Biological Foundations 112, Lecture 7


Leaf Development and Abscission

Chapter 31, pages 668-691
Plant Anatomy-1  27:41-31:27


I.   Leaf Anatomy

     A.   There are three main regions in the leaf

            1.   Epidermis

            2.   Mesophyll

            3.   Veins

     B.   Epidermis

            1.   Epidermal cells

                  a.   Contain no chloroplasts in the cells

                  b.   Secretes cuticle on the outer surface of the cell

                  c.   Generally epidermal cells are one layer thick

                  d.   Irregular in shape (like a jigsaw puzzle)

             2.  Stomata

                  a.   Composed of the guard cells and pore

                  b.   Vary in size from species to species

                  c.   Vary in number per unit area from species to species

                  d.   Usually more on the lower surface than the upper surface

                  e.   Algae and fungi lack stomata

             3.  Guard cells

                  a.   Occur in pairs around the pore

                  b.   Contain chloroplasts

                  c.   Secretes cuticle on their outer surface

                  d.   Crescent-shaped or semicircular in form

                  e.   Walls unevenly thickened:

                        (1)  Thicker walls are adjacent to pore

                        (2)   Part of the opening and closing mechanism

             4.   Trichomes

                    a.   Unicellular to multicellular

                    b.   Simple to branched

                    c.   Scale-like

                    d.   Glandular

       C.   Mesophyll

              1.   Two bands of cells with veins embedded within the two layers

                    a.   Palisade parenchyma

                    b.   Spongy parenchyma

              2.   Palisade parenchyma

                    a.   Chlorenchyma cells making up the upper layers of cells just below the upper
                          epidermis

                    b.   One to several layers thick

                    c.   Elongated cells that are tightly packed

                    d.   Generally above the veins and veinlets

              2.   Spongy parenchyma

                    a.   Chlorenchyma cells making up the lower layers just above the lower epidermis

                    b.   Many large intercellular spaces in this layer, called substomatal chambers

                    c.   Loosely packed cells that are spherical in shape

                    d.   Generally below the veins

       D.   Veins

              1.   The veins have the same structure as the vascular bundles in the stem

              2.   Forms a network through the leaf

              3.   Cell types found in the vein:

                    a.   Xylem - occurs above the phloem

                    b.   Phloem - occurs below the xylem

                    c.   Fibers and collenchyma many times associated with the vein

                    d.   The Bundle Sheath is composed of transfer cells that surround the veinlet  
                          through which all materials pass that move in and out of the veinlet

                    e.   The Border Parenchyma is composed of transfer cells that are immediately
                          adjacent to the free ends of the veinlets and pass water and photosynthetic
                          products in and out of the veinlet

              4.   The function of the veins:

                    a.   Carry water and mineral salts and photosynthetic products

                    b.   Mechanically support the mesophyll so it won't collapse

              5.   Difference between large and small veins:

                    a.   Large veins contain all the different vascular cells

                    b.   Small veins have few vascular elements and very little strengthening tissue

        E.   Petiole

              1.   The xylem and phloem maintain their relative positions as in the leaf and the stem

                    a.   Xylem is above the phloem in the petiole

                    b.   Xylem is internal and phloem is external in the stem

              2.   One or more vascular bundles are embedded in parenchyma

              3.   Fibers may be associated with the vascular bundles for strength

              4.   Collenchyma is frequently contained just under the epidermis for strength

II.    Leaf Abscission

       A.   Process of separation of leaves from the parent plant

       B.   Abscission zone

              1.   Special region of petiole where the petiole attaches to the stem

              2.   Mostly parenchyma cells in this region

              3.   Lignified cells are either smaller or lack lignin or both

              4.   The elongated vascular cells tend to be shorter

                    a.   Fibers are usually absent 

                    b.   Thus an area of weakness

       C.   Process by which leaves separate from the parent plant

              1.   Just before abscission occurs a layer of brick-shaped cells form across the petiole
                    in the spot where the petiole will separate from the stem by cell division

              2.   Next the petiole separates from the stem by one of three mechanisms depending
                    upon the species of plant

                    a.   In some species the middle lamella dissolves away by the plant

                    b.   In other species a layer of cells is dissolved away by the plant

                    c.   In still a third group of species, a layer of cork forms across the petiole, the
                          leaves wither in place and are mechanically blown off by the wind and rain

