3) One of the four megaspores survives.
4) The remaining one of the four goes through 3 rounds of mitosis, producing eight
haploid nuclei. Plasma membranes and cells walls then form, dividing the cytoplasm
into the seven cells that make up the female gametophyte.
5) There are 3 small cells at each end (each with one nucleus), and one large central
cell with two nuclei. (The egg is one of the 3 cells at the lower end, located near an
opening in the integuments of the ovule.
The larger center cell becomes the primary endosperm cell.
POLLINATION AND FERTILIZATION (see fig 44-8)
1) Pollination starts when pollen leaves an anther and lands on a stigma. The pollen absorbs
water from the stigma.
2) The tube cell breaks through the pollen coat, and the pollen grain grows a tube down
through the style towards the ovary. It reaches the opening in the integuments of the ovule.
3) The pollen grain releases 2 sperm cells from the generative cell move down the tube to the
ovary where a double fertilization occurs.
- One sperm fuses with the egg cell to form the diploid zygote.
- The other sperm fuses with the polar nuclei in the primary endosperm cell making this
triploid endosperm (3 sets of chromosomes) tissue.
DEVELOPMENT OF SEEDS ANDS (see fig 44-9)
The seed develops from the ovule.
1) The integuments become the seed coat.
2) The primary endosperm divides. The daughter cells absorb nutrients from the parent
plant, forming a food-filled endosperm (acts as food for the new plant).
3) The zygote becomes the embryo.
The walls of the ovary turn into the flesh of the fruit.
As the seed grows, the embryo differentiates into shoots and roots. The shoot portion
includes one or two cotyledons (seed leaves) (see fig 44-10).
In dicots there are 2 cotyledons in the embryo, and monocots have only 1 cotyledon.
In the monocots, the cotyledon is protected by a tough sheath called the coleoptile (see fig