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Spore-Forming Plants. The Lower Plants. Classification of Life.
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Spore-Forming Plants The Lower Plants
Classification of Life • If we go back to ancient Greek times, Aristotle (350 BC) was the first and most influential classifier of the world. He classified living things into plants and animals. Three sub-groups for each: for plants: its size, and for animals, where it primarily lives. • This matches our naïve view of the world: living things come in two basic varieties: those that move around (animals) and those that are rooted to the ground (plants). • he wasn’t thinking about evolution or phylogeny! • Aristotle’s views were considered definitive, about this and everything else in the physical world, up until the development of science in the 1600’s.
Linnaeus Carolus Linnaeus (1707-1778) a.k.a. Carl Linne Swedish naturalist: Latinized name Like Aristotle, he classified the world into plants and animals, but divided these groups up by the presence of various traits. Plants were grouped into genera and given multi-word Latin names. Linnaeus shortened this to the binomial: genus followed by species. (like Homo sapiens) He also grouped them into larger groups (classes) based on sexual characteristics: the Sexual System of Classification For example: "Nine men in the same bride's chamber, with one woman“. This meant 9 stamens with 1 pistil in the same flower. (All in Latin, of course) although this system was invented for convenience, it fit evolutionary reality fairly well, and we still basically use it (with modifications and corrections) He also invented the kingdom-phylum-class-order-family-genus-species hierarchy.
Five Kingdom Model • New things kept being discovered, and several things became obvious: • fungi aren’t plants, • bacteria and other prokaryotes are fundamentally different from plants and other eukaryotes • single celled eukaryotes (protists) are not easily classified as plants or animals or fungi • This led to the Five Kingdom model of R. H. Whittaker, in 1969. • The Monera are the prokaryotes: the domains of Bacteria and Archaea. Monera is not a term used today. • Plants evolved from protists, but where is the boundary between them?
Three Domain System • The Three Domain system was invented Carl Woese, based on sequencing ribosomal RNA. • It shows a more realistic view of the world of life: • two very different types of prokaryote • Most eukaryotes are protists • Protists aren’t a monophyletic group. • Classification today is based on DNA sequencing. • Relatively easy to do • DNA is what is actually inherited between organisms • We understand the mechanisms of change (mutation) in DNA • DNA-based phylogeny matches traditional phylogeny reasonably well, but lots of changed details. • Especially in the less familiar groups and in very ancient branchings.
What is a Plant? • Somewhere there is a boundary between photosynthetic algae (protists) and plants, a logical place to divide them. • We want to distinguish between plants and algae on the basis of two things: • Plants should be a monophyletic group (a single common ancestor and all descendants) as shown by solid DNA evidence. • Plants should match our naïve view of the world as primarily photosynthetic land-dwelling organisms rooted to the ground. • Not that we can't have some exceptions, but they need good explanations.
Some Possible Divisions • Start with the protist group the Archaeplastida. This group came from the primary endosymbiosis of a cyanobacterium that created the plastid (chloroplast). • We could call this whole group “plants”: anything containing a plastid that is not the result of secondary endosymbiosis. (recall that many protists developed a secondary endosymbiosis with eukaryotic red or green algae). • Quite early, this groups split into the red and green algae. • Red algae contain phycoerythrin, a red pigment that helps gather light deep in the water. • Green algae contain chlorophyll b and in many ways is very similar to land plants. Green algae have long been thought to be ancestral to plants, and DNA evidence has confirmed this. • Some would like to include green algae and plants in a monophyletic group called Viridiplantae: “green plants”. • Probably not widely accepted due to the tradition of keeping algae as a separate group.
Green Algae • Green algae come in many varieties: mostly unicellular, but some are colonial, and a few are genuinely multicellular. • All green algae have: • two membranes surrounding the chloroplast (i.e. primary endosymbiosis) • Chlorophyll a and b, along with carotene (yellow) and xanthophylls (orange-brown) • Cellulose and pectin in the cell wall. • Starch for food storage (something to live on when it’s dark). Red algae use a different form of starch called “Floridean starch”.
More Green Algae • Most are aquatic (marine or fresh water), but some live on land. • There’s a type of algae that lives on snow fields. Has a red pigment to block UV light. • Some land algae are symbionts with fungi (lichens). • Most have both sexual and asexual reproduction, with gametes having 2 flagella. • Most are primarily haploid, although a few have a diploid phase as well. • Possible use of green algae for biofuels. They are very efficient at converting sunlight into chemical energy, and easy to grow.
