Growth and Development
I. Terminology: Growth, Differentiation, and Development
A. Growth: an irreversible increase in size
1. Fresh Mass (Weight)
2. Dry Mass (Weight)
B. Differentiation: process of cellular specialization
C. Development or Morphogenesis: formation of tissues,
organs, and organisms
II. Patterns of Growth and Development
A. Terminology
1. Growth Regions in Plants
a. Apical Meristems: Root Tip, Apical Bud, Axillary Buds)
(1) Primary Meristems: root tips, apical and Axillary
buds. Produce and increase in vertical growth of
a plant (shoots grow up, root grow down)
(2) Primary Growth: tissues produced from cell
divisions of the primary meristems.
b. Lateral Meristems (Cork Cambium, Vascular Cambium)
(1) Secondary Meristems: cambiums that give an
increase in the girth of a plant. Typically
found in dicots but not in monocots. Cork
cambium produces cells that add to the phellem
(cork) on the outer surface of the plant. The
vascular cambium adds cells to the xylem and
phloem.
(2) Secondary Growth: those tissue that are derived
from the cork and vascular cambiums.
2. Plant Growth Patterns
a. Determinate: Having definite limits. A type of
growth in which the plant stops growing after it
reaches a certain size.
b. Indeterminate: Having no set limits. A type of
growth characteristic of plants in which the
organism continues to grow as long as it lives.
3. Temporal Growth Patterns:
a. Monocarpic species (single reproductive event),
typical of many annual and biennial plants that
flower once, then die.
Polycarpic species (many reproductive events),
typical of perennial plants that grow for many
years and flower each year.
b. Annual: plants that have seed germination, growth
and reproduction in a one year cycle.
Biennial: plant that have seed germination, growth,
and reproduction in a two year cycle. Typically
the first years growth is a rosette of leaves
around an underground stem. Flowering occurs
during the second year.
Perennial: plant grow and flower over several years.
B. Steps in Cell Growth and Development
1. Cell division: mitosis, cytokinesis, and the cell
cycle (review these topics from Bgy 11, Bgy 30, and
Bgy 33)
a. Cells divide in different plants:
(1) Periclinal: parallel to the perimeter (closest
surface) of the plant
(2) Anticlinal: perpendicular to the perimeter
(closest surface) of the plant
b. Cell Plate formation in dividing cells
(1) Vesicles pinched off from the Golgi vesicles.
Contain pectins that make up the middle lamella.
Middle lamella cements the walls of adjacent cells
together.
(2) New cell wall formed. Primary Cell wall
(3) Vesicles migrate along the rodlike microtubules
that extend toward opposite poles of the dividing
cell
2. Cell Enlargement:
a. In part controlled by the plane of cell division and
the formation of the primary wall.
b. Cellulose laid down like a spring around the new
cell. Allows elongation of the cell as it takes up
water in the central vacuole
3. Cell Differentiation: specialization of the new cells.
Controlled in part by growth hormones.
C. Primary Wall Changes During Growth
1. Primary Walls: cellulose microfibrils embedded in an
amorphous matrix of noncellulosic polysaccharides and
some protein.
2. Cellulose microfibrils behave like a multistranded cable
that minimizes extension in the direction of the long
axis.
3. Growth can occur at right angles to the orientation of
the microfibril axis.
4. The plant cell wall retains a near-uniform thickness
during growth. The newest deposited cell wall has the
greatest control over cell growth.
5. If new microfibril orientation is random, growth will be
equal in all directions.
6. If new microfibril orientation is not random, but
occurs along one axis, growth will be favored in a
direction perpendicular to that axis. This is seen in
primary tissues of elongating roots, stems, and
petioles.
7. Cellulose formation is controlled by enzymes located on
the plasma membrane (plasmalemma): globules on the
outer face, rosettes on the inner face of the
membrane.
D. Physics of Growth: Water Potentials and Yield Points
1. Uptake of water stretches the cell walls. New cell wall
and membrane materials are synthesized so that
the wall does not become thinner.
2. Water pressure (turgor) drives growth by forcing the
wall and membranes to expand. Rate of water movement
into the cell is governed by two factors: the water
potential gradient and the permeability of the
membrane to water. The rate of cell enlargement is
also proportional to these factors.
