Nazreen Zainul's Blog

• After double fertilization

–        ovule develops into a seed

–        ovary develops into a fruit enclosing the seed

–        As the embryo develops, the seed stockpiles proteins, oils and starch.

• These nutrients are stored in the endosperm, but later in seed development in many   species, the storage function is taken over by the swelling storage leaves (cotyledons) of the embryo itself.
• The endosperm is rich in nutrients, which it provides to the developing embryo.

In most monocots and some dicots, the endosperm also stores nutrients that can be used by the seedling after germination.

• In many dicots, the food reserves of the endosperm are completely exported to the cotyledons before the seed completes its development, and consequently the mature seed lacks endosperm.
• The first mitotic division of the zygote is transverse, splitting the fertilized egg into a basal cell, and a terminal cell which gives rise to most of the embryo.
• The terminal cell divides several time and forms a spherical proembryo attached to the suspensor.
• Cotyledons begin to form as bump on the proembryo.
• A dicot with its two cotyledons, is heart-shaped at this stage.
• Only one cotyledon develops in monocots.
• After the cotyledons appear, the embryo elongates.
• During the last stages of maturation, a seed dehydrates until its water content is only about 5-15% of its weight.
• In the seed of a common bean, the embryo consists of an elongate structure, the embryonic axis, attached to the fleshy cotyledons.

• Below the point at which the fleshy cotyledons are attached the embryonic axis is called the hypocotyls and above it is the epicotyl.
• At the tip of the epicotyl is the plumule, consisting of the shoot tip with a pair of miniature leaves.
• The hypocotyls terminates in the radiacle, or embryonic root.

The embryo of a grass seed is enclosed by two sheath, a coleorhiza, which covers the young root and a coleoptile, which cover the young shoot.

Double fertilization is a complex fertilization mechanism that has evolved in flowering plants, known as angiosperms. This process involves the joining of a female gametophyte (embryo sac) with two male gametes (sperm). It begins when a pollen grain adheres to the stigma of the carpel, the female reproductive structure of a flower. After a pollen grain has landed on an accessible stigma, the pollen grain takes in moisture and begins to germinate, forming a pollen tube that extends down toward the ovary through the style. The tip of the pollen tube then enters the ovary and penetrates through the micropyle. The micropyle is an opening in the protective layers of the ovule. The pollen tube proceeds to release the two sperm in or near the embryo sac.

One sperm fertilizes the egg cell and the other sperm combines with the two polar nuclei of the large central cell of the embryo sac. The sperm and haploid egg combine to form a diploid zygote, while the other sperm and two haploid polar nuclei form a triploid nucleus (some plants may form polyploid nuclei). The large cell of the embryo sac will then form the endosperm, a nutrient-rich tissue which provides nourishment to the developing embryo. The ovary, surrounding the ovules, develops into the fruit, which is used for protection and dispersion of the seeds.

The two central cell maternal nuclei (polar nuclei) that contribute to the endosperm arise by mitosis from a single meiotic product. Therefore, maternal contribution to the genetic constitution of the triploid endosperm is different from that of the embryo.

Click HERE to see the video of double fertilization process

Pollination is the process of moving pollen (male gametophyte) from the anther to a receptive stigma.

Some angiosperm can self- pollinate, but such species are limited to interbreeding in nature. Most angiosperm species rely on a living (biotic) or non- living (abiotic) pollinating agent that can move pollen from the anther of a stamen of a flower on one plant to the stigma of a carpel of a flower on another plant. Approximately, 80% of all angiosperm pollination is biotic, employing animal go- betweens. Among abiotically pollinated species, 98% rely on wind and 2 % on water.

If the pollen and stigma are from the same flower it is self pollination. Usually cross pollination occurs because it insures variation in the offspring, a distinct evolutionary advantage.

Since self pollination is not advantageous a number of factors help prevent it such as:

• position of the anthers relative to the stigma
• shape of petals (bilateral symmetry)
• timing of pollen or egg production
• physiological incompatibilities including dioecious species. This may involve a separation of the male and female parts of a flower into separate imperfect flowers on the same plant or on entirely separate dioecious plants.

Flowers have coevolved with insects for millions of years so many characteristics of flowers — shape, color, nectar, and odor — are designed to attract pollinators (not to be beautiful in the eyes of humans).

Click HERE to see the video of pollination process.

Flowers are part of the sporophyte generation in angiosperms. It’s the flower’s job to produce the male and female gametophytes.

• male gametophyte 2 cells (pollen grain)
• female gametophyte 7 cells (embryo sac)

Male spore development begins with:

microsporangia or microspore mother cell

• is diploid
• is found in a pollen sac of the anther
• undergoes meiosis
• and produces 4 microspores (haploid)
  • each microspore undergoes mitosis to produce
  • microgametophytes (pollen grains each with two cells)

The pollen grain has a hard bumpy outer cell wall containing 2 cells, a tube cell and a generative cell (suspended in the tube cell)

A pollen Grain

Female egg development begins with:

megasporangia or megaspore mother cell

• is diploid
• is found in the ovary
• undergoes meiosis
• and produces 1 megaspore (the other 3 die)
  • the megaspore undergoes mitosis 3 times to produce the
  • megagametophyte (Embryo sac composed of 8 nuclei and 7 cells)

  Embryo sac or megagametophyte

Once fertilized the embryo sac or ovule composed of nucellus, integuments, and embryo will develop into the seed. The ovary –tissue immediately surround the seed or seeds– will develop into a fruit.

To view detailed discussion on gametogenesis click HERE

Plants have mitosis occurring in spores, which are produced by meiosis. The spores germinate into the gametophyte phase. The gametophytes of different groups of plants vary in size. For example, angiosperms have as few as three cells in pollen, and mosses and other so called primitive plants may have several million cells. Plants have an alternation of generations where the sporophyte phase is succeeded by the gametophyte phase. The sporophyte phase produces spores within the sporangium by meiosis. Sexual reproduction in plant mainly involves four stages; formation of gametes, pollination, double fertilization and development of embryo and seed.