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Seed Plant Synapomorphies Introduction to Non-Flowering Seed Plants (Gymnosperms) a Seed Is a Highly Modified Megasporangium, So Seed Plants Are Heterosporous

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Seed Plant Synapomorphies Introduction to Non-Flowering Seed Plants (Gymnosperms) a Seed Is a Highly Modified Megasporangium, So Seed Plants Are Heterosporous LAB 06: Seed Plant Synapomorphies Introduction to non-flowering seed plants (Gymnosperms) A seed is a highly modified megasporangium, so seed plants are heterosporous. We will review important differences between heterosporous non-seed plants and seed plants. There are five lineages of extant seed plants, four of which are gymnosperms (seeds not enclosed in a fruit) and one lineage of angiosperms (seeds in a fruit). We will also review some potentially confusing differences in what the terms dioecious and monoecious refer to when applied to homosporous versus heterosporous plants. MICROSPORANGIA, MICROSPORES, MALE GAMETOPHYTES Microsporangia. There is no fundamental difference in the function of microsporangia in heterosporous non-seed and seed plants. The structure of the endosporic male gametophytes and the way they function, however, is drastically different in the two groups of plants. Microspores and Male Gametophytes: In heterosporous, non-seed plants an entire antheridium develops within the microspore wall (review the structure of the Selaginella male gametophyte). It has jacket cells surrounding a substantial number of spermatocytes. The microspore wall eventually cracks open and many sperm are released and swim to the female gametophyte (in dew, rainwater, pond water). Because seed plants are heterosporous, the gametophytes are endosporic just as they are in heterosporous non-seed plants. But the male gametophyte has undergone substantial reduction so that there is no longer any trace of an antheridium. Male gametophytes of seed plants consist of about 2-6 cells. The microspore, with its tiny internal gametophyte, is carried by wind or animals to somatic tissues in the vicinity of the female gametophyte. It germinates there by producing a tubular outgrowth. Two sperm are ultimately released from the male gametophyte and swim or are conveyed to the female gametophyte. In seed plants the movement of sperm to the female gametophyte is independent of water in the environment. The tiny male gametophytes of seed plants are given a new name because they function in a new way. We call them pollen. The transport and arrival of pollen in the vicinity of the female gametophyte is called pollination. A male gametophyte (pollen grain) usually has one to several vegetative cells. It has a single generative cell that produces two sperm. The male gametophyte also has a single tube cell that directs the development of the pollen tube. In some pollen grains, a distal weak spot on the exine (outer wall) of the pollen grain is the site of pollen tube emergence. The proximal side of a pollen grain is the side that was in contact with other microspores at the completion of meiosis (tetrad stage). The distal weak spot where the pollen 66 tube emerges is usually seen as an aperture (opening) in the exine. In many angiosperms, there are multiple apertures and a single pollen grain may develop more than one pollen tube. There are two ways in which pollen tubes function. Haustorial pollen tubes have an exclusively nutritive function. They digest the surrounding tissue for a period of weeks or months. The food obtained is absorbed and used to support continued growth of the pollen tube and maintenance of the male gametophyte. Eventually two flagellated, swimming sperm are produced by the generative cell. They are released from the proximal end of the pollen grain - not from the pollen tube itself. They swim to the egg and fertilization is accomplished. Haustorial pollen tubes are found in cycads and Ginkgo. Haustorial pollen tubes represent the ancestral condition in seed plants, indicating that the pollen tube may not have originally evolved as a sperm delivery system. The haustorial process allows pollen grains to be light and easily transported because food storage in the grains is unnecessary; sperm transport likely evolved later as a secondary function. All other seed plants produce siphonogamous pollen tubes. Siphonogamous pollen tubes grow at varying rates and do varying amounts of digestion of the surrounding tissue, but ultimately also serve the function of conveying the sperm to the egg. The sperm are not flagellated and are therefore non-motile. A comparison of male gametophyte structure and function in heterosporous non- seed plants and seed plants. 