Bio Lab Wksht & HW Questions 51

Bio Lab Wksht & HW Questions 51 - Selaginella...

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46 Some ferns and some members of the Lycophyta have evolved heterospory , meaning that they produce two sizes of spores ( microspores and megaspores ) in two kinds of sporangia ( microsporangia and megasporangia , respectively). Heterosporous plants are thought to have evolved from homosporous plants. They demonstrate life cycles intermediate between homosporous plants and seed plants, which we will examine in detail in the next lab. In this part of the exercise, we will examine the features of a heterosporous plant life cycle using Selaginella ( Phylum Lycophyta ). Examine the living Selaginella (ground pine) plant on display. When the plants are ready to reproduce sexually, they produce small cones called strobili on the tips of their branches. The strobili contain the microsporangia and the megasporangia. You may need to use a hand lens to view these strobili as they are not easily observed. Are any visible? Consult prepared slides of
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Unformatted text preview: Selaginella strobili to gain further insight into these structures. Each strobilus, or cone, is made up of many fertile leaves, each of which bears a sporangium on its upper surface. Note that some of these sporangia contain a few large spores, while others contain many small spores. Sketch a strobilus and label the micro- and megasporangia and micro- and megaspores. Are the spores haploid or diploid? Are the microspores and megaspores produced by mitosis or by meiosis? Heterosporous plants differ from homosporous plants in that the spores, although released from the sporangium, do not germinate into free-living gametophytes. Instead, gametophytes develop inside each spore. Would you predict that these gametophytes are bisexual or unisexual? What evidence do you have which would help you answer this question? Water is still required to complete the life cycle in these plants. Why?...
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