Laboratory Notes for BIO 1003, 1016, & 3001
© 30 August 1999, John H. Wahlert, Mary Jean Holland, & Donald McClelland
|© D. McClelland|
Lilium anthers x.s., pollen tetrads. Haploid microspores are produced inside the pollen sacs by meiosis, and they usually are in tetrads (groups of four). Each microspore divides by mitosis to form a male gametophyte or pollen grain. Pollen consists of a generative cell which floats in the cytoplasm of the tube cell; each of these cells has its own nucleus. Pollen can be transported by wind (grasses usually), insects (most of the colorful flowers), and by night flying moths and small mammals (bats and mice). Flowers usually contain a nutritious reward of extra pollen or sweet nectar for the pollinator. Pollinators brush against the ripe anthers and get covered with pollen.
Lilium: stigma and pollen tubes. Pollen brushes off its carrier onto the stigma of a pistil. The tube cell grows through the style to the ovary; the generative cell flows into the tube and divides by mitosis to form two sperm. The male gametophyte has matured.
Stigma, pollen, and germinated pollen grain
Lilium ovary: megasporocyte. The ovary contains several ovules. Initially each contains a big, diploid megaspore mother cell in a chamber called the megasporangium. Tissue on either side, called integuments, grow and surround the megasporangium; they do not meet completely, and a tiny hole called the micropyle remains as a passage into the ovule for the pollen tube. The integuments will eventually become the seed coat.
Lilium: first four-nucleate. The diploid megaspore mother cell undergoes meiosis without cytokinesis, so it then contains four nuclei. These nuclei undergo movement, combination, and mitosis to form the four-nucleate female gametophyte. Fertilization by the two sperm cells from the pollen is double. The egg is at the micropylar end of the ovule and is fertilized to become a diploid zygote. The other sperm enters the central cell and the resulting polyploid tissue is called endosperm. The zygote divides by mitosis to form an embryo sporophyte. The endosperm is a nutrient tissue within the seed (it is most of what you eat in corn).
Seeds and Fruits
Individual plants, excepting some algae, do not have the ability to move from place to place as many Protista and animals do. Instead they rely on spores and seeds for dispersal of their kind over the earth. Haploid spores (n), which are produced by meiosis, are tiny and easily carried by wind and water; seeds, which contain a diploid sporophyte embryo (2n) and its initial nutrient supply, are comparatively large; both spores and seeds are resistant to extremes of the plant's environment, e.g., cold and dryness.
Recall that in bryophytes and pterophytes the sporophyte and gametophyte are distinct phases in the life cycle; sporophytes are diploid (2n) and gametophytes haploid (n); sporophytes produce haploid spores by meiosis; gametophytes produce haploid gametes by mitosis and cellular differentiation.
Conifers and anthophytes, the seed bearing plants, have ovules; each ovule is a megasporangium surrounded by protective layers known as integuments. These complex structures are produced by the sporophyte plants. What has happened to the gametophytes? Pollen, which is transported mostly by wind (conifers and grasses) and insects (plants with obvious flowers) is the male gametophyte. The female gametophyte lies hidden in the megastrobilus of a conifer or ovary of an anthophyte. The production of seeds requires a parental investment of nutrients; it is only the life stage with vascular tissues, the sporophyte, that is designed to fill this demand. But not all plants with vascular tissue produce seeds: recall that ferns and their relatives produce spores and gametes.
A pine seed consists of a tough outer integument (2n) and nucellus or megasporangium (2n) that are part of the ovule of the parent sporophyte, haploid gametophyte tissue (n) and the embryo sporophyte (2n).
The seed of a flowering plant is similar to that of a conifer in that it has a tough, seed coat integument (2n) that is part of the parent's ovule wall. Within this is a special, triploid (3n) nutrient tissue called endosperm that was formed by a separate fertilization from that of the egg. Monocots have a large endosperm in the seeds, and the embryo sporophyte has one cotyledon; in dicots the nutrients of the endosperm have been transferred into the two cotyledons of the embryo sporophyte.
The seeds of flowering plants are not exposed as in conifers but are enclosed within another tissue, the fruit coat (2n) from the parent plant. In grain and corn the fruit coat is very thin and fused to the seed coat, but in many dicots the fruit is thick and often nutritious:
Fruit is delicious and prized by many animals as food. The seeds resist attack by acid and enzymes in animal digestive systems. The seeds of fruit that is eaten get free transportation (dispersal) and are deposited in a rich packet of fertilizer. Some kinds of seeds are unable to germinate until the seed coat has been eroded by passage through an animal's digestive system.
Slides and specimens:
Zea mays: kernel l.s. (slide). The bulk of this monocot seed consists of endosperm, a triploid (3n) nutrient tissue. The single cotyledon is between it and the embryo sporophyte near the outside of the seed. The sporophyte consists of plumule (embryonic leaves), hypocotyl (embryonic stem), and radicle (embryonic root).
Examine a dicot seed such as a peanut. The embryo sporophyte consists of plumule, hypocotyl, and radicle, as in corn, but the bulk of the seed is the pair of cotyledons, which are also diploid sporophyte tissue. The seeds of dicots break in half between the cotyledons. Recall that the dark seed coat is the old integument of the ovule. In dicots the nutrients of the endosperm are transferred into the cotyledons, and the endosperm is not seen.
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