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Download 1 2006S Bio153 Lab 4: Seedless Vascular Plants July 11th / July
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1 2006S Bio153 Lab 4: Seedless Vascular Plants July 11th / July 13th After the appearance of land plants, the next big evolutionary step was the development of vascular tissue. Vascular tissue allows for the movement of water and nutrients around the plant body, meaning that plants could be larger. Vascular tissue also provides support, which means that plants could grow upright. This is important, because upright growth allows a plant to escape competition for space and light by overtopping its competitors. Plants that could keep part of their body in contact with moist soil and have access to sunlight would have a great advantage over those with a sprawling growth form (such as is seen in the bryophytes). To colonize land and compete with other plants, it was necessary for plants to be able to grow into large structures that could anchor themselves in the ground, capture light efficiently, and extract water and nutrients from the soil and transport it around the body. This has involved the evolution of leaves, stems, (collectively called the shoot), and roots. Thus, in the seedless vascular plants, we see the evolution of vascular tissue, true roots, and tracheids. The earliest known vascular plants appeared in the late Silurian period about 420 million years ago. These were simple plants with a dichotomously branched horizontal stem system bearing erect dichotomously branched stems. Fossil evidence shows that these early plants contained elongated cells organized into tissues. These cells had walls thickened with an extremely strong substance called lignin. These lignin rings would have allowed these plants to support erect stems and conduct water throughout the plant. By about 380 million years ago, fossil 2 plants show an elaboration of these elongated cells into more complex tissues known as tracheids. Tracheid cells die after maturing, leaving no cytoplasm and only cell walls. This increased the efficiency of water transport in these primitive plants. The aboveground and belowground parts of early vascular plants showed little differentiation; in later groups we see the evolution of highly differentiated plant parts. It appears that these structures have evolved independently in different plant lineages – current evidence suggests that roots evolved at least twice and leaves perhaps six times. Leaves: Simple vascular plants such as Psilotum do not have true leaves, but instead possess small projections called prophylls. Prophylls differ from true leaves in that they lack vascular tissue. The two types of true leaves found vascular plants – microphylls and megaphylls – are thought to have independent evolutionary origins. Microphylls are found in Lycophyta and contain a single unbranched strand of vascular tissue. It is hypothesized that microphylls arose from sporangia (spore-bearing structures). Most other vascular plants have leaves with an elaborately branched vascular system. These leaves are known as megaphylls. The currently accepted hypothesis is that megaphylls evolved from branched stems. Originally, most plants branched dichotomously (the stem apex divided to produce two equal branches). In some Devonian plants the two branches became unequal, so only one continued the growth of the main axis of the plant, while the other remained short and spread horizontally. This photosynthetic tissue eventually joined the parts of the short branch. Primitive megaphylls often show dichotomous branching of the vascular tissue. Reproduction: In early vascular plants we see the shift from the dominance of the gametophyte generation to that of the sporophyte generation. Rather than being a temporary structure permanently dependent on the gametophyte, the 3 sporophyte eventually emerges as a separate, independent entity. However, the early vascular plants are still dependent on water to complete their life cycle – sperm must swim through water to reach the egg and produce the diploid embryo. Most early vascular plants are homosporous. In homosporous plants, the sporophyte produces a single type of spore that gives rise to a bisexual gametophyte. This structure bears both the female (archegonia) and male (antheridia) sex organs, which produce female (egg) and male (sperm) gametes. A later evolutionary trend is the emergence of heterosporous plants. In heterosporous plants, 2 types of sporangia (spore-bearing structures) give rise to 2 types of spore - microspores that make male gametophytes, and megaspores, that make female gametophytes. 4 Early vascular plants: 1. Psilophyta (whisk ferns) The living genus Psilotum is remarkably similar to early fossil land plants although there is much debate over whether there is a direct relationship. Examine a specimen of Psilotum and note that it consists only of a horizontal underground stem (called a rhizome) without roots and an erect over-ground stem. It bears small projections on the over-ground stem resembling small leaves. However there is no vascular tissue in these projections so they are named prophylls to distinguish them from the leaves of other vascular plants. Examine a cross-section of the stem of Psilotum. Find the xylem tissue in the centre (stained red) and locate the tip of the stars. The small blue cells between these tips are the phloem tissue. Xylem transports water from the roots, while phloem transports the products of photosynthesis form the leaves. 2. Lycophyta (club mosses) Lycophyta is a division of vascular plants that originated early in the Devonian period, about 390 m. y. ago. Plants very similar to modern Lycopodium occur in the late Devonian period about 350 m. y. ago. Many fossil lycopods were enormous plants up to 40m tall, but living lycopods are all small. This group of plants shows two important advances; they have leaves with a strand of vascular tissue and they produce rather small relatively simple roots. Roots differ from rhizoids in that they contain vascular tissue. In the Lycophyta the leaves are spirally arranged and branching is dichotomous. The leaves are usually long and narrow and do not have branching vascular tissue. Simple leaves of this type are known as microphylls. Lycopodium is homosporous, meaning that meiosis in the sporangium forms spores that give rise to bisexual gametophytes bearing both archegonia and antheridia. The bi-flagellated sperm swim to the egg, and early development 5 of the embryo occurs in the archegonium. The sporophyte in club mosses grows a root and becomes an entity separate from the gametophyte. Another member of the Lycophyta is Sellaginella. is the only genus in this group, but includes 700 known species. Most live in moist places, but a few are desert plants that lie dormant during the driest parts of the year (such as the “resurrection plant”). Selaginella is heterosporous; its megaspores and microspores germinate to form separate male and female gametophytes. Heterosporous reproduction in Selaginella represents an evolutionary step forward. Selaginella needs water for reproduction, as its sperm must swim to the egg in the archegonium. The young embryo is nourished by the megagametophyte, but eventually emerges from the gametophyte and becomes independent. The sporangium is able to produce microor megaspores depending on its nutritional status. The slides of Selaginella cones that you will examine will show the random distribution of micro- and megaspores within a cone. Examine a longitudinal section of a Selaginella cone (known as a strobilus). Note the presence of microspores and megaspores, the micro- and megasporangia, and the sporophylls (the modified leaves that bear the sporangia). 3. Sphenophyta (horsetails) Sphenophyta is another division of plants originating in the Devonian period. Basically similar to the Lycophyta in organization, but in the Sphenophyta the stem is organized into nodes and internodes. In the Sphenophyta the leaves are arranged in rings (whorls) at each node and branches arise at the same 6 point. The branches arise alternately with the leaves, an arrangement unique to this division. Observe a stem of the horsetail, Equisetum and note the arrangement of the tiny leaves and the origin of the branches at the nodes. 4. Pteridophyta (ferns) Ferns are an enormously successful group with over 12,000 species. They are the largest and most diverse group of seedless vascular plants. Many of them are epiphytes, growing on other plants (particularly in the tropics). The have large well developed megaphylls called fronds. Most ferns are homosporous, bearing spores in sporangia that are often clustered in sori on the edges or undersides of leaves. The gametophyte resembles a liverwort, while the sporophyte is usually structurally complex and large. In ferns, the sporophyte depends upon the gametophyte during the early phases of its development, and in some species the gametophyte is capable of supporting several young sporophytes. Reproduction again requires water, because the motile sperm must swim to the egg. Examine a fern and note the sori, which contain the sporangia and the spores. Make sure that you see gametophytes. You may see young sporophytes growing on them. 7 Fig. 4. Life cycle of a homosporous fern.