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IB 201 LABORATORY EXERCISE 10 PLANT EVOLUTION AND DIVERSITY Goals – Experience plant diversity, learn about important evolutionary innovations in plant history, understand how the modular body form of most plants has led to a different evolutionary pattern than is seen in animals (lack of body plans). Be able to identify examples of adaptive radiation and convergence using a phylogenetic framework.. This lab will introduce you to major groups of plants other than algae. You will use the information on plant taxa and phylogeny to trace the evolution of plant structures. The lab will take place at the Plant Biology Greenhouse. Directions to the Greenhouse are located on the previous page. The public parts of the conservatory consist of the conservatory (a large, landscaped area with a circular path) and a series of rooms off of the hallway running out of the conservatory. Each room contains plants with a particular theme (desert plants, aquatic plants, etc.). PART I: PLANT EVOLUTIONARY HISTORY AND THE MAJOR PLANT TAXA. Origin of Plants – The plants are among the most diverse and certainly the most conspicuous of terrestrial organisms. What we think of as plants are more technically known as embryophytes. The embryophytes are a monophyletic group that fall within the green algae. Algae is a general name for organisms capable of photosynthesis (other than the plants). Most algae are simple organisms composed of a single cell or a simple colony of cells. The green algae are a monophyletic group (if the embryophytes are included), united by a number of cellular and biochemical characteristics, most notably the presence of the photosynthetic pigment, chlorophyll b. There are several other groups of algae with different photosynthetic pigments. Some of the these are only distantly related to the green plants (green algae and embryotphytes). Green algae green algae embryophytes green algae green algae The phylogeny above shows the relationship to the green algae in a schematic fashion. The important point is that the green algae and the embryophytes form a monophyletic group but that the green algae are paraphyletic without the embryophytes. Embryophytes are distinguished by the presence of embryos, offspring in early stages of development that grow within one parent. This was a crucial adaptation that allowed plants to live in terrestrial environments. 1 Important Characteristics of Embryophytes Before continuing with the lab you should briefly review the terms that follow. Material explaining these terms in more detail will be available, along with illustrations, on the course web site shortly before spring break. Make sure you are familiar with alternation of generations and what is meant by a sporophyte and a gametophyte. You should also familiarize yourself with these parts of a plant: antheridia, archegonia, sporangia, strobili, microphyll, macrophyll, petal, sepal, anther, stigma, style, and ovary. Reproduction. All embryophytes have two different multicellular stages in their life cycle. The haploid stage is known as the gametophyte while the diploid stage is known the sporophyte. A plant life cycle is shown schematically in the following diagram Spore meiosis mitosis Sporangia Sporophyte Gametophyte Archegonia Antheridia Egg Sperm mitosis Zygote The stages in bold are diploid, the stage that are underlined are haploid. A considerable amount of detail has been removed. The archegonia and antheridia are gamete producing structures. They are roughly equivalent to the ovaries and testes of animals, except, being already haploid, they produce gametes through mitosis rather than meiosis. The sporangia are structures that produce spores. In some plants all spores produce identical gametophytes that contain both antheridia and archegonia. In other plants there are two kinds of sporangia that produce two kinds of spores. Microspores produce microgametophytes that have only antheridia (i.e. they are male). Megaspores produce megagametophytes that only have archegonia (i.e. they are female). All seed plants have both micro and megagametophytes. The Plant Body. A plant is composed of two major sections: the root and the shoot. The shoot can be further divided into stems and leaves. Movement of water and nutrients between different parts of the plant is accomplished through the vascular system, which is composed of xylem and phloem cells. Plants can take on a variety of growth forms. Herbaceous plants have no wood (secondary growth) and are generally fairly small. Vines have elongated stems that use 2 other plants for support. Shrubs have multiple woody stems and grow to a moderate size. Trees have a single woody stem and grow to a large size. Succulents have leaves, stems, and/or roots that have been modified to store water and nutrients. In the following sections we will discuss the major groups of embryophytes. Sections that are preceded by ** are not monophyletic groups but are used to present general characteristics. Monophyletic taxa are indicated by an alphanumeric code (e.g. A1, B3) **NONVASCULAR PLANTS Three different taxa of plants lack true vascular tissue (xylem and phloem) that transports food and nutrients within plant bodies. They lack roots and do not absorb nutrients or water from the soil. These taxa do not form a monophyletic group. The most familiar group of non-vascular plants is the mosses. Mosses have leaf like structures and cells that are capable of limited transportation of water and nutrients (these cells are not the same as the phloem and xylem of vascular plants). The other major group of nonvascular plants is the liverworts. Some liverworts resemble mosses but the most commonly observed species have a flattened green body called a thallus. All nonvascular plants are small, low-growing organisms. All non-vascular plants have a dominant gametophyte. In other words the plant is haploid and the sporophyte is a small structure attached to the gametophyte. In mosses the sporophyte is a stalk that projects upwards from the top of a ‘stem’. A1 - Bryophyta – Mosses B1 – Hepatophyta – Liverworts (Marchantia is one example in the greenhouse) C1 – Anthocerophyta – Hornworts (no examples in greenhouse) **TRACHEOPHYTA OTHER THAN ANGIOSPERMS All of the rest of the plants have vascular systems and are known collectively as the Tracheophyta. The great majority of the Tracheophyta are in a single group, the angiosperms or flowering plants. In this section we will consider all the other groups. D1 - Lycopodiophytes – These plants mostly resemble large mosses (common names include club moss and spike moss). The Lycopodiophytes have a true vascular system and their reproductive system is similar to the ferns (see next section). The dominant form of the plant is the sporophyte. They are widespread and often common plants (particularly in the moist tropics) but fairly inconspicuous. The most obvious difference between the Lycopodiophytes and most ferns are that the club and spike mosses have microphyllous leaves that only have a single vein. Representatives of both Lycopodium and Selaginella are in the greenhouse. E1 - Pteridiophytes and other seedless tracheophyta – The Pteridiophytes are the ferns. They are vascular plants that reproduce without seeds. Closely related to the ferns are the horsetails (Spenophyta, genus Equisetum) and Psilotum, which is probably an unusual fern but has historically been classified in its own group. All the seedless vascular plants 3 have similar reproductive systems. The sporophyte is dominant (ferns are diploid unlike mosses). Spores are released into the environment and develop into small inconspicuous gametophytes that produce eggs and/or sperm. Sperm has to swim into archegonia to fertilize eggs making the plants dependent on water for sexual reproduction (non-vascular plants also need water for sperm to travel from antheridia to archegonia. There are many ferns in the greenhouse – most are in the fern room. The remaining groups are all seed plants. Unlike the plants described above, the gametophytes are not free living organisms. The megagametophytes are encased in ovules (part of the maternal plant). The microgametophytes are tiny structures consisting of only 2-3 cells known as pollen. The embryonic sporophyte develops inside the ovule which becomes a seed after fertilization. F1 - Cycads – These plants resemble palm trees but reproduce using cones (different cones produce pollen and ovules). They are very slow growing plants that are restricted to the tropics and are generally not very common or widespread. Cycads were the dominant group of plants during the Mesozoic (the age of dinosaurs). They differ from all other seed plants except Gingko in having sperm that can swim. Cycads also differ from all other woody plants in having a single unbranched stem. They have extremely slow rates of growth and very soft wood. Cycas, Ceratiozamia, Zamia, and Dioon are examples in the greenhouse. G1 - Conifers and H1 Gingko. These two groups of plants are closely related although in many ways they seem quite different. Unlike cycads their growth form is very similar to that of trees in the Angiosperm group. Gingko was a common plant in the Mesozoic but only a single species exists today and it is extinct in nature (it is a common urban tree in cool climates). There are no specimens of Gingko in the greenhouse but it can be seen on campus – look for it later in the semester when it has leaves. Each leaf is fan shaped with un-branching veins that radiate out from the base. Like cycads Gingko has separate male and female trees. Male trees are much more commonly grown than female trees because the fleshy covering of the ovule has an unpleasant smell. The conifers are the conspicuous non-flowering plants. They include many common garden trees and shrubs as well as abundant trees of northern forests. Any number of species can be seen on or near campus (Spruce, Fir, Pine, Juniper, Yew, etc.). Most conifers have leaves that are modified into needles or flattened scale-like structures. Conifers do not have mobile sperm and reproduce using sperm nuclei like flowering plants. Both pollen and ovules are produced in cones. Araucaria (Norfok Island Pine) and Podocarpus (a broad-leaved conifer) are both in the greenhouse. J1 - Gnetophytes. This is a small but bizarre group containing three very different kinds of plants: Welwitschia, Ephedra, and Gnetum. All three of these can be seen in the greenhouse (in the desert room, the economic plant room, and the tropical shrub room respectively). The Gnetophyta are very closely related to the angiosperms. They share some similarities in the structure of their vascular tissue as well as details of fertilization with the Angiosperms. 4 K1- ANGIOSPERMS The angiosperms are by far the dominant group of plants on earth with over 90% of the species (most of the rest of the species are either ferns or mosses). They include all the plants not mentioned above. The defining characteristic of the flowering plants is that the ovules are encased in an ovary (part of the flower). This ovary becomes the fruit when the ovule becomes the seed. Major Groups of Angiosperms The flowering plants were traditionally divided into 2 major groups: the dicots and the monocots. The characteristic that distinguished the two groups is the number of cotyledons (the embryonic leaves that are seen in seedling plants): monocots having one cotyledon and dicots having two. A number of other characters (floral structures, leaf venation, distribution of vascular bundles in stems and roots) also tended to separate the angiosperms into these two groups. However more detailed phylogenetic analyses have consistently found that the dicots are not a monophyletic group and that the dicot condition evolved prior to the origin of the angiosperms (all of the other seed plant groups with the exception of conifers are composed of plants with two cotyledons). These analyses have consistently supported two major monophyletic groups of angiosperms: the monocots and the eudicots (tricolpates). There are a number of other smaller groups of flowering plants whose relationship to each other and the monocots and