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Lycophytes Biodiversity Background the Vascular Plants, Or Tracheophytes, Are a Monophyle

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Lycophytes Biodiversity Background the Vascular Plants, Or Tracheophytes, Are a Monophyle LAB 3- VASCULAR PLANT LIFE CYCLES: Lycophytes Biodiversity background The vascular plants, or tracheophytes, are a monophyletic subgroup of the land plants, characterized by: 1-independent, long-lived and branched sporophytes, 2-lignified secondary walls, 3-xylem and phloem (transport/vascular tissue), 4-roots and an endodermis. As you look at the materials in lab, try to identify examples of these four synapomorphies (shared, derived traits). Phylogenetic tree of vascular plants (includes extinct lineages) From Cronk, Q. 2009. The molecular organography of plants. 32 Laboratory Exercise 3 In this lab you will study the synapomorphies of vascular plants by: - Observing the water-conducting cells of an early vascular fossil - Identifying cell types in a cross-section of a stem of Lycopodium. - Studying members of the earliest lineage of vascular plants, the Lycophytes, including the recently sequenced model lycophyte Selaginella moellendorffii. Pre-vascular tissue: A cross section of the stem of the sporophyte fossil plant Aglaophyton (Rhynia major) shows that it has an epidermis with stomata, a cortex where some of the cells were associated with a fungus (water and mineral absorption), and a central core of vascular tissue with phloem and xylem-like cells (lacking secondary wall thickenings so they are not considered tracheids). Look at the slide of a cross section of this plant on demonstration and identify the tissues shown below. Label the Aglaophyton Stem cross-section Primary vascular arrangement. The arrangement of vascular tissue in the stem and root is referred to as the stele. A summary of stelar types can be found in the cronk book (p. 35). In virtually all tracheophytes, roots have a protostele. STEM AND ROOT ANATOMY Locate the following tissues in the cross-section of a Lycopodium Shoot (slide): 1) epidermis, 2) cortex, 3) stele, 4) endodermis, 5) pericycle, 6) protoxylem, 7) metaxylem and 6) phloem region. Label each on the illustration below. 33 Lycopodium is an early vascular plant that has similar anatomy in stems and roots. This type of stele is typical of most roots, but other vascular plants have a distinct type of stele in stems. Homosporous Lycophytes- Lycopodium (clubmoss) The most common and conspicuous species in our part of the world are terrestrial plants with long runners, but some local species are also small and grow erect (e.g. Huperzia in field trip site). In the tropics, it is common for Lycopodium species to be epiphytic. Observe a Lycopodium plant and note the following: Branching. 1. Note the long runner (horizontal stem) with its side branches. The growing point of the runner is at the actively growing end and at the other end older tissues are starting to senesce. 2. Note the dichotomous branching. At most branch points, branching is unequal. One branch is indeterminate and continues as the main axis. The other branch grows more slowly and may be determinate if it produces a strobilus. 3. Geotropic behavior of the indeterminate and (usually) determinate branches is different. The indeterminate branch at each dichotomy grows neither up nor down, but along the ground while the determinate branch tends to show negative geotropism (turns upward). Leaves. 1. Note the spiral phyllotaxy 2. Note the single vein in each leaf, typical (but not exclusive) of microphylls. 34 Roots. 1. This plant has only shoot-borne (adventitious) roots. Nonseed plant embryos are not distinctly bipolar. The first root often forms late and may be associated with a leaf. Subsequent roots clearly originate on the shoot. 2. Look at the wiry roots. They branch dichotomously (easier to see in young roots). Dichotomous root branching is unusual, even in non-seed plants. Strobili. Note that the strobili (cones) form on determinate side branches. Depending on which species you are looking at, the cones may be solitary (one per side branch) or there may be several cones on a branched stalk. In some species, there is a clear distinction between the form of sporophylls (modified leaves associated with sporangia) and vegetative leaves. Examine a strobilus under the dissecting scope. How many sporangia per sporophyll? __________________ Look for the short stalk of the sporangium. Where is it attached? (axil, leaf surface?)_______ How does dehiscence occur (opening of? (by pore? slit?) ________________________ Examine spores with the compound scope. Use fresh spores from the dissection material. The spores may not wet properly unless you use soapy water in making the slide (in dropper bottle). Put a drop of soapy water on a slide, touch a dissecting needle to the drop and use the wet needle to pick up some spores. Examine with the compound scope. GAMETOPHYTES. Gametophytes of most Lycophyte species are subterranean and mycotrophic (mutualistic association with fungi). They are not generally available fresh for study. See demo of preserved gametophytes with attached sporophyte. 