       D.   Tyloses as well as gums are formed inside the vessels in order to plug them up before
             abscission occurs

       E.   Reasons for the abscission zone:

             1.   Causes the dropping of the leaf

             2.   Protects the plant from insect damage and disease at the point of abscission

             3.   Protects the plant from dehydration and water loss

III.  Leaf Development

       A.   Steps in leaf formation

              1.   A small glob of cells remains meristematic and stops dividing on the outer margin of
                    the apical meristem - this glob of cells is called a leaf initial or leaf primordium

              2.   The leaf primordium matures into a leaf bud and remains dormant until early spring
                    when it begins to grow to develop into a leaf

              3.   The growing leaf bud:

                    a.   Produces a marginal meristem that forms the lamina

                    b.   Produces a central meristem that forms the leaf veins

                    c.   Both meristems undergo cell division to produce the leaf cells

                    d.   When all the leaf cells are formed and it is the right time in the spring, the leaf  
                          cells begin to differentiate

                           1.   The cells expand in size

                           2.   The cells differentiate and mature

                           3.   Thus, the expansion of the leaf is due to cell growth

       B.   Phyllotaxy

              1.   Alternate

              2.   Opposite

              3.   Whorled

IV.  Leaf Shape

       A.   What directs the leaf to take a particular shape?

              1.   Under genetic control

              2.   Influenced by the environment

              3.   Coordinated by plant hormones

       B.   Physiological age of plants

              1.   The age of the plant can be measured in two ways

                     a.   Chronological age - by how old the plant is in years

                     b.   Physiological age - by the kind of leaves it puts out

               2.   Adult and Juvenile Leaves

                     a.   A seedling always puts out juvenile leaves

                     b.   As the plant gets chronologically older it stops putting out juvenile leaves and
                           begins to put out adult leaves

                     c.   If however, the chronologically old plant is injured, the new branch that it puts
                           out next to the injury will generally put out juvenile leaves

                   d.   Thus, even though the plant is chronologically old, it will put out
                         physiologically old leaves

               3.   Difference between juvenile and adult leaves

                     a.   It varies from species to species

                     b.   In the Canyon Live Oak, adult leaves have an entire margin (smooth around
                           the margin) and juvenile leaves have very sharp pointed teeth

       C.   How the environment modifies leaf shapes

              1.    Light

                     a.   Factors in light quality that affects leaf shape are:

                           (1)  Intensity of the light

                           (2)  Wavelength of the light

                           (3)  Daily duration of the light

                     b.   Light is necessary for proper chloroplast development.  A lack of light
                           produces yellowish leaves and stems - called etiolation

                     c.   Leaves have different morphologies depending upon how much light they get

                           (1)   Sun leaves

                                   (a)   Tend to be in full sunlight all day long

                                   (b)   Are smaller leaves than shade leaves

                                   (c)   Are thicker leaves than shade leaves

                           (2)   Shade leaves

                                   (a)   Tend to be in the shady area of the plant most of the day

                                   (b)   Are larger leaves than sun leaves

                                   (c)   Are thinner leaves than sun leaves

              2.   Water

                    a.  Submerged leaves are different morphologically from aerial leaves

                    b.  Leaves that have developed under water stress show the same morphological
                         characteristics as sun leaves

                    c.  Reduced CO2 and lower temperature can cause aerial leaves to have the same                                   morphology as submerged leaves

              3.  Nitrogen stress - leaves that have developed under nitrogen stress will show the
                                             same morphological characteristics as sun leaves

V.   Modified leaves

       A.   Bud scales

       B.   Spines on cacti

       C.   Tendrils

       D.   Petiole can be leaf-like in Acacia

       E.   Storage organs

             1.   Daffodil bulb - the scales are thick and store food

             2.   Succulent plants store water

       F.   Insectivorous plants

             1.   The leaves are modified for catching and digesting plant tissue to remove nitrogen
                   from it

             2.   Examples:

                   a.   Drosera - sundew

                   b.   Darlingtonia, Nepenthes, Sarracenia - pitcher plant


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