Charophytes • The green algae can be split into two main groups: the chlorophytes and the charophytes. • Most are chlorophytes, including single celled forms and colonial forms. • The charophytes, especially a group called the stoneworts (Charales), are the green algae group most closely related to plants. Based on DNA evidence. • They have whorls of short branches surrounding a central stem. No leaves, just photosynthetic branches. • They live in shallow water and lake margins: occasionally dry out. • Charophytes are the most structurally complex of the green algae.
Charophytes as the Sister Group to Plants • Some characters shared by charophytes and plants, and no other groups: shared derived characters, used to define monophyletic groups. • Charophytes protect their embryos with a layer of sporopollenin, a tough polymer whose composition isn’t precisely known. Land plants use it to cover spores and pollen grains. • Phragmoplast: A structure used in cell division, it is the scaffold on which the new dividing wall between the 2 daughter cells forms.
Embryophyte = Land Plant The current most common definition of “plant” excludes all green algae. The group is called the Embryophyta. The defining characteristic of an embryophyte is that they carry the multicellular embryo within the mother’s body. That is, the female gamete is fertilized inside the mother plant’s body, and it continues to develop there, using nutrients from the mother (a placenta). Embryophyte and land plant are synonyms: they refer to the exact same group of organisms.
Evolutionary Trends • By moving onto the land, plants had to deal with 2 big issues: gravity ( or lack of buoyancy) and dryness. • Major trends: • development of roots, shoots, vascular system. Roots needs to absorb nutrients, not just hold onto the surface. Shoots need to support photosynthetic system off the ground. Vascular system to transport materials between parts of the plant. • Waxy cuticle on the plant surface to prevent desiccation. • Increasing the diploid phase of the life cycle, and decreasing the haploid phase. Diploid gives a backup copy of each gene, as a defense against random mutations. Allows a larger, more complex body. • Spore, seed and pollen protection and dispersal. How can they be protected, how can the male gametes find the females, and how can new individuals disperse to new locations.
Major Plant Groups • Four groups, some of which have more than one phylum: • Bryophytes. Mosses, liverworts and hornworts. No vascular system. The most primitive plants. • Seedless vascular plants. Ferns of various types. Have vascular system but use spores to reproduce. • Gymnosperms. Conifers, cycads, ginkos. Have seeds, a major innovation (so the next set of lecture notes starts here. • Angiosperms. Flowering plants: most of the common plants. Seeds develop in an ovary. • Note that some groups aren’t monophyletic. They are probably independent branches off the main evolutionary line. We will largely ignore this, however.
Alternation of Generations • The sexual cycle in eukaryotes has a diploid phase and a haploid phase. • Diploid: 2 copies of each chromosome, one from each parent • Haploid: only 1 copy of each chromosome. • Fertilization: two haploid cells (the gametes) combine to form a new diploid cell, the zygote. • Meiosis: a diploid cell undergoes a special form of cell division that results in 4 haploid cells. • Alternation of generations means that both the diploid and haploid phases are multicellular. • Humans do not have alternation of generations: haploid phase is 1 cell only • Most fungi do not have alternation of generations: diploid phase in 1 cell only. • Plants do have alternation of generations
Alternation of Generations in Plants • The diploid phase is called the sporophyte. Some cells in the sporophyte undergo meiosis, which produces haploid spores. This occurs in a multicellular structure called a sporangium. • Spores are unicellular and packaged to survive harsh conditions. • Spores germinate into a new haploid plant, the gametophyte. Some cells in the gametophyte develop into haploid reproductive cells, the gametes. The gametes are the equivalent of human sperm and eggs. Production of gametes occurs in a multicellular gametangium. • Two gametes fuse together during fertilization, producing a zygote, a single diploid cell that is the first cell of the new sporophyte. • The zygote develops into an embryo (multicellular diploid) attached to the mother. The embryo is then released from the mother. It starts growing as an independent sporophyte.