35 Heterosporous Lycophytes- (Sellaginella and Isoetes) Vegetative Morphology of Selaginella kraussiana. This plant is maintained in our greenhouse where it grows vigorously and regularly escapes as a weed. Observe the potted plants. As in many species of Lycopodium this species of Selaginella is characterized by anisotomous branching (unequal dichotomies). Pick off a healthy green axis, which has 5 or 6 dichotomies in it. Study with the naked eye and dissecting scope. Note adventitious roots. Use transmitted light (from below) so you can see the vascular traces of main axes, side branches and roots. Illustrate below Reproductive Morphology of Selaginella. Strobili of S. kraussiana. Strobili are usually numerous on these plants, but the young strobili that you need to dissect are very inconspicuous because the sporophylls resemble vegetative leaves. If you cannot locate them ask your instructor for help! 1. Examine a branch with 4 or 5 dichotomies, using the dissecting scope. Look for young strobili out near the tips of the branches. Larger, older strobili are easier to find, but will have megasporangia that have already dehisced. 2. Using forceps and needles on the dissecting scope stage, pry apart the sporophylls and look for mega- and microsporangia. S. kraussiana usually has a single large megasporangium and numerous microsporangia in each strobilus. 3. Find both young strobili with undehisced megasporangia and older strobili in which the megaspores have been shed. 4. Tease out some megaspores into a drop of water and make a temporary slide mount (do not squash them). Observe their surface features using the compound scope. 5. Microspores are too small to handle separately, but a temporary slide mount can be made of an entire microsporangium. Squash the sporangium under the cover slip so its spores will be released. Note the spines on the spore wall, possibly functioning to help the microspores stick to megaspores. Illustrate your observations in the space below. 36 Challenge: Search for young Selaginella sporophytes in the pots, they are usually still attached to the (endosporic) gametophyte, as in the picture below. Label: megaspore, young sporophyte. Observe other Species of Selaginella on Display – look at the pattern of branching, phyllotaxy, and leaf shape and form. Specimens include S. moellendorfii, whose genome was fully sequenced in 2011. Isoetes (quillwort). This is an unusual plant, most people would mistake it for a monocot! (but you’ll know better…) A. Gross Morphology. Look at live plants from the greenhouse. B. Reproductive structures (slides). The drawings below will help you with interpretation. 1. Isoetes microsporophyll and microsporangium l.s. (Triarch A-223-5) 2. Isoetes megasporophyll and megasporangium l.s. (Triarch A-223-7) Isoetes plant with microsporophylls and megasporophylls. 37 VASCULAR PLANT LIFE CYCLES: ferns Introduction Fern Phylogeny (Cronk, 2010) LEPTOSPORANGIATE FERNS (“FILICOIDS”). This is the largest group of ferns. You will see many species in the field trip. Sporophyte: Some fern species have above ground vertically oriented stems and are often referred to as tree ferns, although they lack secondary growth and are relatively short compared to arborescent seed plants. Most fern species are characterized by the presence of relatively large, compound leaves (usually called “fronds” and composed of pinnae) and an inconspicuous, often subterranean, rhizome. Ferns with a short, erect rhizome tend to do very little branching and to bear numerous leaves arranged in a tight spiral. An example of the latter is our local sword fern (Polystichum munitum). Species with creeping rhizomes often bear relatively few leaves at each branch tip and internodes are relatively long. The ubiquitous bracken fern (Pteridium aquilinum) has an underground, horizontal rhizome and falls into this category, as does the licorice fern (Polypodium glycyrrhiza) which is a common local epiphyte on maples and alders Sporangia. Leptosporangiate sporangia are generally tiny, delicate, stalked structures with relatively few spores (16-64) that are initiated from a single cell and have an annulus, a specialized group of cells involved in dehiscence. The sequence below shows a basic type of leptosporangium. Note the unevenly thickened cell walls of the annulus. Shrinkage and shape change of the annulus cells causes the sporangium to tear open at a weak point, the stomium. Water loss continues until the tension (negative water potential) inside these cells is so great that air bubbles develop. Sudden appearance and expansion of air bubbles causes the cells of the annulus to quickly expand, in turn causing the capsule to spring back towards the closed position - hurling the spores for some distance (Figure 5.1.) 38 Figure 5.1: Sporangium structure and dehiscence. A. mature sporangium; B, drying of cells of annulus causes them to shrink, tearing open the sporangium; C, sudden release of tension causes the sporangium to snap forward, throwing out spores. A defining characteristic of ferns is the presence of sporangia on either the abaxial leaf surface or along the leaf margin. Sporangia usually occur in distinct clusters known as sori (Fig.
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