466 CHAPTER 12 • PLANT AND FUNGI DIVERSIFICATION

Plants Are a Branch of Eukarya Mosses, Ferns, and Their Relatives The Advent of the Seed Flowering Plants

The process of fertilization—the fusion of two gametes to form a zygote—begins when a pollen grain lands on a stigma of a fl ower, but that is far from the end. In angiosperms, the process is called double fertilization because there are two separate fusions of male nuclei (each carrying a complete set of the organism’s genetic material) from the pollen grain with female nuclei in an ovule. As we will see, double fertilization is a more effi cient system than fertilization in gymnosperms because whenever an embryo is produced at fertilization (and only then), so, too, is a more substantial, ready-made food source.

When we left off in the previous section, a pollen grain—which will deliver the haploid male gamete—had just arrived at the stigma, in the female reproductive structure. The pollen grain forms a pollen tube that extends downward through the stigma and style to the ovary, and ultimately enters an ovule. The pollen grain forms two haploid sperm by mitosis, both of which move down the pollen tube (FIGURE 12-22).

While the pollen tube is growing, a haploid spore in the ovule divides mitotically to form a total of seven haploid cells, but only four of these participate in fertilization and formation of a seed. The remaining three are degenerated. One of the fi ve cells is the female gamete (the egg), and this remains near the place where the pollen tube will be guided into the ovule by two of the other haploid cells, which aid in fertilization. The remaining haploid cells of the four participating in fertilization move into the middle of the ovule. So, when the pollen tube reaches the ovule, one of the two sperm fuses with the egg to form a zygote. The other sperm fuses with the two nuclei in the middle of the ovule to form the endosperm, which has three sets of chromosomes (i.e., is triploid). The process is called double fertilization because two sperm enter the ovule and combine with haploid female cells in two separate events, forming (1) a zygote (with two sets of chromosomes) and (2) an endosperm (with three sets of chromosomes).

The fi nal steps in producing a seed occur as the diploid zygote cell undergoes multiple mitotic divisions to form an embryo, while the triploid cells multiply mitotically to produce the endosperm that will provide nutritional support for the seedling through its initial growth stages. The outer layers of the ovule form the seed coat that will protect the seed until it sprouts. The enclosure of the seed within the ovule is a distinction between angiosperms and gymnosperms. The seeds of a gymnosperm are unenclosed and are sometimes referred to as “naked.”

12 • 10 -------------------------------------------------Angiosperms improve seeds with double fertilization.

ANGIOSPERM LIFE CYCLE

1 Male anthers release haploid pollen grains that are delivered to the stigma of another flower.

2 A pollen grain produces a tube that extends through the stigma to the ovary. Within the ovule of the ovary, a spore divides into seven haploid cells. One cell becomes the egg, another cell—with two nuclei—will form the endosperm following fertilization by a sperm cell.

3 Two sperm are released by the pollen grain. In a process called double fertilization, one sperm fuses with the egg to form a zygote, while the other sperm fuses with the two nuclei of the endosperm-forming cell.

4 The zygote and the endosperm continue to develop within the ovule, forming a seed that will eventually be released and grow into a mature plant.

Seed

Pollen

Diploid flowerOvule

Ovary

MEIOSIS

FERTILIZATION

HAPLOID STAGE

DIPLOID STAGE

12

3

4

FIGURE 12-22 Double fertilization fortifi es the seeds of angiosperms.

467

Plant and Animal Relationships Fungi

What is the advantage of such a complex method of fertilization? Gymnosperms do not do this—a single sperm fuses with an egg

to form a zygote in an ovule that contains hundreds of other female cells that are used as a source of nutrients for the resulting embryo.

There are two important advantages to the angiosperm method.

1. Double fertilization initiates formation of endosperm only when an egg is fertilized, whereas gymnosperms use hundreds of haploid female cells to make the endosperm before fertilization occurs. Waiting to be sure that an egg is fertilized is a good strategy, because making endosperm is a large energy investment for a plant. Gymnosperms invest that energy up front, and nutrients in ovules that are not fertilized are wasted because those “seeds” do not contain an embryo. In contrast, angiosperms do not waste energy forming endosperm in ovules that will not contain embryos.

2. Angiosperms can produce smaller gametes than gymnosperms, because the large energetic reserves will be produced only after fertilization occurs. The small size of the male and female gametes of angiosperms ensures that seeds are produced quickly. FIGURE 12-23 shows that as different reproductive strategies have evolved in plants, their gametes have become progressively smaller. Rapid production of seeds allows angiosperms to grow as annual plants (i.e., plants that complete their life cycle from sprouting to seed production in one growing season), which is something gymnosperms cannot do.

Outbreeding, the combination of haploid cells from two different individuals, produces offspring with greater genetic diversity (i.e., carrying a greater diversity of alleles) than offspring that result from inbreeding, the combination of a male and female gamete from the same individual. Plants have a variety of ways to ensure that only sperm from another individual will fertilize the female gamete. Plants that have male and female reproductive structures in different fl owers or even on different individuals increase the chance that a pollen grain landing on a stigma comes from a different plant. But many fl owers with both male and female parts also have mechanisms to prevent self-fertilization. For example, the anthers may mature before the stigma, so the stigma is not ready to receive pollen when the anthers are active. And when the stigma becomes functional, the anthers are no longer producing pollen. Other plants use a molecular recognition system to prevent inbreeding: proteins on the surface of the stigma will not allow pollen from the same individual to form a pollen tube.

Angiosperms undergo a process of double fertilization, which ensures that a plant does not invest energy in forming endosperm for an ovule that has not been fertilized. Angiosperms have also developed methods to reduce the occurrence of self-fertilization, thereby ensuring greater genetic variation among offspring than through inbreeding.

TAKE-HOME MESSAGE 12 • 10

What advantage does double fertilization give to angiosperms?

Q

FIGURE 12-23 Overview of the haploid and diploid stages of plant life cycles.

Q Do fl owers with both male and female structures fertilize themselves?

HAPLOID AND DIPLOID LIFE STAGES

NON-VASCULAR PLANTSThe majority of the life cycle is spent in the haploid stage.

VASCULAR SEEDLESS PLANTSThe haploid and diploid stages are both multicellular and physically independent from one another.

GYMNOSPERMSThe evolution of seeds in gymnosperms almost completely eliminates the prominent haploid stage seen in mosses and ferns.

ANGIOSPERMSHaploid gametes are further reduced in size, enabling more rapid seed production.

As plants have developed different repro-ductive strategies, they have progressed from having a prominent haploid stage of life to simply having haploid gametes.

Haploid life stageDiploid life stage