BSC 2010 Dr. Bowes Period 6

Reproduction – Angiosperms (continued)

Development of male gametophyte (pollen). See Fig. 38.4.

• Occurs in the anther pollen chambers (sporangium).
• Microspore mother cells (diploid, 2n) divide by mitosis to form microsporocytes (2n).
• Microsporocytes divide by meiosis to give 4 haploid (n) microspores. They separate and develop into pollen.
• Each microspore divides again by mitosis to produce two cells: a generative cell and a tube cell which together make a pollen grain (male gametophyte).
• Pollen grain develops a thick, very resistant wall with markings characteristic of the species.  Pollen (walls) preserved in lake sediments provide a means to determine the composition and abundance of plant species in that area many thousands of years ago.
• Anther dries out and pollen chambers split open to shed mature pollen grains.
• Release of pollen from anther. Pollen chamber walls break open – pollen dehisces. Dispersed by either wind or animals - depends on the species of plant.
• Generative cell divides again by mitosis to give 2 sperm cells.
• After pollination the pollen grain germinates. Its tube cell grows a long tube down the female style.  The 2 sperm cells move down the tube to fertilize (syngamy) the egg cell and polar nuclei in the ovule in a unique double fertilization event.

The 2 sperm cells and the pollen tube (with nucleus) comprise the whole of the male gametophyte.

Development of female gametophyte (embryo sac). See Fig. 38.4

• Each young ovule inside an ovary contains a diploid (2n) megasporocyte (mother sporophyte tissue).
• It undergoes meiosis to produce four haploid (n) cells. Three usually disintegrate (unlike in pollen production) and the fourth becomes a megaspore (n).
• The megaspore undergoes three divisions by mitosis to form the embryo sac - the female gametophyte.
• The three divisions produce 8 haploid nuclei and seven cells - one large cell has two nuclei in it, the other six cells each have one nucleus.
• When it is ready for fertilization the embryo sac is composed of:
• Three antipodal cells (n) at one end of the embryo sac.
• At the other end (micropyle end) there is one egg cell (n) and two synergid cells (n).
• In the center are the two polar nuclei.

• This is the complete female gametophyte.

Pollination and Fertilization (Syngamy).

What is the difference?  The two are not synonymous.

• Pollination is arrival of the pollen at the stigmatic surface.
• Syngamy is actual fusion of the male and female gametes.
• Pollination can occur without syngamy, e.g. some plants prevent self fertilization by not allowing their own pollen to germinate on their own stigma.
• How might they stop it once it arrives?
• In some gymnosperms the pollen arrives at the stigma (pollination) but syngamy does not occur until the following year.

The process of syngamy.

• Pollen germinates on the stigma and a pollen tube grows down through the style until it reaches the micropyle end (opening between the surrounding two integuments) of the ovule.
• The two sperm nuclei move down the tube.  Note these are NOT swimming sperm and in this respect angiosperms differ from most other organisms.
• The pollen tube digests its way into the ovule and releases the two sperm nuclei into the embryo sac. The two synergid cells guide the pollen tube.

The process of syngamy - Double Fertilization. See Fig. 38.9.

• One sperm fuses with the egg nucleus to produce a zygote which is the new 2n daughter sporophyte.  It will develop into the embryo and eventually produce shoots and roots of the new plant.
• The second sperm nucleus fuses with the two polar nuclei at the center of the embryo sac.  It gives triploid (3n) tissue that develops into the endosperm of the seed.  The endosperm in many species is used as a food reserve for the developing embryo.
• The double fertilization is unique to angiosperms.
• At this stage the ovule is a complicated organ.  It has 2n mother sporophyte tissue (e.g. integuments); haploid cells (antipodal + synergids); 2n daughter sporophyte (zygote) and triploid cells (endosperm).

  Fruit and Seed Development, Structure, & Function. See Figs. 38.10 and 38.11.

• The ovule develops into the seed.  Thus the seed is also a complicated organ in terms of tissue origins.
• The ovary surrounding the ovule becomes the fruit. This makes a true fruit.  If other tissues form part of the “fruit” it is a false or accessory fruit.

What is the function of the seed?

• Protection and nourishment for the developing embryo. Well packaged food source (major food source for humans).
• In many species it is an over-wintering structure (low water content and resistant to adverse conditions).
• Also disperses species in time to improve survival - not all seeds germinate at once.  Some can remain viable for many years.
• See Fig. 38.10 & 38.11 for seed development and seedling structure. Know the parts of a seed and their function.

What is the structure and function of fruit? See p792 and Fig. 38.12.

•  It does not nourish the embryo as many erroneously think.  It may nourish animals though.
•  It does provide protection for the seed, but more important it is a dispersal device.
•  Fruits are dispersed by animals (apples are good to eat, but some fruits stick to animals), wind (dandelion), sometimes by water (coconut), and explosive dehiscence that shoots out seeds as fruit dries and ruptures explosively.
• There are several basic types of fruits: simple, aggregate, multiple, and accessory (false) fruits.

The Angiosperm Life Cycle.

See Fig. 38.1.  This is something with which you should be familiar.
So after all this you should know the parts of a flower that develop into various structures - and know their functions.

Plant Biotechnology.  Read pages 797-799

Be familiar with the following terms:  GM (genetically modified); transgenic plants; Bt toxin protein.
Know the examples of high lysine maize,  Bt transgenic maize, transgenic papaya (Fig 38.20), and "Golden Rice" containing daffodil genes.
List the benefits of biotechnology, and some of the concerns.

Think About It….

The gametophyte generation of angiosperms is not very large or conspicuous, though the male can sometimes make its presence felt - allergies!

At its maximum size, how many cells (and nuclei) make up the male gametophyte?

How many cells (and nuclei) make up the female gametophyte?

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