Biological Foundations 112, Lecture 17


Seeds

Chapter 34, pages 733-738
Basic Botany-5 20:45-25:51


I.   Seed Development

     A.   Recall the mature ovule

            1.   Structure of the embryo sac at fertilization

                  a.   Egg Cell

                  b.   Polar nuclei

                  c.   Synergids

                  d.   Antipodals

            2.   Embryo sac is surrounded by the nucellus

            3.   The nucellus is surrounded by the inner and outer integuments

      B.  Double fertilization of the embryo sac

            1.   Fusion of egg and sperm to form the diploid zygote

            2.   Fusion of the two polar nuclei with the sperm nucleus to form the triploid primary
                  endosperm nucleus

      C.   Embryonic Development

             1.   Primary endosperm nucleus (3N) divides rapidly by mitosis, forming a
                   multinucleated mass of endosperm

                   a.   No further divisions of the primary endosperm nucleus occurs

                   b.   Zygote nucleus remains quiescent until the primary endosperm has fully                                             developed

             2.   Zygote grows into a full embryo

                   a.   First few mitotic divisions produces a filament 4-8 cells long (filament called a
                         suspensor)

                   b.   Cell at the micropylar end elongates, pushing the rest of the filament further up
                         into the endosperm

                   c.   Cell at chalazal end begins mitotic divisions at right angles to the filament (this
                         cell called a proembryo)

                   d.   Further divisions of the proembryo produce a globular structure at the chalazal
                         end  (called globular embryo)

                   e.   Globular structure grows into a heart shaped structure (this is the beginning of  
                         cotyledon formation)

                   f.   Each branch of the heart grows into a cotyledon

II.  Three Types of Seeds

      A.   Common Bean Seed

             1.   Characteristics

                   a.   Two cotyledons - the food source for the seed

                   b.   No endosperm present

             2.   Structure

                   a.   Hilum - oval scar where funiculus attached to the seed

                   b.   Funiculus - stalk or "umbilical" cord of the seed

                   c.   Micropyle - small opening in the seed coat below the hilum

                   d.   Raphe - ridge above the hilum.  The main conducting tissues of the
                                       developing seed go through this area.

                   e.   Chalaza - area above the raphe where the conducting tissue of the developing
                                         seed fans out over the seed

                   f.    Integument - seed coat

                   g.   Embryo

                         (1)  Shoot - the part of the plant that will grow up and above ground

                                (a)  Cotyledons  - seed leaves, two of them, containing all the nourishment
                                                              for the seed.

                                (b)  Hypocotyl - portion of shoot below the cotyledons that attaches to the
                                                           embryonic root of the radicle

                                (c)  Epicotyl - portion of shoot above the cotyledons that contains the
                                                       apical meristem

                         (2)   Radicle - embryonic root that will turn down and grow into the ground

      B.   Castor Bean Seed

            1.  Exactly like the common bean seed EXCEPT:

                 a.   Cotyledons are thin and leaf-like

                 b.   Cotyledons become the first foliage leaves after the seed germinates

                 c.   Cotyledons are embedded in the endosperm and the cotyledons do not contain
                       any food material

                 d.   The endosperm is the food source of the seed

      C.   Wheat Seed

            1.   Characteristics

                  a.   One cotyledon

                  b.   Endosperm present - the source of nourishment for the seed

            2.   Structure

                  a.   Endosperm

                        (1)  Two layers

                               (a)  Aleurone layer

                                     i)   Single layer of cells

                                     ii)   Contains proteins and fats but little or no starch

                               (b) Starchy endosperm

                                     i)   Nutrient region of the seed

                                     ii)  Filled with starch grains

                  b.   Embryo

                        (1)  Shoot apex

                               (a)  Contains rudimentary (early, non-functional leaves)

                               (b)  Coleoptile - sheath covering the shoot and leaves

                        (2)   Root apex

                                (a)  Contains the root tip and the root cap

                                (b)  Coleorhiza - sheath covering the root apex

                        (3)   Scutellum

                                (a)   Shield-shaped structure that occurs between the endosperm and
                                       shoot and root apicies

                                (b)   This is the single cotyledon of the seed

                                (c)   Function

                                       i)   Outer cells secrete enzymes that digests the food in the endosperm

                                       ii)  Digested food moves through the scutellum to the growing embryo

                                       iii) Remains inside the seed during germination and does not emerge 
                                            and become a green leaf as the castor bean seed does

