
b. cork cambium- forms pericycle & poeriderm
c. older roots have annual rings & bark just like stems do
III. Mechanism of plant growth and development
A. Growth morphogenesis & differentiation produce the plant body
1. morphogenesis- organization of cells into organs & tissues; develops body form
2. differentiation- creates cells w/ specific structures & fxns
3. developmental phase of the plant affects destiny of cells that are formed
4. Plane & symmetry
a. asymmetrical cell division- one daughter cell gets more cytoplasm than the other
b. plane of cell division- decided by where prophase band (microtubules & filaments, etc.) are set
can be horizontal or vertical
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5. orientation of cell expansion
a. 90% of cell expansion is due to water uptake- weakened cell wall allows turgor pressure to
drop as cell expands; encourages further osmosis & further expansion
b. elongates the shoots & roots rapidly
c. elongation is perpendicular to prophase band
B. The importance of cell location to its future development
1. pattern formation-the development of structures in specific locations
2. positional information – signals that indicate the position of a cell in a plant
a. axial polarity- asymmetric first division indicates the root/shoot orientation of the plant
b. KNOTTED 1 homeotic gene affects leaf shape (morphology)
C. Genes controlling transcription influence meristematic changes
1. Cells of the meristem are not committed to any particular cell type
2. In the very young area of the plant up by the meristem, new cells are made
a. Most become nodes or internodes
b. modules- sometimes the cells are grouped into mature, specialized knots called modules & left
behind on the sides of the stems; the future of the module is set when it gets formed
c. phase change- refers to whether a the module, when it forms a branch (meristem), will be vegetative
(just a branch with leaves) or reproductive, producing a flower & later a fruit with seeds
 Meristem identity genes- when expressed trigger genes needed for flowers to develop
 Organ identity genes- determine the floral pattern, when triggered, they make sure the right
flower part develops where and as it should
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