eudicots is uncertain. These plants are typically divided into two informal groups: the ‘paleoherbs’ and the ‘magnoliids’. K2 - Monocots The monocots are the most well defined group of angiosperms. In addition to the single cotyledon.the monocots typically have parallel veined leaves (secondarily lost in many species with large leaves), and have vascular bundles that are scattered throughout the stem. The root system is adventitious (not developing from embryonic root tissue but growing from the stem). Monocots lack the ability to form true wood (secondary growth) and are typically herbaceous with floral parts in sets of three. Grasses, palms, orchids, lilies, and the aroids (many common house plants including Philodendron) are all examples of monocots. The monocots contain close to 100,000 species. K3 - Eudicots (Tricolpates) The eudicots are the largest single group of plants in the world. Evidence of the monophyly of this group largely comes from molecular data. The only morphological character uniting the group is the structure of the pollen grains. The eudicots have many 5 typical dicot characters (e.g. two cotyledons, stem vascular bundles in a ring, branching veins in leaves) but these are presumed to be basal angiosperm characters. There are well over 100,000 species of eudicots. They range from tiny herbs to large trees and virtually every other type of growth form. Any angiosperm with a woody stem or highly branched leaves is likely to be a eudicot. **‘Paleoherbs’ The paleoherbs are a group of dicots that share some monocot characteristics. Relationships among the four main groups are uncertain and it is not known if they form a monophyletic group or if they are several groups closely related to the monocots. The paleoherbs are similar to monocots in being mostly herbaceous. They are also primarily found in aquatic and moist forest habitats, similar to plants in the earliest branches of the monocot phylogeny. Waterlilies are the most familiar example of a ‘paleoherb’ **‘Magnoliids’ The magnoliids are several groups of trees and shrubs that have what appear to be ancestral characteristics for the angiosperms. Most notable are the large number of floral parts with little fusion of those parts. It is uncertain whether they are a monophyletic sister taxon to the eudicots, a paraphyletic group that is basal to the eudicots or a paraphyletic group that is basal to the rest of the angiosperms. Exercise 1: Scavenger Hunt to Trace Plant Evolution. Using the material above as well as the phylogeny and list of taxa in the appendix do the following for the following list of plant taxa. Bryophyta (mosses) – find 1 specimen Hepatophyta (liverworts) – find 1 specimen Anthocerophyta (hornworts) – none in greenhouse Lycopodiophytes – find 1 specimen Pteridiophytes – find 2 specimens from different families Psilotum – find one specimen Sphenophyta – find 1 specimen Cycadophyta– find 1 specimen Gingko – none in greenhouse Coniferophyta - – find 1 specimen (in greenhouse or outside) Gnetophyta – find all three taxa Monocots – find three specimens from different families Eudicots – find three specimens from different families Find an example of either a paleoherb or a magnoliid. A. Find the specimens as listed above and create a table listing them with a brief description of each in your lab notebook. Include all the taxonomic information on the 6 label plus whatever else you need to oomplete the rest of the exercise. Have your species checked by your TA. B. Go to the Tree of Life web page for embryophytes http://tolweb.org/tree?group=Embryophytes&contgroup=Green_plants And to the Tree of Life Web page for seed plants http://tolweb.org/tree?group=Spermatopsida&contgroup=Embryophytes Use the phylogenies on these pages to produce a phylogeny for your non-angiosperm plants from exercise A. Based on your observations mark down the points of any major evolutionary transitions (vascular tissue, wood, seeds, etc.). C. Based on your observations and out of class reading write a one paragraph description for each of two plant families (one non-angiosperm, one angiosperm). Include information on reproduction, growth form, and basic ecology. PART II: CONVERGENCE AND ADAPTIVE RADIATION. In comparison with animals plants are both more and less variable in structure. Plants of the same species show much more variation in form than animals. The size and number of stems, branches, leaves, etc. are highly variable in plants. The number of limbs and other organs is generally much less variable in animals. In contrast the diversity animal form is high across different taxa. An arthropod is clearly distinguishable from a vertebrate and an amphibian is clearly distinguishable from a mammal based on overall body plan as well as numerous more subtle details. In contrast plants that appear similar may be completely unrelated. The evolution of similar form in evolutionary distant organisms is known as convergence. Convergence is known in animals, for example in the evolution of similar body shape and digging limbs in three different groups of mammals: moles (family Talpidae, order Insectivora), golden mole (family Chrysochloridae, order Insectivora), and marsupial moles (family Notoryctidae, order Marsupialia). Convergence is very common in plants. Growth forms such as vines, succulents, and epiphytes have evolved repeatedly. The inverse of convergence is adaptive radiation. The Galapagos finches used in the computer simulation in lab 8 are an example of adaptive radiation. So are the many different forms of Drosophila that have evolved on the Hawaiian islands and elsewhere. Many different forms evolve from a single common ancestor. This has occurred commonly in plants. Exercise 2. While touring the greenhouse identify two examples of convergence (members of different families with the same body) form and one example of adaptive (one family with at least three members showing different growth forms). Show these examples to your TA before leaving. Do some research and write a paragraph on each of your three examples describing the pattern of evolution in each case. 7