Lecture 35 April 25 2003 R.Jones Chapters 17 and 31
Angiosperm Reproduction and Anatomy of the plant body
1. In the flowering plants (Angiosperms) there is a further evolutionary advance in reproduction that made this group of plants very successful. Angiosperms have ovules enclosed within the ovary. Whereas the ovule produces the seed, the ovary produces the fruit. The ovary wall provides additional protection for the seed. Double fertilization is the other feature where an endosperm and zygote are formed. The endosperm is an important source of food for the embryo.
2. The flower consists of concentric whorls of sepals, petals, stamens and carpel (Fig 17.10 and 31.10)). The sepals and petals are specialized for attracting pollinators in many species. The stamens are composed of anthers supported by a stalk called a filament. The anther produces haploid microspores that develop into pollen. The mature pollen grain has a thickened and ornate cell wall that contains allergenic proteins. Enclosed by the wall are a cell that contains another cell. The larger cell has a nucleus called the tube nucleus and the cell contained in the larger cell is the generative cell.
3. The carpel consists of the ovary with one-to-many ovules, and a style that supports a stigma. The ovule contains an embryo sac that was produced from a haploid megaspore. Mitosis produces 8 nuclei and seven cells that make up the embryo sac. One of the haploid cells is the egg and there are two nuclei in the center of the embryo sac. These two nuclei are called polar nuclei and they are important in double fertilization (see Fig. 31.10).
4. Pollen can be brought to the stigma of a compatible plant by wind or pollinators. Pollen germinate on the stigma and the pollen tube begins to grow down the style toward the ovule. When the pollen tube forms the haploid generative cell divides once more (by mitosis) producing 2 non-motile sperm. The pollen tube enters the ovary and then seeks out an ovule.
5. The pollen tube discharges two sperm into the embryo sac. One sperm combines with the egg to give the diploid (2N) zygote and the second sperm combines with the two polar nuclei to give the 3N endosperm. The zygote grows and differentiates into the embryo and this is often surrounded by the nutritive tissue of the endosperm.
6. Double fertilization produces an endosperm which provides the next generation with a food supply that supports the young seedling. This allows the next generation to be easily established even under the most adverse conditions. The endosperm of cereals are the most important source of food for humans.
7. Whereas the evolution of the seed and fruit, delivery of the non-motile sperm to the egg by pollen tubes, and double fertilization are all adaptations that made the seed plants and flowering plants well adapted to dry land, their anatomy also evolved to cope with a dry land existence.
8. Plants differ from animals in having indeterminate growth patterns that are driven by groups of cells called meristems. Cells in the meristems of plants undergo continuous rounds of cell division and do not themselves specialize. They give rise to cells that become increasingly more specialized. Plants undergo two types of growth, primary growth and secondary growth. We'll first deal with primary growth.
9. Primary growth in plants is brought about by the activity of APICAL MERISTEMS located at the tips of stems and roots. The apical meristem is a group of undifferentitated (i.e., unspecialized) cell that divide to give more specialized cells. Cells of the apical meritem give rise to three primary meritsems, the protoderm, the procambium and the ground meristem. These primary meristems in turn give rise to the dermal tissues, the ground tissues and the vascular tissues.
10. The dermal tissues are all referred to as the epidermis and this surrounds stems, leaves and roots. The epidermis is specialized in the above ground parts into guard cells (more details later) and trichomes or hairs. The epidermis of the above ground shoot is also covered by a cuticle. The epidermis of the root has specialized root hairs.
11. Encosed within the eipdermis is the ground tissue in which the vascular tissue is embedded. The ground tissue of the shoot is divided into cortex and pith and in most roots only a cortex is found. The ground tissue consists of three cell types. Parenchyma, involved in photosynthesis or storage, collenchyma specilaized by cellulosic thickenings that give strength (found in celery petiole "strings") and sclerenchyma that are lignified cells. Sclerenchyma can be elongated and called fibers as in the fibers from flax from which linen is made or the long fibers of hemp from which rope is made. The thickenings of fibers are of lignin. Sclereids are another form of sclerenchyma and these cells are often branched and give the gritty testure to pears.