Lecture 6 Notes
The Plants
| I. About 266,000 species known
II. Two major groups A. Bryophytes B. Vascular Plants III. Plants are dominant large organisms in the world - may exceed all other groups in terms of biomass. Largest things on earth are plants (General Sherman tree, a sequioa in California is over 47 feet in diameter with branches larger than most eastern trees) IV. Plants defined by following characteristics: A. cellulose-enriched cell walls B. chloroplasts with chlorophylls a and b and accessory pigments, carotenoids C. starch is the primary food reserve V. Plants evolved from the green algae A. Major task was to find a way to avoid dessication when colonizing land 1. Plants evolved cuticle, a waxy coating over the epidermis to prevent water loss 2. Stomata (pores in the leaf) allow gas exchange, since diffusion through cuticle is largely prevented 3. As plants grew larger, they needed new ways to distribute food, nutrients and water a. vascular systems developed (xylem and phloem) b. hormonal systems developed that helped coordinate growth and development VI. Plant life cycles A. All plants go through an alternation of generations - going from gametophyte (cells have only 1 copy of each chromosome, i.e. gene, to sporophyte (multiple copies of each chromosome, i.e., gene) B. In some plants, the gametophyte is free-living (mosses, liverworts and ferns, for example) C. Early plants - all spores identical in size and appearance. In more advanced plants male spores smaller than female spores. First step towards formation of pollen and ovaries D. Mosses and other Bryophyte Life Cycles 1. Bryophytes (about 10,000 species of mosses) a. mosses, liverworts, and hornworts (wort is an old botanical term for leaf) Thus a liverwort is a plant with leaves that look like a liver! Hornworts thought to be most closely related to vascular plants. b. Most are small plants (< 2m tall) because of their lack of a vascular system. c. all are non-vascular (no phloem or xylem) d. Require freshwater to reproduce (allows sperm and egg to mix together) e. Step-by-step life cycle: i. male gametophyte produces structures called antheridia ii. antheridia site of male sperm production iii. female gametophyte (separate plant) produces eggs in structure called an archegonium (archegonia is plural) iv. drop of water splashes sperm onto archegonia, and sperm fertilizes eggs v. developing egg now has chromosomes from male and female parents, so this embryo is now called a sporophyte (spore producing plant) vi. sporophyte develops and grows attached to female gametophyte In this case, sporophyte is sort of a parasite on the gametophyte vii. mature sporophyte produces a spore case viii. Meiosis occurs and haploid spores are produced. Germinate and form gametophytes (haploid plants). Now, go back to step i and life cycle is repeated. 2. Seedless Vascular Plants - Ferns, Club Mosses, Horsetails, Whisk Ferns i. Life cycle similar to that for mosses, but sporophyte dominates now This is the part of the fern that we usually see. Change over to domination by sporophyte continues throughout plant kingdom until the gametophytes are reduced to an ovule and pollen grain. ii. Ferns are vascular plants - have phloem and xylem. Been around for a long time - some are quite large (tree ferns), others very small (curly grass fern in New Jersey Pine Barrens is less than 1" tall!). a. xylem - transports water,hormones and nutrients - moves primarily from roots to stem to leaves to atmosphere b. phloem - transports water, hormones, and photosynthates (products of photosynthesis) - can move both up and down the plant iii. Now plants have true leaves, stems and roots iv. But ferns are seedless vascular plants - they still produce spores, not seeds 3. Vascular Plants that Produce Seeds i. What is a seed? a. a seed is an embryo surrounded by nutritive tissue and a protective coat b. seeds allow plants to get through bad times (go dormant) c. seed coat protects embryo, and helps regulate timing of germination 1. some seed coats rot at a specific rate so the seeds germinate at the right time of the year 2. other seed coats allow seeds to pass through animal guts (which scarifies them - abrades the seed coat) and prepares them to germinate - animal serves as a disperser of seeds without consuming them 3. Seeds contain food storage organs (cotelydons - first leaves) and endosperm (nutritive tissue to help embryo germinate and survive) Most of the nutritive value of the grains that we eat is derived from endosperm. Coconut milk is endosperm. Corn is mostly endosperm. ii. Gymnosperms a. Four major groups 1. Conifers - pines, firs, spruces, larches 2. Cycads - palmlike compound leaves 3. Ginkgo - single species genus 4. Gnetophyta - very primitive group, few species left b. Gymnosperms have an ovule that is not completely enclosed by parental tissues. c. Takes 2 years in pines to complete reproductive cycle. Study your book for the life cycle, which is hard to illustrate in lecture notes. d. Pollen is now produced - can consider the cells that make up pollen as the gymnosperm male gametophyte. Ovule and associated cells are the female gametophyte. See how reduced the gametophytes are compared to the sporophyte (the tree). e. Endosperm is derived from female gametophyte - is haploid, that is, it has only one set of chromosomes, none from the male parent. iii. Angiosperms - the flowering vascular plants a. Arose about 125 MYA - when dinosaurs were prevalent b. Comprise about 235,000 species out of 266,000 known species - most successful group of plants in terms of species known. c. Only this group has true flowers d. Major advancement - ovule is surrounded completely by parental tissue - much better protection for developing embryos. e. Flowers derived from vegetative tissues (all parts of flower can be traced back to leaves at some point). i. small inconspicuous flowers - most likely are wind pollinated ii. showy flowers - pollinated by animals - insects, birds, bats, even possums or rodents in some cases! f. Seeds enclosed in a capsule, which technically is a fruit. Fruit from a consumer point of view is different from fruit from a botanical point of view. Fruits protect maturing seeds and are also used to enhance dispersal (through consumption, or by attaching to animals that pass by). For example, Velcro was invented after a botanist studied how seeds of beggar's ticks (a type of plant) attached to animal fur. 1. Consumer view point - fruit is sweet, vegetables less so 2. Botanical view point - all seed capsules are fruits (beans, peas, apples, kiwi, etc.) g. What are the parts of the flower? i. sepals - outermost whorl of petal-like structures (sometimes colorful and sometimes just green or nondescript) ii. petals - next inner whorl - showy portion of the plant iii. anthers - structures that produce pollen iv. gynoecium - innermost whorl where eggs are located in ovules 10. Pollen grains land on surface of stigma (receptive surface of gynoecium). If same species, pollen germinates and grows down the style (long neck-like structure above ovules) until ovules are reached. Then sperm are released and eggs fertilized. 11. Angiosperms have double fertilization - eggs are fertilized, and tissue that will become endosperm receives DNA from pollen. Since endosperm tissue is mostly 2N (diploid), the addition of DNA from the pollen (which is N, or haploid) adds another set of chromosomes, making endosperm 3N!! Triploid endosperm. http://www.nsf.gov/od/lpa/news/02/pr0209.htm
12. No other group of organisms on earth has double fertilization!!!
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