*Modifications in reproduction were key
adaptations enabling plants to spread
into a variety of terrestrial habitats.
* Water has been replaced by
wind and animals as a means
for spreading gametes.
* Embryos are protected in
SEEDS.
• Vegetative (asexual)
reproduction is used for
propagation of some plants
in a variety of environments.
**Our focus in this chapter:
SEXUAL REPRODUCTION!
GAMETOPHYTE: reproductive structure
of plants
-where haploid GAMETES (n) are produced
(sperm and egg cells)
-gametes fuse to form a ZYGOTE (2n) that
develops into a multicellular plant
FLOWERS:
-the reproductive structure of angiosperms
-four sets of modified leaves:
1) Sepals
2) Petals
3) Stamens:
-anther, filament
4) Carpel (a.k.a. pistil):
-stigma, style, ovary
FLOWERS:
Female gametes develop in EMBRYO
SACS, which contain the EGG CELLS
this occurs at the base of the carpel /pistil,
inside the OVARY
Male gametes develop in the stamen
inside POLLEN GRAINS
this occurs at the stamen tips within
chambers of the ANTHERS
FLOWER VARIATIONS:
Complete flower = flower with sepals, petals, stamens, carpels
Incomplete flower = flower missing one or more of these parts
(e.g. most grasses do not have petals on their flowers)
FLOWER VARIATIONS:
Perfect flower = flower having both stamens and carpels (a.k.a. bisexual)
Imperfect flower = flower that is either STAMINATE (male) or CARPELLATE (female) – a unisex flower. (a.k.a. unisexual)
Monoecious = “one house”; plants with
either perfect flowers or with both
staminate (male) and carpellate (female)
flowers on the same plant
(ex: star lily, rose, dandelion)
Dioecious = “two houses”;
plants having staminate and
carpellate flowers on separate
plants (ex: willow, juniper)
Development of the Male
Gametophyte (POLLEN):
POLLEN GRAIN = the immature male
gametophyte; develops within the anthers
of stamens in an angiosperm
extremely durable;
tough coat to
prevent
biodegradation
At the time of pollination:A pollen gain consists of 2 cells (both
haploid):
-GENERATIVE CELL: divides to form 2
sperm cells
-TUBE CELL: produces the pollen tube;
contains the generative cell
Embryo Sac:
egg cell (OVUM) at one end
ovum flanked by 2 syngerids (help to attract sperm cells)
two nuclei in center (“polar nuclei”)
POLLINATION:
-occurs when wind- or animal-born pollen
released from the anthers lands on the
STIGMA at the tip of the carpel
POLLINATIONPOLLINATION = the placement of pollen
onto the stigma of a carpel
Methods of Pollination / spread of pollen:
• wind
• animals / insects
• self-pollinate
(most plants
cross-pollinate)
To prevent self-pollination (and thus
ensuring sperm and egg from different
plants meet), some plants:
-have stamens and carpels that mature at
different times;
-have structural arrangement of flower parts that
reduces the chance of pollen getting
transferred from stamen to carpel;
-are SELF-INCOMPATIBLE (a biochemical
block that prevents the pollen grain from
developing and fertilizing the egg)
SELF-INCOMPATIBILITY: the rejection of
pollen from the same, or closely related,
plant by the stigma
after POLLINATION…
-pollination is followed by this sequence:
1) a pollen tube grows from the pollen grain, down
the carpel, into the embryo sac;
2) sperm are discharged from the pollen grain
resulting in fertilization of the egg;
3) the zygote develops into an embryo;
4) as the embryo grows, the ovule surrounding it
develops into a SEED;
5) while seed formation is taking place, the entire
ovary is developing into a FRUIT, which will
contain one or more seeds.
DOUBLE FERTILIZATION:
When a compatible pollen grain lands on a
stigma of an angiosperm, double
fertilization occurs.
DOUBLE FERTILIZATION = the union of
two sperm cells with two cells of the
embryo sac
DOUBLE FERTILIZATION:
Steps:
1) pollen grain adheres to stigma, germinates, and extends a pollen tube toward the ovary;
2) generative cell divides (mitosis) to form 2 sperm;
3) directed by a chemical attractant, pollen tube enters and discharges its 2 sperm nuclei into the embryo sac;
4) 1 sperm unites with the egg ZYGOTE;
5) other sperm combines with the 2 polar nuclei to form a 3n (triploid) nucleus in the large central cell of the embryo sac.
this central cell will give rise to the ENDOSPERM (a food storing tissue for the seed/embryo)
**after double fertilization, each ovule develops into a seed and the ovary into a fruit surrounding the seed(s).
Structure of the mature seed:
in mature seeds, the embryo is quiescent
(dormant) until germination
• the seed dehydrates until its water content
is only 5-15% by weight.
• the embryo is surrounded by endosperm,
enlarged cotyledons, or both.
• the seed coat is formed from the outer
layers / integuments of the ovule.
Dicot seed structures:
-cotyledons =
embryonic leaves
-hypocotyl =
embryonic axis
below cotyledons
-radicle = embryonic
root
-epicotyl =
embryonic axis
above cotyledons
*Seeds are dispersed from the source plant when fruits are moved about by
the wind or animals.
*Seeds deposited in soil of the proper conditions (moisture, nutrients), will
GERMINATE.
*Fruits ripen about the time seeds are
becoming fully developed
Ripening fruits are characterized by:
1) fruit becomes softer as a result of enzymes digesting the cell wall components;
2) colors usually change and fruit becomes sweeter as organic acids or starch are converted to sugar;
3) these changes produce an edible fruit which entices animals to
feed, thus dispersing the seeds!