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Plants to land - What is a plant? - What are the challenges of land? - How were these challenges met? Refer to chapter 23 in text. - What is a plant? Short form: a truly multicellular photoautotroph. photoautotroph: self energy and carbon capture (it photosynthesizes) truly multicellular: showing tissue specialization Contrast with unicellular photoautotrophs, algae, (which, being unicellular eukaryotes, are protists) Kelp are (very) large communal algae, without actual tissue specialization↓ wildflowers.jdcc.edu/Common%20Wood%20Violet.html http://www.smbaykeeper.org/images/site_images/Giant_kelp_adult.jpg Prior to 500 million years ago the land was mostly barren: - bare rock and sand, with maybe prokaryotes scattered about. There had been colonial algae in the seas for about 2 billion years, and many complex animals were present, all marine. ← Artist’s rendition of the Cambrian explosion (505 mya) based on Burgess Shale fossils discovered in western Canada in 1909 by Charles Walcott. http://www.cartage.org.lb/en/themes/sciences/Paleontology/Paleozoology/EarlyPaleozoic/cambdiorama.gif What are the challenges of land? 1. Dehydration While living in water there is still the problem of water balance, but not like being surrounded by air. 2. Support Aquatic algae are supported by the water. 3. Nutrient acquisition/gas exchange These algae are surrounded by the nutrients they need, suspended in the water itself. 4. Nutrient dispersal Algae are generally small, or thin, and don’t have far to move materials internally. 5. Dispersal of reproductive structures Again, surrounded by water, reproductive cells are hydrated and washed away, but on land… These challenges were met in a step-wise progression, reflected in modern examples. Charophyceans are the algae most closely related to plants: - similar cellulose-making apparatus (for cell walls) - same peroxisome enzymes (take care of photosynthesis byproduct) - sperm same as those of sperm-producing plants - similarity in cell plate formation in mitosis - apical growth, lengthening filaments, like stems - gaps through cell walls (plasmodesmata) for cell-cell communication http://www.life.umd.edu/labs/delwiche/Strp/Chlorophyta/charophyceae/Coleo-irregularis.jpg But these were still all living in the water… Vascular Roots Leaves Seeds Flowers No (very thin/flat) No (rhizoids, to anchor only) No No (spores) No They do have a water-retaining outer surface. http://taggart.glg.msu.edu/bot335/bryo.htm These were the dominant plants for 100 million years. Most of their life they are haploid (more later). Sperm have to swim in moisture ←haploid gametophyte with diploid sporophyte sticking out http://www.davidlnelson.md/Cazadero/CazImages/Moss_sporangium2.jpg http://flickriver.com/photos/imbala/6101817600/ Bryophyte (e.g. mosses↓ liverworts and hornworts→) the earliest land plants Lycopodium venustulum ↓ Selaginella plana → (note sporangia-bearing strobili… ‘clubs’) http://courses.bio.indiana.edu/B300-Knox/Images/Selaginella_plana.jpg Lycophytes (e.g. “club mosses”.. NOT actual mosses) Vascular Yes (strong, hydrophobic lignin in cell walls) Roots Yes, off of rhizomes (underground stems) Leaves Sort of (single vein microphylls) Seeds No (windblown spores and flagellated sperm) Flowers No - Leaf forms evolved independently (not direct ancestor of modern branched-vein leaves) - Forests of lycophytes contributed to coal deposits from Carboniferous era; most genera are extinct. - Spores are flammable… http://www.palaeos.com/Plants/Lycophytes/Images/Lycopodium.jpg http://nouveaufauves.files.wordpress.com/2008/04/new-fern-fronds5.jpg Pteridophyte/ Filicophyta (e.g. ferns) Vascular Roots Leaves Seeds Flowers Yes Yes (rhizome and root) Yes (megaphylls, or “fronds”) No (spores) No Two other groups, horsetails and whisk ferns, seem more like lycophytes – They have lost traits through devolution. http://www.davidlnelson.md/Cazadero/CazImages/Fern_structure.jpg Vascular Roots Leaves Seeds Flowers http://www.pinegenome.org/i/redwood.jpg Gymnosperms (e.g. conifers and cycads) “naked seeds” Yes Yes Yes (often needles) Yes (not in fruit) No Redwoods are conifers → Sagos are cycads ↓ http://www.biologyreference.com/images/biol_02_img0213.jpg - Taxonomically not one clade. - Wind-borne pollen Angiosperms (i.e. all flowering plants) Yes Yes Yes Yes (in fruit) Yes Google images Vascular Roots Leaves Seeds Flowers - - http://www.sky-bolt.com/images/curcumaflower.jpg Evolution of flowers and fruit has provided for a wide range of reproductive strategies (e.g. coevolution of animal pollinators) Two broad divisions: ←monocots (e.g. ginger) and eudicots (e.g. dogwood)↑ (More on this later.) Alternation of generations Along with the trend to gain the traits noted, there has been a trend to spend less time in a haploid state. (recall: diploid (2n) is 2 sets of chromosomes, usual for most eukaryotes, while haploid (n) is one set, as is gametes in sexual reproduction.) The next series of slides goes over the life cycles showing this: There is a lot of stuff. Don’t panic. We will have more details on angiosperm fertilization later Bryophytes note -The gametophyte (n) is the conspicuous stage. -Sporophyte is the only diploid (2n) stage, depends on the gametophyte. -Sperm (upper right) have to swim in moisture to egg. Pteridophytes note - Dominant frond is diploid (2n). - Haploid gametophyte still vegetative. - Sperm still need to swim Gymnosperms note - Megaspore, microspore, and food reserves are the haploid structures - Microspore (pollen) is wind blown. Angiosperms note - As with gymnosperms, little is haploid. - A triploid (3n) food storage structure (endosperm) has been added. - Pollen may be carried by wind or pollinators. Alternation of generation, in summary: What do you think are the advantages of haploidy? Of diploidy? Describe how each of the 5 major plant groups represents adaptations to life on land. Charophyceans have a lot in common with land plants: Why aren’t they aquatic plants, then? photoautotroph. conifer Charophycean cycad cellulose Angiosperm Bryophyte alternation of generations rhizoid diploid lignin haploid Lycophyte rhizome gametophyte Pteridophyte sporophyte Filicophyta pollen Gymnosperm endosperm