Lecture 34 April 23 2003 R.Jones Chapters 17, 31
Reproduction in Seed Plants (contd) and in Flowering Plants
1. The bryophyte body is that of the gamete producing part of the life cycle, called a gametophyte. These gametophytes are haploid (N) plants and make gametes by mitosis! Compare this with gamete production in animals where meiosis produces gametes. The sperm is motile and water is required for fertilization to occur. The resulting diploid (2N) zygote develops into a spore-producing structure that is attached to the female gametophyte (see Fig. 17.5). Haploid spores are produced by this 2N sporophyte and the haploid spores germinate to give new male and female gametophyte plants. The fact that motile sperm are required for sex in bryophytes makes them relatively poorly adapted to land.
2. Vascular plants, i.e. plants having meristems, and phloem and xylem (vascular tissue) comprise a more successful group of plants whose vegetative parts (i.e., non-reproductive parts) are well adapted to land.
3. Ferns, whisk ferns, club mosses and horsetails are non-seed vascular plants. Their relatives flourished in the carboniferous period ad their remains have given rise to coal and oil. The non-seed vascular plants have (normally as in ferns) roots, shoots and leaves, a well developed transport system, cuticles on leaf surfaces and stomates.
4. Sexual reproduction in ferns and allies is poorly adapted to dry land (see Fig 17.6). Mature, non-seed vascular plants are diploid (2N) sporophytes, i.e. they make spores. They produce haploid (N) spores by meiosis and these are shed from the underside of leaves and germinate on moist soil. The spore gives rise to a gametophyte, a small and insignificant plant that has no leaves stems or roots but looks very similar to liverworts but is smaller. These haploid gametophyte plants make eggs in flask-shaped archegonia and motile sperm in antheridia. Sperm must have liquid water to swim toward the egg in the archegonium The resulting diploid zygote grows to a mature sporophyte plant. This mode of sexual reproduction makes the ferns poorly adapted to land since sex is difficult. For plants to evolve they need the recombination of genetic information that occurs during sexual reproduction in order to adapt to the environment. Ferns can not do this effectively in today's biosphere so they evolve slowly.
5. The seed plants are the gymnosperms and angiosperms. Gymnosperm (lit. naked sperm) are the conifers, cycads, ginkgo (one genus, Ginkgo biloba), and gnetophytes (Gnetum, Ephedra and Welwitschia). Gymnosperms are distinguished from the Angiosperms because angiosperms have flowers.
6. Seed plants are very well adapted to dry land. For reproduction, the sperm are delivered to the egg that contained in an ovule by pollen. Pollen is formed from microspores. Haploid microspores are produced by meiosis in the male cones. The mature pollen shed by conifers is a simple structure consisting of 4-5 cells. Eggs are formed in female cones from a megaspore. The haploid megaspore is produced by meisosis and develops after mitosis into the female gametophyte, a multicellular structure, also called the ovule that contains the egg (see Fig. 17.8).
7. Pollen of conifers are winged and are spread by wind. If they land on a female cone they stick to the surface of the cone because it is sticky because of the presence of a pollination drop. As the pollination drop dries, it pulls the pollen grain into the space between the cone scales. The pollen grain germinates in the female cone and the generative cell of the pollen tube divides by mitosis to give 2 non-motile sperm. These move into the pollen tube as the pollen grows. Fertilization usually occurs 15 months or more after the pollen landed on the surface of the female cone. The 2N zygote resulting from fertilization develops into the new conifer plant.