3. Y = Ys + Yp = Ys + P
Y = water potential
Ys = osmotic or solute potential
Yp = pressure potential which is a real pressure, P,
called turgor pressure
Difference of water potential (DY) inside and outside a
cell is:
DY = (Yse + Pe) - (Ysi + Pi)
e = external
i = internal
Equation for the rate of cell enlargement:
(1/V)(dV/dt) = L(DY) = L(Yse - Ysi- Pi)
L = hydraulic conductivity of the membrane to water
V = cell volume
dV = incremental change in the volume of the cell
dt = infinitesimal increment of time
dV/Dt is the rate of cell increase in volume, or the
rate of growth. Growth is a geometric or logarithmic
function of time. To express this, divide dV/dt by
the cell volume to give a relative growth rate of the
cell which is defined as r.
4. These two equations show that there are two ways for
water potential inside a cell to be more negative
than water potential outside the cell. This makes
water uptake and growth possible
a. Solutes inside the cell may increase, making the
osmotic potential inside the cell more negative
However, solute concentrations inside growing cells
remain constant. Therefor, the driving force for
growth must be the turgor pressure.
b. Pressure inside the cell can decrease.
Pressure in the cell is caused by the mechanical
resistance of the cell wall to being stretched. If
this mechanical resistance is decreased (the wall
relaxes due to stretching) it results in lower
pressure, which reduces cell water potential, leading
to a larger water potential gradient (DY) and
movement of water into the cell.
5. Elastic Growth: cell walls stretch
Plastic Growth: cell walls do not return to original
dimensions. Plastic stretching increased by auxin.
Extensibility: both elastic and plastic components.
6. Plastic wall stretching is achieved as the cell wall is
loosened, so cellulose microfibrils can slide past
each other (shear) by breaking bonds between adjacent
microfibrils.
7. Plastic and elastic elements in series with each other.
As the plastic elements relax and stretch, they allow
the elastic elements to shorten. This only happens if
cell wall stress and turgor pressure are reduced. If
water enters as pressure decreases in response to
relaxation of the plastic element, the elastic
elements shorten only slightly, resulting in pressure
decreases only slightly. This is called creep: a
steady-state, growth process, where wall stress and
turgor remain constant.
8. Relative cell growth rate (r) is proportional to turgor
pressure (P) when it exceeds a yield threshold or
yield point (Y).
r=F(P-Y) for P>Y
Proportionality factor F is the wall extensibility.
Yield point (Y) is expressed as the minimum pressure
necessary to cause cell growth. Relative cell growth
rate depends upon five interrelated factors:
a. Conductivity of walls and membranes to water around
the cells
b. Difference in osmotic (solute) potential inside
and outside the cell
c. Cell turgor pressure
d. Two wall-yielding properties: extensibility and
yield threshold
9. Wall relaxation is the reduction in wall stress at
constant cell-wall dimensions.
10. As wall relaxation occurs, turgor pressure drops
exponentially until the yield point (Y) is reached at
some value above zero. The rate of turgor-pressure
decrease can be used to evaluate the wall
extensibility (F).
11. In vivo stress relaxation uses the natural stresses on
the wall (cell turgor). Turgor applies hydraulic
stress in all directions from within the cell.
12. In enlarging cells, water uptake dilutes the solute and
lowers Ysi. However, solutes are absorbed from the
surroundings or synthesized in the tissue.
III. Growth Kinetics: Growth Through Time
A. Whole Organs: The S-Shaped Growth Curve
1. Phases of a Sigmoidal Growth Curve:
a. Logarithmic phase: size (V) increases exponentially
with time (t).
b. Growth rate (dV/dt) is slow at first, but the rate
continuously increases. Rate is proportional to the
size of the organism; the larger the organism, the
faster it grows.
c. Linear phase: increase in size continues at a
constant, usually maximum rate for some time.
d. Senescence phase: decreasing growth rate as the
plant reaches maturity and begins to senesce.
2. Typically, elongation is more rapid during the long
days of late spring and early summer but there are
exceptions.
3. Growth in stem diameter continues at a decreasing rate,
until well after growth in height stops.
4. Dormancy does not occur in the roots.
B. The Flow Analogy of Plant Growth
1. Dividing cells are found in the apical meristems.
Elongating cells are found farther from the tip.
Differentiating cells are found even farther from the
tip.
2. Phyllotaxy: history of leaf production can be understood
by examining pattern formed by leaves or leaf scales
along the stem. Future leaf development of a leaf
primordium at the stem tip can be determined by
examining older leaves farther down the stem.
3. These characteristics of a stem indicate that
indeterminate growth in plants is a flow process.
