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Pollen (male gametophyte)
Within this group study the illustrations in
Lily anther x40. Four pollen sacs and a
cross section of the fillament are visible. Diploid (2N chromosomes)
microspore mother cells in the middle of the pollen sacs undergo
meiosis to make microspores which later become pollen grains. The large
green cells lining the pollen sacs provide nourishment for the
developing pollen grains.
tetrads x400. Meiosis produces groups of
four haploid (N chromosomes) megaspores which are initially attached to
each other as shown here. In some of the illustrated megaspore groups
you can't see all four meagaspores because some are out of the plain of
celled microspores x400 which will soon
divide by mitosis to form two nucleate pollen grains. One of the
illustrated spores has just finishing this division. An elaborate
pollen wall is forming at this stage of development.
pollen grains x400 have two haploid nuclei
such as the specimen on the left, and an elaborate wall. Multinucleate
pollen grains are young male gametophytes.
anther x40. When the pollen is fully
formed the pollen sacs break open and pollen is released.
and pollen x100. When pollen is carried by
the wind or an animal to the hairy stigma of a flower the pollen
germinates and makes a pollen tube. Several purple staining pollen
grains are visible with green staining pollen tubes emerging from them.
grain with pollen tube x400. This grain
has been germinated in sugar water. When a pollen grain germinates, one
of its two nuclei divides by mitosis to produce two sperm nuclei. The
generative nucleus and then the two sperm nuclei follow the tip of the
pollen tube as it grows toward the ovary and the ovules inside. A
germinated pollen grain with pollen tube and three haploid nuclei is
the mature male gametophyte in flowering plants.
tubes grow down the hollow center of the style
as they head toward the ovary.
Female Gametophyte development, Meiosis, Fertilization
Within this group study the illustrations in the
ovary xs x20 has three chambers that
result from the fusion of the three modified leaves (carpels) that
formed the ovary. Each of the three chambers has two rows of ovules
(unfertilized seeds). Thus, in an ovary cross section like the one
illustrated you can see six ovules in various stages of development.
mother cell x100. This large diploid (2N)
cell will undergo meiosis to form four nuclei, each a haploid (N)
megaspore. The flaps of tissue on either side of the megaspore mother
cell are seed coats. They will grow around (growing toward the upper
left in this illustration) the mother cell almost but not quite
completely surrounding it.
I x400. Meiosis of the megaspore mother
nucleate stage x100. The first two nuclei
formed by meiosis. The micropyle is the slit immediately above the old
megaspore mother cell with the two nuclei.
II x400. Meiosis of the megaspore mother
cell. You can see two groups of chromosomes, each at metaphase II, and
Megaspores x100 within the old megaspore
mother cell. The slit-like micropyle is to the right of the megaspores.
This tube like opening, where the seed coats failed to fuse as they
grow around the megaspore mother cell, is where a pollen tube will
penetrate the ovule.
female gametophyte x100. In most species
of flowering plants three of the four megaspore nuclei die. The
remaining megaspore, the one closest to the micropyle, divides several
times by mitosis to form the female gametophyte. Illustrated is the 4
nucleate state of this gametophyte
nucleate Embryo Sac x40. This is the Lily
mature female gametophyte consisting of 7 cells and 8 haploid nuclei.
Such a flowering plant female gametophyte is usually called the "embryo
sac". The egg and two flanking synergid cells are at the lower right.
The large central cell contains two large "polar nuclei", one at the
lower left and one at the upper right. The three antipodal cells are at
the upper left.
Sac x400. The polar nuclei and egg are
labeled. The two "synergid" cells flank the egg. The three "antipodal"
cells are above and to the right of the upper polar nucleus.
Fertilization x400. Lily caught "doing
it"! Two sperm nuclei exit the pollen tube. In most species of
flowering plants one sperm nucleus (N) fuses with the two polar nuclei
(each N) to form the 3N nucleus of the endosperm mother cell. The other
sperm nucleus (N) fuses with the nucleus of the egg (N) to form the
Seed and fruit development
MONOCOT SEEDS AND FRUITS
early embryo x40. The zygote, near the
micropyle of the seed starts to divide by mitosis to form the first few
cells of the (2N, sporophyte) embryo. The endosperm mother cell divides
by mitosis to form the (usually 3N) multicellular nutritive tissue
known as "endosperm". These are surrounded by the seed coat and the
(upper left corner) fruit (ovary) wall.
mature seed x20. The endosperm contains
large dark staining structures that store reserve food for the embryo.
A cross section of the embryo shoot/root axis is in the center.
"seed" x12. Actually this is a one seeded
fruit of a type known as a "grain". This fruit type is characterized by
an ovary wall fused to the seed coat. The embryo's single cotyledon,
which gives the monocot group its name, is easily seen. Mature monocot
seeds usually have lots of starch filled endosperm.
DICOT SEEDS AND FRUITS (Study the Capsella
illustrations in sequence)
young embryo x100. At this stage the
embryo consists of a string of cells (suspensor) that pushes a
heart-shaped ball of cells (the proembryo) in toward the center of the
seed. Later, all the rest of the embryo's growth will be done by the
proembryo. The two lobes of the proembryo will become the two
cotyledons. Endosperm is starting to fill the rest of the seed cavity.
young seed within its fruit x40.
embryo with bending cotyledons x100. This
intermediate stage in embryo development is an enlargement of the
mature embryos x40. Several mature seeds
within their fruit. The embryos have bent into the fetal position
filling the entire cavity of the seed. The nutrients of the endosperm
have been absorbed by the embryo, mostly into their two cotyledons.
Unlike most monocots which have lots of endoserm in their mature seeds,
most dicot seeds have little or no endosperm when mature.
fruit xs x20. A "green bean" which is
actually a fruit with seeds inside. The three layers of the ovary wall
can be seen. The seed is obviously a dicot seed, with the two
cotyledons filling most of the inside space of the seed.
(strawberry) fruit x30. You can see a
dicot embryo in one of the tiny little achene fruits on the surface of
the strawberry. Strawberries are aggregate achenes. The fleshy part
that you eat is swollen receptacle (flower stalk).
flower bud xs x12. This is a Lily bud with
(from the outside toward the center) 3 sepals, 3 petals, 6 stamens each
bearing an anther with 4 pollen sacs, and a single central pistil
sectioned through the ovary. You can see 6 ovules in the ovary
flower bud xs x20. You can see from the
outside toward the center 4 sepals, 4 petals, 6 stamens, and a single
central pistil that was formed by the fusion of two carpels. The pistil
is sectioned through the ovary and several ovules are visible inside.
docot flower bud x30. You can see 5
sepals, 5 petals, 5 stamens, and one pistil. The sepals look very leaf
like, with venation, mesophyll, etc.
flower bud ls x30. Sepals petals and
stamens are all partially fused to each other forming a cup which
surrounds the pistil. This type of fusion is called "perigynous". The
sepals petals and stamens separate from each other at the level of the
style. Since this is a slice through a closed bud you can see cross
sections of sepals and petals near the top of the illustration.
flower bud xs x12, sectioned through the
style at a level where the sepals petals and stamen are still more or
less fused. The outer layer consists of fused sepals. There are
numerous stamens and a single pistil. This is the same species of
flower as the previous illustration.
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