Biological Foundations 112, Lecture 27

Bryophytes II

Chapter 27, pages 552-555
Basic Botany-3 0:00-9:52
Five Kingdoms-5 0:00-7:51

I.   Class Anthocerotae - Hornworts

     A.   Thallus

            1.   Gametophyte only, forms a thallus

            2.   Small and green with little internal or external structure

            3.   Slightly lobed

            4.   Numerous multicellular rhizoids

     B.   Antheridia - similar in structure to Hepaticae except that they are embedded in the thallus
                               and not on an elevated disc

     C.   Archegonia - similar in structure to Hepaticae except that they are embedded in the        
                                 thallus and not on an elevated disc

     D.   Sporophyte - unique part of this plant group

            1.   Epidermis - contains stomata

            2.   Subepidermis - contains chloroplasts

            3.   Sporangium

                  a.   Upright and elongated green structure

                  b.   Sporogenous tissue

                        (1)  Forms an elongated cylinder parallel to the plane of the sporangium

                        (2)  Spores mature from the top (apex) down toward the gametophyte thallus

                  c.   Meristematic region

                        (1)  Adds new sporophyte cells to the base of the sporangium

                        (2)  Spores produced over several months in the spring

                  d.   With favorable conditions

                        (1)  Sporophyte lengthens greatly (2-4 inches in species on Howell Mountain)

                        (2)  Some sporogenous tissue may be replaced by conducting strands

                        (3)  Foot enlarges

                        (4)  Gametophyte may die and decay with the sporophyte coming in direct          
                               contact with the soil and growing on its own for a few weeks in late            
                               spring and early summer

II.  Class Musci - Mosses

      A.  Characteristics

            1.   Habitat

                  a.   More common and conspicuous than liverworts or hornworts

                  b.   Grows on rocks, trees, and sometimes in streams

                  c.   Some can resist drought, however water is needed for growth and reproduction

            2.   Growth stages

                  a.   Protonema - filamentous stage that grows out of a spore

                  b.   Thallus

                        (1)  Horizontal and upright "stems"

                        (2)  "Leaves" spirally arranged

                        (3)  Rhizoids

                               (a)  Multicellular filaments of cells

                               (b)  Can give rise to protonema

                        (4)  Gametangia grow at the tips of the "stems"

            3.   Structure of the "stem"

                  a.   Epidermis

                  b.   Cortex

                  c.   Central cylinder - contains cells that resemble sieve-tube members

      B.   Life Cycle

             1.   Gametophyte

                   a.   Development of mature thallus

                         (1)  Spores germinate to form protonema

                         (2)  Protonema differentiate into a mature "stem"

                   b.   Mature thallus

                         (1)  Plants are either monoecious or dioecious depending upon the species

                         (2)  Antheridia

                               (a)  Borne at the tips of the "stems"

                                     i)  Sterile jacket contains photosynthetic pigments

                                     ii) Turns orange or red when ripe

                               (b)  Surrounded by paraphyses

                         (3)  Archegonia

                               (a)  Borne at the tips of the "stems"

                               (b)  Have a long neck

                               (c)  Have a long stalk between the venter and the gametophyte

                               (d)  Surrounded by paraphyses

                   c.   Fertilization

                         (1)  Ripe antheridia open when water is present

                         (2)  Sperms swim to the archegonia by a chemotactic response

                         (3)  Archegonia open like those of Hepaticae

             2.   Sporophyte

                   a.   Zygote develops into an embryo

                   b.   Embryo differentiates into a foot, seta and sporangium

                   c.   Growth of the seta and the formation of the calyptra:

                         (1)  Seta grows rapidly

                         (2)  Raises the sporophyte 1/2 inch or more above the venter

                         (3)  Old venter is stretched out of shape and finally the top breaks off

                         (4)  The top remains on the capsule and is called the calyptra

                         (5)  The calyptra is composed of the neck and the upper portion of the venter

                         (6)  The calyptra remains on top of the capsule for a time and finally blows off

                   d.   Sporangium

                         (1)  Quite small in size

                                (a)  1/16 to 1/8 of an inch in diameter

                                (b)  1/8 to 1/4 inch long

                         (2)  Capsule

                                (a)  Theca - upper region of capsule where the spores form

                                       i)   Amphithecium - outer sterile tissue

                                            a)   Epidermis - outer protective layer

                                            b)   Air chambers - large spaces in the sporangial wall

                                       ii)  Endothecium - inner tissue where spores are formed

                                            a)   Archesporium - layer where spores are formed just inside the
                                                                             air chambers

                                            b)   Columella - tissue in the very center of the capsule

                                (b)  Apophysis - enlarged sterile base on which the theca rests

                                (c)  Operculum - lid of sporangium

                                (d)  Annulus - thickened ring of tissue around the sporangium just under    
                                                        the operculum to which the peristome teeth attach

                                (e)  Peristome teeth - finger-like structures that aid in spore dispersal

                         (3)  Spore formation

                                (a)  Occurs in the archesporium

                                (b)  Sporogenous cells increase by mitosis to form many spore mother      

                                (c)  Spore mother cells undergo meiosis to form a tetrad of tetraspores

                                (d)  When mature, the tetrad breaks apart into separate haploid spores

                         (4)  Spore dispersal

                               (a)  Peristome teeth bend in when the humidity is high and pick up spores

                               (b)  Peristome teeth bend out when the humidity is low and the wind blows  
                                      the spores off the peristome teeth

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