Male and Female • In the fungi (and many protists), the gametes are almost identical. There is no “male” or “female”, just different mating types. • Plants (and animals) have distinctly different male and female gametes. • The male gamete (sperm) is dispersed out into the world, and must find the female. • The female gamete (egg) stays inside the mother’s body, and is fertilized there. It is usually larger than the male gamete. • The gametes are made by the gametophyte, the haploid plant. In some plant groups, separate male and female gametophytes are produced, and in other species, a single gametophyte produces both male and female gametes. • The structures that produce gametes: • Male gametes (sperm) are produced in antheridia. • Female gametes (eggs) are produced in archegonia.
Examples • In this fern, the sporophyte produces just one kind of gametophyte, which is bisexual. A single gametophyte plant produces both antheridia and archegonia.
Another Example • In this moss, a single kind of sporangium produces gametophytes that are unisexual: either male or female. The female gametophyte produces archegonia, which make the eggs. The male gametophyte produces antheridia, which make sperm.
M+F continued • In the seed plants, the gametophytes are differentiated into male and female: the male gametophyte is the pollen grain and the female gametophyte is the ovule. These plants have two different kinds of sporangia, one for each sex. “micro-” refers to the male and “mega-” to the female. • The microsporangium produces microspores (microgametophytes), which then produce antheridia, which make sperm (male gametes). • The megasporangium produces megaspores (megagametophytes), which then produce archegonia, which make eggs (female gametophytes). • In seed plants, the antheridium and archegonium have been reduced to very small structures that are not identified as separate from the rest of the gametophyte. The gametophyte as a whole generates the sperm and eggs.
Example In this pine tree (a gymnosperm), the sporophyte plant produces separate megaspores and microspores. The megaspores develop into female megagametophtyes and the microspores develop into male microgametophytes. The antheridium and archegonium have been reduced to very small structures that are not identified as separate from the rest of the gametophyte. The gametophyte as a whole generates the sperm and eggs.
Move to Sporophyte Dominance • A major change over the evolutionary history of plants is a move from a dominant haploid gametophyte to a dominant diploid sporophyte. • Bryophytes: mostly haploid, with small diploid sporophyte growing out of the gametophyte. • Ferns: the main plant body is a diploid sporophyte, but there is a small free-living haploid plant • Flowering plants: the plant is diploid, and the gametophyte is reduced to 3 cells for the male and 8 cells for the female. • Being diploid means there is a second copy of each gene. So, if one copy gets mutated (mutation happens all the time), the other copy can continue to fill its role, so the cell lives on. Otherwise, organisms just can’t get too complex. Animals are also diploid for the same reason.
Photosynthesis • Photosynthesis uses energy from light to convert carbon dioxide (CO2) into sugar. • Occurs in the chloroplasts, which were once free-living bacteria that got swallowed up by endosymbiosis. • In other parts of the plant, chloroplasts get used for storage of food or other pigments (like in flowers). Called plastids. • Two parts to photosynthesis: light reactions (occur only in the light) and the Calvin cycle (occurs in both light and dark). • Light reactions: Light energy is captured by chlorophyll and used to extract high energy electrons from water, which converts it to oxygen. • Calvin cycle: The high energy electrons are used to convert carbon dioxide into sugar. This is called carbon fixation.
Cell Walls • The cell wall is mostly made of cellulose. • Cellulose is a molecule made of many glucose sugar molecules linked in long chains • Starch is also made of many glucose units, but the linkages between the glucoses is different in cellulose and starch. This gives them different chemical properties. • Notably, almost all organisms can easily digest starch, but very few can digest cellulose. • Mostly just some types of bacteria and protists • Cellulose is probably the most common organic compound on Earth. • In cells needed for support or water conduction, the cell wall is thickened and strengthened bylignin, a complex organic compound that is even harder to digest than cellulose.
Cells, Tissues, and Organs • A big difference between plants and their protist ancestors is that plants are multicellular and have different organs. • Multicellular organisms have many different types of cell. • Tissue: a group of cells with a common structure • Organ: a group of different tissues organized for a common purpose. • The main plant organs: leaves, stems, roots. Plus various reproductive organs. • All the cells in an individual have the same genes. Different cell types occur because different sets of genes are active. • An example: the stem of a plant has two main functions: to support the upper parts of the plant, and to conduct fluids between the roots and the leaves. Stems have several tissues in them: epidermis (the outer covering), xylem and phloem (vascular tissues), and fibers (for support) and general body cells. In turn, the vascular tissues are composed of several different cell types: tracheid and vessel elements for the xylem, and sieve tubes and companion cells for the phloem.