4. New cells being produced in the meristem pass through
the hook at the top of the developing stem. These
cells enlarge, elongate, and eventually differentiate
below the hook
a. Cell divisions stop before the cells reach the hook
b. Hook must form when cells on the outside elongate
more than those on the inside.
5. Hook straightening is controlled by light of different
irradiance and wavelength.
IV. Plant Organs: How They Grow
A. Roots
1. Organization of the Young Root.
a. Seed germination begins with protrusion of the
radicle. In some species, cytokinesis occurs in the
radicle before germination is complete. In other
species few mitotic divisions occur before radicle
protrusion. Elongation occurs by growth of cells
formed when the embryo developed.
b. Oldest root cap cells are distal (farthest from the
meristem). Proximal (closer to meristem) root cap
cells are younger. Root cap protects the meristem
and is important in the perception of gravity. The
root cap also exudes a polysaccharide rich slime or
mucigel that lubricates the root to help in soil
penetration. The root cap may also harbor
microorganisms, influence formation of mycorrhizae,
root nodules, and ion uptake.
c. Cells divide in the apical meristem. Develop into
epidermis, cortex, endodermis, pericycle, phloem, and
xylem. In-between the root cap and the apical
meristem of the root is the quiescent center where
cell division seldom occurs.
2. Formation of Lateral Roots begins several millimeters to
a few centimeters distal to the root tip. The
pericycle is the tissue responsible for initiation of
lateral root formation. Thus, a lateral root must
penetrate the cortex of the root before emerging into
the soil.
3. Radial Growth of Roots
a. In the root-hair zone, the vascular cambium between
the primary phloem and primary xylem produces new
xylem and phloem cell. These tissues increase with
of the root. Most monocots do not form a vascular
cambium and thus do not have secondary growth.
b. After the vascular cambium initiates secondary
growth, a cork cambium (phellogen) is initiated in
the pericycle. Cork (phellem) is produced toward the
outside and secondary cortex (phelloderm) is produced
to the inside of the cork cambium. The new cell
become suberinized.
c. Suberinized roots can absorb water and mineral salts
through: (1) lenticles, (2) tiny crevices formed by
penetration of branch roots, (3) holes left when
branch roots die.
B. Stems
1. Apical meristems of shoots form in the embryo and are
responsible for origination of new leaves, branches,
and floral parts.
2. Intercalary meristems: intercalated (inserted) between
regions of older, nondividing cells.
C. Leaves
1. Leaf Primordium: do not develop randomly
a. Phyllotaxis
b. Recent model: one substance diffuses from the apex
and another from the ends of the vascular system;
the second substance is consumed by formation of a
leaf.
2. Leaf Development: outward extension occurs by both
periclinal and anticlinal divisions at the primordium
tip. Meristematic activity begins throughout the
length of the leaf.
a. Grass leaf and conifer needle development stops first
at the distal end and eventually stays at the leaf
base.
b. Angiosperms: meristems produce new cells along
margins of the leaf axis; cease activity before the
leaf matures.
c. Grasses: meristem is intercalary: active for long
periods, stimulated by defoliation (i.e. mowing the
grass!)
3. Cell division in a leaf is complete when the leaf has
reached less than 1/5 its final area. Final 80% of
leaf expansion is caused by growth of performed cells.
Growth occurs over the entire leaf area, but not
uniformly. Cells in the young leaf are compact. As
the leaf expands, the mesophyll cells stop growing
before the epidermal cells. The expanding epidermis
pulls the mesophyll cells apart, causing development
of intercellular air spaces in the mesophyll.
D. Flowers:
1. Perfect: containing female and male parts
Imperfect: staminate (male) or pistillate (female)
flowers
Dioecious: separate male and female plants
Monoecious: staminate and pistillate flowers at
different positions along a single stem
Cones (Strobili): most conifers are monoecious;
Junipers and others are dioecious.
2. Anthesis: opening of flowers:
a. Some flowers remain open from anthesis until
abscission. Other flowers open and close at specific
times for several days. Permanent opening is caused
by differential growth of the inner verses the outer
part of the petal. Continued opening and closing
is a response to temporary changes in turgor pressure
across the two sides. Influenced by temperature,
atmospheric vapor pressure, internal clock.
b. Withering is associated with extensive transport of
solutes from the flowers to other parts of the plant,
often to the ovary, with rapid water loss.
E. Seeds and Fruits
1. Chemical changes in growing seeds and fruits.
a. The zygote, embryo sac, and ovule develop into the
seed. The ovary develops into the fruit (pericarp).
