The Life Cycle of a Gymnosperm

How does fertilization take place in gymnosperms in the absence of water?

The word gymnosperm actually means “naked seed.” The name reflects the fact that gymnosperms produce seeds that are exposed on the scales within cones. Gymnosperms alive today include relatively rare plants such as cycads and ginkgoes and the much more abundant plants known as conifers, which include pines and firs. DOL•24–DOL•25

Pollen Cones and Seed Cones Reproduction in conifers takes place in cones, which are produced by the mature sporophyte plant. Conifers produce two types of cones: pollen cones and seed cones. Pollen cones, also called male cones, produce the pollen grains. As tiny as it is, a pollen grain makes up the entire male gametophyte stage of the gymnosperm life cycle. One of the haploid nuclei in the pollen grain will divide later to produce two sperm nuclei.

The more familiar seed cones, or female cones, produce female gametophytes. Seed cones are generally much larger than pollen cones. Near the base of each scale of the seed cones are two ovules (AHV yoolz), the structures in which the female gametophytes develop. Within the ovules, meiosis produces haploid cells that grow and divide to produce female gametophytes. These gametophytes may contain hundreds or thousands of cells. When mature, each gametophyte contains a few large egg cells, each ready for fertilization by sperm nuclei.

FIGURE 22–16 Pollen Cone This pollen cone on a pine tree is shedding pollen, which will be carried by wind to seed cones.

Pollination and Fertilization The conifer life cycle typically takes two years to complete. The life cycle of a pine is shown in Figure 22–17. The cycle begins in the spring as male cones release enormous numbers of pollen grains that are carried away by the wind. Some of these pollen grains reach female cones. There, pollen grains are caught in a sticky secretion on the scales of the female cone and pulled inside toward the ovule. In gymnosperms, the direct transfer of pollen to the female cone allows fertilization to take place without the need for gametes to swim through standing water.

Development Inside Seeds If a pollen grain lands near an ovule, the grain splits open and begins to grow a structure called a pollen tube, which contains two haploid sperm nuclei. Once the pollen tube reaches the newly developed female gametophyte, one sperm nucleus disintegrates; the other fertilizes the egg contained within the female gametophyte. Fertilization produces a diploid zygote, which grows into an embryo—the new sporophyte plant. The embryo is then encased to form a seed. The seed is ready to be scattered by the wind and grow into a new plant.

In Your Notebook Make a flowchart that records the events leading up to fertilization in a gymnosperm.

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