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 
        (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 
        (2) New cell wall formed.  Primary Cell wall
        (3) Vesicles migrate along the rodlike microtubules 
             that extend toward opposite poles of the dividing 
    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 
    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 
    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 
  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 
    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 
    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 
    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 
    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 
      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 
    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) 
       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 
        (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 
      d. Chemical composition of edible fruits and 
          transformation of carbohydrates during ripening 
      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 
      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 
  C.  Several cells cooperate to form primordia from which the 
      while plant arises.
    1. Frederick Stewart, Cornell university 1950's:  work on 
    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.