Sucrose, glucose, and fructose accumulate in ovules
until the endosperm nuclei are surrounded by cell
walls. Sugar content decreases as they are used in
cell-wall formation and starch or fat synthesis.
Nitrogen is present in proteins, amino acids,
glutamine and asparargine. Amino acids and
amides decrease in concentration as storage proteins
are formed in the protein bodies.
b. Role of enzymes and nucleic acids in developing
seeds: must posses enzymes necessary for
germination.
(1) Enzymes essential to germination are produced in a
stable form during seed development
(2) Translated from stable messenger RNA, tRNA, and
rRNA molecules synthesized during seed maturation
(3) Other proteins are formed from newly transcribed
RNA molecules after the seed germinates.
c. Dehydration: loss of water during seed maturation is
critical
d. Chemical composition of edible fruits and
transformation of carbohydrates during ripening
changes.
e. Transformation of chloroplasts to carotenoid-rich
chromoplasts, accumulation of anthocyanin pigments,
and accumulation of flavoring components.
2. Importance of seed for fruit growth
a. Development of fruits usually depends on germination
of pollen grains on the stigma (pollination) or on
pollination plus fertilization. Developing seeds are
essential for normal fruit growth.
b. Normal production of fruits lacking seeds is called
Parthenocarpic Fruit Development. Ovary development
without pollination. Fruit growth stimulated by
pollination without fertilization or form
fertilization followed by abortion of the embryos.
3. Relations between vegetative and reproductive growth
a. Topping: removal of flowers and fruits.
b. Competition for nutrients among vegetative and
reproductive organs. Mineral salts, sugar, and amino
acids Nutrient accumulation in developing flowers,
fruits, or tubers occurs largely at the expense of
materials in nearby leaves.
c. Mechanism by which fruits can divert nutrients out of
leaves and into their on tissues. Against
concentration gradients. Controlled by phloem
unloading. Various hormones, cytokinins, involved.
d. Factor that stimulate shoot growth retard flower,
tuber, and fruit development. High nitrogen
fertilization causes stem and leaf growth of tomato
plants, may reduce fruit development. There is no
real evidence that high nitrogen inhibits fruit
production per plant is other nutrients, phosphorus
and potassium are adequate.
e. Heavy pruning, drought , typing branches to the
ground, or various other mutilation procedures
stimulate flowering. Commercial growth retardant
inhibit growth of stems, stunting is sometimes
accompanied by early appearance of flower buds or a
greater number of flowers per plant.
V. Morphogenesis: Juvenility
A. After seed germination, annual and perennial seedlings
enter a rapid growing phase in which flowering cannot be
induced.
Characteristic morphology established.
Juvenile phase as opposed to the mature or adult phase.
B. Juvenile phase with respect to flowering varies in
perennials. One year to 40 years.
"Ripeness to respond": long juvenile phases in conifers
and other trees can pose obstacles to genetic programs
designed to improve quality. Typically stem cuttings are
used to form adventitious roots in juvenile trees. In
the adult phase, the rooting ability is reduced and
sometime lost.
C. Heterophylly: juvenile and adult morphologies of leaves.
Balance of gibberellins and ABA may be involved in the
transition from one state to another.
VI. Morphogenesis: Totipotency
A. Totipotent: a nonembryonic cell has the potential to
dedifferentiate into an embryonic cell and then to
develop into a complete new plant.
B. Totipotency illustrated by development of cultured callus
tissues into new plants. Partial totipotency occurs
when adventitious roots develop from stem cells and when
xylem and phloem are regenerated from wounded cortex
cells.
C. Several cells cooperate to form primordia from which the
while plant arises.
1. Frederick Stewart, Cornell university 1950's: work on
cytokinins
2. Vasil and Hildebrandt 1965: produced entire plants
from isolated single cells.
D. Haploid pollen grains develop into callus tissues and
then whole plants.
1. Endoreduplication (doubling of chromosomes in mitosis,
with lack of subsequent cytokinesis)
E. Somatic embryogenesis: somatic haploid or diploid cells
develop into differentiated plants through characteristic
embryological stages. Process occurs from cells
associated with seed development, such as nucellus or
synergid cells. Apomixis: embryo in a seed is not formed
from a union of gametes
F. Somatic embryogenesis: more easily occurs in tissues that
are embryonic.
G. Isolation of a cell from its surroundings can predispose
the cell to embryogenesis. The role of growth hormones
in this process is not well understood. Auxins
can induce callus formation on many stems, and specific
growth regulators, particularly cytokinins, often must
be added to culture media to induce embryogenesis.