Meristems • Meristemsare special regions in the plant where cell division occurs. Cells in other parts of the plant don’t divide. Meristemsproduce all of the new cells; once a cell leaves the meristem, it can enlarge but not divide. • Apical meristem: at the tip of the plant shoots and at the tip of the roots. This is where growth occurs, producing new leaves, branches, flowers, etc. • Lateral meristem: in the stems of woody plants: they produce lateral growth. Also called cambium layers. • Once a cell has been produced in a meristem, it goes through a process of differentiation, which turns it into some particular type of cell. • Xylem, phloem, epidermis, etc. • Meristems are the equivalent of stem cells in animals.
Vascular Tissue • Two basic types: xylem and phloem • Xylem conducts water and mineral nutrients up from the roots. • Xylem cells are dead and hollowed out. • Wood is made of xylem, but even non-woody plants have xylem. • Water is pulled up by transpiration: water molecules evaporating from the leaves pull other water molecules up the tubes, because water molecules stick together. • Phloem cells carry organic matter (mostly sugar) from the leaves to other parts of the plant. • Unlike xylem, phloem cells are alive. • The cells are connected by many pores, so material flows easily between the cells. • Flow of material in both directions
Xylem and Phloem • Xylem and phloem occur together, in bundles that also include supporting cells. • Xylem in the inside, phloem on the outside, with the meristem (cambium) between them. • So, in a tree, the xylem becomes wood, and the phloem is the layer just under the bark. Removing the bark from a circle around the tree kills it because the phloem has been disrupted: the roots are not connected to the leaves. • A meristem layer (called vascular cambium) lies between the xylem and phloem, and generates new cells.
Leaves • Leaves are the main site of photosynthesis. • Photosynthesis mostly occurs in the layer of cells just below the epidermis. (palisade layer) • The sugars are then transported to other parts of the plant through the vascular system. • The spongy tissue below the palisade layer carries the sugar (dissolved in water) to the veins of the leaf, which are part of the vascular system. • Leaves are coated with a waxy layer called the cuticle. The leaf epidermis cells secrete the cuticle, which helps prevent the leaf from drying out.
Stomata in the Leaves • A big development in bryophytes is stomata: openings in the leaves that open and close in response to conditions. (singular=stoma) • The leaves are covered with the waxy cuticle, which is impermeable to gases • Photosynthesis needs CO2 from the atmosphere, and it releases oxygen • Transpiration needs water vapor to evaporate out of the leaves, but in hot, dry climates, too much evaporation would kill the plant. • Stomata can open and close in response to the need for carbon dioxide and the need to avoid drying out. • All plants except liverworts have stomata.
Evolution of Leaves • The basic structure of having a flat surface to maximize exposure to light for photosynthesis is found in many algae as well as in the plants. However, these are not considered “true leaves”, because they have no vascular tissue. • Microphylls are leaf-like structures found in some of the seedless vascular plants: lycopods and horsetails. Microphylls have a single strand of vascular tissue down the middle. Seem to have evolved as outgrowths from the stem. • Megaphylls are the type of leaf seen in ferns and seed plants. Megaphylls have a branched system of veins (vascular tissue). They seem to have evolved independently from microphylls, in response to a drop in atmospheric carbon dioxide in the late Paleozoic. Maybe by filling in the spaces between small branches (webbing).
Bryophytes Liverwort pore: always open, in contrast to stomata, which open and close. Bryophytes are also called the non-vascular plants. They do not have xylem and phloem to conduct fluids and nutrients between different parts of the plant body. The bryophytes are not a monophyletic group (clade). Instead, we use the name for 3 phyla: the liverworts, the hornworts, and the mosses. All three groups spend most of their life cycle as haploid gametophytes. The sperm are produced in antheridia. They have flagella (several), and they have to swim through drops of water to find the eggs inside the archegonia of another plant. No vascular system means that the plants must be small and low to the ground, and that they are mostly found in moist environments. Roots are called rhizoids: they just hold the plant down and don’t extract water and nutrients for the rest of the plant. Each one is a single elongated cell.
More on Bryophytes • The fertilized egg is contained within the archegonium of the gametophyte. This zygote grows into a sporophyte without leaving. The sporophyte grows out of the gametophyte. • The sporophyte is composed of a foot that anchors it to the gametophyte, a stalk, and the sporangium, where meiosis and spore production occurs. • The sporophyte is not photosynthetic, and it is completely dependent on the gametophyte fro survival.
