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Lecture 10

BIOL120 Lecture Notes - Lecture 10: Sporophyte, Gametophyte, Embryogenesis


Department
Biology
Course Code
BIOL120
Professor
Simon Chuong
Lecture
10

Page:
of 7
LECTURE 10 – PLANT REPRODUCTION
Plant Reproduction – can occur asexually or sexually.
Asexual reproduction occurs through mitosis (vegetative fragmentation, spore
formation): offspring are genetically identical to the parent
Sexual Reproduction results in genetic variation (fusion of gametes) – there is
usually a cross-over event before the gametes come together
Asexual Reproduction – asexual or vegetative reproduction involves cell divisions by
mitosis such that offspring are genetic ‘clones of parent. In a stable environment, a plant
that reproduces vegetatively will quickly propagate itself (selective advantage)
Examples: when these plants fragment and leaves break off they begin to germinate and
grow a new plant that will be identical to the original. This means that if a disease comes
in that negatively affects the plant, they will all be affected.
Quaking Aspen – Adventitious shoots arise from buds that form on the roots
Kalanchoe adventitious plantlets – Produce adventitious plantlets at the edges of
leaves. The plantlets fall off the leaf and readily root in the soil.
Cholla cacti – produce stem segments that easily break off that fall to the ground
and start new plants. Segments frequently hitch rides on clothes or animal fur,
thereby distributing the species
Water hyacinth – produces many new plants on short stems that break to release
them. Reproduces so rapidly that it clogs waterways in tropical and semi-tropical
regions like Florida.
Piggyback plant – starts new plants at the base of every leaf.
Plant Life Cycles: Sexual Reproduction – leads to offspring that are genetically different
from the parents and each other. It results in genetic variation (diversify the genetic
toolbox). Fusion of gametes (sperm and egg) give rise to a zygote. Postulated to enhance
adaptability to changing environments and facilitates colonization of new environments
All plants have two multicellular forms (gametophyte – 1n, and sporophyte – 2n)
Bryophytes are gametophyte dominant
Seedless vascular plants are sporophyte dominant but they are independent of
each other.
Seed plants are sporophyte dominant and the gametophyte is significantly reduced
Plant Sexual Cycles – 5 steps
1. Sporophyte (diploid): some cells undergo meiosis producing haploid spores
2. Spore undergoes mitosis to produce multicellular haploid gametophyte
3. Gametophyte (haploid): some cells give rise to sperm and egg
4. Fertilization occurs when sperm and egg fuse to produce a diploid zygote
5. Zygote undergoes mitosis and develops into a mature diploid sporophyte
Plant Sexual Cycles – variation in the types of spores produced
Bryophytes: single type of spore gives rise to separate male and female
gametophytes
Ferns: single type of spire gives rise to a bisexual gametophyte
Seed Plants: two types of spores are made – megaspore produce the female
gametophyte and the microscope that produces the male gametophyte
-In most bryophoytes, such as mosses: Sporophytes is dependent on the independent
gametophyte.
-In most seedless vascular plants, such as ferns: Sporophytes are independent and the
gametophyte is much smaller but is also independent
-In most gametophytes, such as conifers: sporophytes is independent an the microscopic
female gametophytes are attached to female cones and are dependent. The microscopic
male gametophytes are attached to male cones and are also dependent on the sporophyte.
-In angiosperms (flowering plants): the sporophyte, which is the flowering plant is
independent while the microscopic male gametophytes are inside the male parts of
flowers and the microscopic female gametophytes are inside the female parts of the
flowers. Both are dependent on the sporophyte.
Gymnosperm cone structures
In the first cone, the pollen cones, form each season and have papery sporophylls
containing sporangia that release pollen in the spring.
In the second cone, the ovulate cone, takes several seasons to mature and have woody
ovulate scales typically bearing two ovules that become seeds. Each ovulate scale is
formed from fused sporophylls derived from a highly modified axillary bud forming in
the axil between the bract (a modified leaf) and the cone axis. In this cross section, only
one ovule is visible on each scale.
Gymnosperm reproduction
Male Cones: cone axis gives rise to sporophylls (spore forming leaves) and these produce pollen.
Pollen is shed each season.
Female cones: cone axis gives rise to woody bracts (leaves) and in their axils, a bud gives rise to
sporophylls that fuse to form a scale bearing two ovules. It takes several seasons to mature.
1. Microsporocyte undergoes meiosis giving rise to four haploid microspores
2. Each microspore then give rise to pollen grain with four cells that develops endosporically
a. Two prothallial cells: no known function
b. One generative cell: divides and eventually gives rise to stalk cell, body cell. Body cell
eventually produces two sperm cells.
c. One tube: produces pollen tube
3. Pollen lands on drops of fluid produced by megasporangium on each ovulate scale
4. Evaporation brings pollen through micropyle into contact with megasporangium where it
germinates
5. Germination of pollen stimulates the development of megasporocyte