            3.   Terminology for manufactured wheat products

                  a.   Polished white rice removes the caryopsis coat, aleurone layer and some
                       endosperm

                  b.   White Flour - the ground endosperm of the wheat seed

                  c.   Wheat Germ - the scutellum, shoot and root apicies

                  d.   Bran - the caryopsis coat and aleurone layer that has been polished off the                                                  wheat seed

                  e.   Whole Wheat Flour - the entire wheat kernel ground up into flour

                  f.    Wheat Flour - flour that comes from wheat, generally unbleached white flour

                  g.   Bleached Flour - white flour that has been bleached white with a chemical
                                                    agent

III. Seed Dissemination

      A.  Adaptations for dispersal

            1.   Wind dispersal - various types of wings or plumes on the fruit are used to catch the
                                             wind and carry the fruit to new locations

            2.   Water dispersal

                  a.   Memberanous envelope within the fruit contains air so that the fruit will float -
                        sedges, an example

                  b.   Coarse, loose fibrous outer coat - coconuts

                  c.   Most seeds will float until they become hydrated

            3.   Animal dispersal

                  a.   Spines, hooks, barbs, etc. adhere to the animals coat

                  b.   The seeds of fleshy fruits pass through the digestive tracts of animals and birds
                        and are deposited with the feces of the animal

                        (1)  Weed seeds through the digestive tracts of horses and cattle

                        (2)  Tomato seeds through the digestive tracts of humans

                        (3)  Toyon and Madrone seeds through the digestive tracts of Cedar Waxwings

                  c.   Seeds are buried by animals - squirrels

IV. Seed Dormancy and Germination - Beal Experiment

      A.  Experiment

            1.   Started in 1878

            2.   Buried jars containing seeds of several different species of plants

            3.   Opened jars at 5 to 10 year intervals

      B.   Results of experiment

            1.   Most seeds remained viable for at least 10 years

            2.   Moth mullen germinated after 90 years

      C.  Archeological Deposits

            1.   Oriental lotus from archeological diggings it appears to be viable after 1000 years

            2.   Other species from cold and anaerobic archeological deposits, it appears, are viable
                  after long periods

V.  Factors for breaking seed dormancy

      A.  Moisture - seeds must be hydrated

      B.  Oxygen - seeds need oxygen to burn food for energy

      C.  Light - some seeds need light after they are hydrated to cause germination to occur

      D.  Scarification

            1.   Seed coats that are resistant to water and oxygen permeation must be removed so
                  that water and oxygen can penetrate.

            2.   Scarification is the process of removing the impermeable seed coat

            3.   It can happen naturally as in the intestine of an animal or artificially by etching the
                  seed with acid or mechanically rubbing off the seed coat

      E.   Cold Period

            1.   After being hydrated, the seed needs freezing or near freezing temperatures for a  
                  certain minimum time

            2.   This certain minimum amount of time at this cold temperature signals the seed that the
                  winter is past and it can germinate without being frozen by frost or cold weather

            3.   This happens naturally with most wild seeds that are in the ground over the winter.  
                  They become hydrated in the fall and go through the winter and germinate in the
                  spring as a result of experiencing the cold temperature.

            4.   Stratification is an artificial method of giving seeds a cold period.  The seeds are
                  spread out on wet cheese cloth.  The sheets of cheese cloth are stacked on top
                  of one another and placed in a dark cold refrigerator for approximately two  
                  months depending upon the species.  The wet cheese cloth hydrates the seed
                  and the cold period makes it ready to germinate.

      F.   Chemical Inhibitors - abscisic acid (plant hormone) as well as other chemical inhibitors
                                                exist within the seed coat or fruit that prevents the seed from
                                                germinating.  These must be leached out of the seed before it will
                                                germinate.

      G.   Heat or Fire - Cones of certain of the pines remain closed after the seeds become
                                     mature.  When a fire comes through the area, it heats up the cone.  After
                                     a few days, the cone begins to open, releasing its seeds.  These seeds
                                     begin to germinate as soon as the first heavy rains begin to come after
                                     the fire.  The knobcone pine is a good example of this.


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