Liverworts • Liverworts are probably the earliest branching plant lineage. • The name come from an old and very incorrect idea called the Doctrine of Signatures: a plant resembles the organ it can heal. Liverworts somewhat resemble the liver, and so they were thought to cure liver ailments. • Liverworts are small and low to the ground. They produce a flattened stem that looks like a leaf, but it lacks the different cell and tissue types found in real leaves. • Unlike all other plant groups, liverworts don’t have stomata that open and close in response to different environmental conditions. Instead, liverworts have pores that are always open. (Hornworts and mosses have stomata.)
Hornworts • Another bryophyte group. The sporophyte grows out of the gametophyte, but the sporophyte is photosynthetic. • Another small, low-growing plant. Distinguished by the horn-shaped sporophyte.
Mosses The mosses are the largest group of bryophytes. There are a number of plants called “moss” that are really not bryophytes: reindeer moss is an example Some mosses have a strand of vascular tissue, but not a full xylem-phloem combination as is found in all non-bryophyte plants. Mosses have a use for humans: they absorb water very well, so peat moss is a common soil additive in gardening. It is also used for fuel in some parts of the world. It is also burned to produce the smoky taste of Scotch whiskey. Peat moss bogs are quite acidic, and this preserves dead organisms.
Seedless Vascular Plants • Two phyla: the lycophytes (club mosses) and the pterophytes (ferns and horse tails) • Sporophyte is dominant, with small gametophyte. The eggs are in archegonia, and the sporophyte grows right out of the gametophyte, just like in the bryophytes. Sperm have flagella and move through drops of water to the eggs. • Roots and shoots both form branches (not true in bryophytes).
All are extinct. Known only from fossils, which date to 420 mya. Sporophytes (as shown in the figure) are branched and grow independently of gametophytes. Some had vascular tissues. Up to 50 cm tall, which is HUGE compared to bryophytes! Consisted of stem tissues only (photosynthetic vertical stems and horizontal rhizomes). NO leaves NO roots for absorbing water (but rhizoids for anchoring). Protrachaeophytes and early vascular plants Aglaophyton [29.12/29.11]
Lycophytes • Lycophytes are club mosses (plus spike mosses and quillworts). • Not closely related to the “true” mosses, which are bryophytes with no vascular system. • Leaves are microphylls. • Some called ground pines: they resemble small pine trees, with the sporangia at the tips looking somewhat like pine cones. • Many are epiphytes: they grow off the ground, supported by trees but just using them for support, not as parasites. • Flash powder, used by photographers before electricity-powered flash bulbs, was Lycopodium club moss spores: they are very small and burn very rapidly.
Paleozoic Club Mosses • Club mosses were a dominant form in the Carboniferous period of the Paleozoic. Sometimes called “scale trees” because the bark had scales on it. • They were the first plants to grow into trees, up to 40 meters tall. • They grew in swamps, and most of our coal in Illinois comes from Carboniferous club mosses (plus ferns and horsetails). • These big lycophytes are long extinct, but smaller ones survived and exist today.
Pterophytes • Pterophytes are ferns, horsetails, and whisk ferns. • Have megaphyll leaves. • Gametophyte is a small multicellular structure called a prothallus that is often bisexual. The large sporophyte grows right out of it. • Ferns have roots that can branch at any point. In contrast, lycophyte roots can only branch at the growing tip, by forming a Y with two equal branches. • Azolla (duckweed), a very small fern, grows in rice paddies. It has a cyanobacterium symbiote, which fixes nitrogen, useful to fertilize the rice. • Whisk ferns are just 2 small genera, mostly tropical epiphytes. No true roots, just rhizoids to hold them down. (but roots may have been lost secondarily). Dichotomous branching: Y shaped, 2 equal branches: considered a primitive trait, with unequal branches evolving later.
Horsetails • Horsetails have silica in their stems that makes them good for scouring pots: "scouring rushes". • Horsetails were very common, up to 15 meters tall in the Carboniferous period. Today only a few species exist. • Sporangium at the top of the plant. • Photosynthesis in the stems. • When the stems branch, a whorl of smaller branches appear (i.e. the whorls are branches, not leaves).