67 MEGASPORANGIA, MEGASPORES, FEMALE GAMETOPHYTES Megasporangia and megaspores: There are many important differences between heterosporous non-seed and seed plants in the structure and function of megasporangia and megaspores. They are listed below: (1) As a megasporangium begins to develop, other tissues at its base begin to grow upward and eventually entirely surround it except for an opening, the micropyle, at its distal end. There may be a single such coat or integument around the megasporangium or two. Gymnosperms usually have one integument. In angiosperms the ancestral number of integuments is two, but in derived angiosperm families it has been reduced to one. An integumented megasporangium with its internal female gametophyte is known as an ovule. (2) In heterosporous, non-seed plants the number of meiocytes (megasporocytes) that develop within a megasporangium is variable. Thus there may be as few as four megaspores produced per sporangium or there may be many. In seed plants, a megasporangium rarely develops more than one megasporocyte, thus the potential number of megaspores is four. Meiosis usually occurs so that a linear tetrad is formed. The most common developmental pattern is for the three distal meiotic products to degenerate. The surviving single megaspore produces the female gametophyte. Later we will discuss some variations on this theme where bisporic and tetrasporic female gametophytes occur in some taxa. (3) The single megasporocyte is embedded in a solid tissue called the nucellus. The dominant hypothesis with respect to the evolutionary history of the nucellus is that it represents sterile tissue of the megasporangium. (4) The megasporangium of seed plants is indehiscent. In other words, the single megaspore with its internal female gametophyte is not released. This should be contrasted with the behavior of heterosporous non-seed plants in which the megasporangia dehisce and the megaspores, which are not fused to surrounding tissue, are released to the outside world where fertilization of their internal female gametophytes takes place. Because the megasporangia of seed plants are indehiscent, they evolved mechanisms that allow sperm access to the female gametophyte. (5) After fertilization, the integumented megasporangium (ovule) ripens into a seed. The megasporangium stalk has an abscission zone that ultimately breaks down and frees the seed to be dispersed. In non-seed plants, the only dispersal stage is the spore (in addition to vegetative propagules). Female gametophyte. The endosporic female gametophytes are very different in gymnosperms and in angiosperms. In most gymnosperms, the single megaspore grows into a gametophyte that consists of hundreds or thousands of cells and in most cases two or more archegonia differentiate at the micropylar end. In angiosperms, the gametophyte is reduced to a few cells and lacks archegonia. FERTILIZATION, EMBRYO AND SEED DEVELOPMENT In non-seed plants the basic unit of dispersal is the spore (megaspore in the case of heterosporous, non-seed plants). Megaspores do not have a very great range of sizes, at least in comparison to seeds, and all megaspores function more-or-less the same way. Embryos of heterosporous non-seed plants must grow into a mature sporophyte wherever the megaspore containing them happens to fall. Embryos of non-seed plants may grow continuously without undergoing a period of dormancy or may remain quiescent during a period when environmental conditions are unfavorable for growth (winter, dry season, etc.). But prolonged survival is impossible. This should be contrasted with seeds that often contain considerable stored food and may survive for years in a dormant condition. Maturation of a seed involves not only the development of the embryo, but also numerous changes in the integuments as they ripen into a tough, protective seed coat. 68 MONOECY AND DIOECY. Cruden & Lloyd (1995) have proposed a common terminology to describe sexual phenotypes and breeding systems in all land plants. The terms “monoecy” and “dioecy” necessarily refer to different things in heterosporous and homosporous species. A monoecious species bears both male and female sex organs on the same individual plant (which can be described as bisexual or hermaphroditic), whereas in a dioecious species there are separate male and female plants (individual plants are unisexual). The potential for confusion lies in the fact that in heterosporous plants the "individual" we are referring to is a sporophyte, whereas in homosporous plants the "individual" is a gametophyte. As an example of homosporous plants, consider moss. Mosses and other homosporous plants produce only one kind of sporangium and therefore one kind of sporophyte. The terms monoecious and dioecious in homosporous plants can only refer to gametophytes. A monoecious moss species has archegonia and antheridia on the same gametophyte. A dioecious moss species produces separate male gametophytes (antheridia-bearing) and female gametophytes (archegonia-bearing). In many bryophyte species it has been shown that half of the spores from a particular sporangium produce female gametophytes and half produce male gametophytes. This fact implies that sex determination is a consequence
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