hx1nech25.pdf

Section 25-2ObjectivesSexual Reproductionin Seed PlantsSeed Plants Have Tiny GametophytesDistinguish the male andfemale gametophytes ofseed plants. (p. 552)Describe the function ofeach part of a seed. (p. 553)Summarize the life cycleof a conifer. (p. 555)Relate the parts of a flowerto their functions. (p. 556)Summarize the life cycleof an angiosperm. (p. 558)Key Termspollen grainovulepollinationpollen tubeseed coatcotyledonsepalpetalstamenantherpistilovarydouble fertilizationReproduction in seed plants (gymnosperms and angiosperms) isquite different from reproduction in seedless plants. For one thing,you need a microscope to see the gametophytes of seed plants, asFigure 25-5 shows. Also, seed plants do not release their spores. Thespores remain within the tissue of a sporophyte and develop intotwo kinds of gametophytes—male gametophytes, which producesperm, and female gametophytes, which produce eggs. The tinygametophytes of seed plants consist of only a few cells. A malegametophyte of a seed plant develops into a pollen grain, whichhas a thick protective wall. A female gametophyte of a seed plantdevelops inside an ovule (AHV yool), which is a multicellular structurethat is part of the sporophyte. Following fertilization, the ovuleand its contents develop into a seed.Because the gametophytes of seed plants are very small, seed plantsare able to reproduce sexually without water for sperm to swimthrough. Wind and animals transport pollen grains to the structuresthat contain ovules. The transfer of pollen grains from the malereproductive structures of a plant to the female reproductive structuresof a plant is called pollination. When a pollen grain reaches acompatible female reproductive structure, a tube emerges from thepollen grain. This tube, called a pollen tube, grows from a pollengrain to an ovule and enables a sperm to pass directly to an egg.Figure 25-5Seed plant gametophytesThe tiny gametophytes of seed plants develop within specialized structures thatform in the reproductive parts of a flower.Pollen grainsPollen grains are transferred to a femalestructure during pollination.Male reproductivestructureOvulesA pollen grain consistsof only two or three cells.Female reproductivestructureThe female gametophytewithin an ovule consists ofonly seven cells.552 Chapter 25Copyright © by Holt, Rinehart and Winston. All rights reserved.