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
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
(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
cells
(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
PUC Home Page | Gilbert Muth Home Page | Botany Syllabus Home Page