Lesson Overview 22.3 Seed Plants

THINK ABOUT IT Seeds are so common that they are easy to overlook. What are seeds?   Every seed contains a living plant ready to sprout as soon as it encounters the proper conditions for growth. The production of seeds has been one key to the ability of plants to colonize even the driest environments on land.

The Importance of Seeds What adaptations allow seed plants to reproduce without open water?  

The Importance of Seeds What adaptations allow seed plants to reproduce without open water?   Adaptations that allow seed plants to reproduce without open water include a reproductive process that takes place in cones or flowers, the transfer of sperm by pollination, and the protection of embryos in seeds.

The Importance of Seeds A seed is a plant embryo and a food supply, encased in a protective covering. The living plant within a seed is diploid and represents the early developmental stage of the sporophyte phase of the plant life cycle.  

The First Seed Plants Today’s seed plants are all descended from common ancestors.   The fossil record indicates that ancestors of seed plants evolved new adaptations that enabled them to survive on dry land.

Cones and Flowers In seed plants, the male gametophytes and the female gametophytes grow and mature directly within the sporophyte. The gametophytes usually develop in reproductive structures known as cones or flowers.   Nearly all gymnosperms bear their seeds directly on the scales of cones. Flowering plants, or angiosperms, bear their seeds in flowers inside a layer of tissue that protects the seed.

Pollen In seed plants, the entire male gametophyte is contained in a tiny structure called a pollen grain.   Pollen grains are carried to the female reproductive structure by wind or animals such as insects. The transfer of pollen from the male reproductive structure to the female reproductive structure is called pollination.

Seeds After fertilization, the zygote contained within a seed grows into a tiny plant—the sporophyte embryo.   A tough seed coat surrounds and protects the embryo and keeps the contents of the seed from drying out. The embryo begins to grow when conditions are right. It does this by using nutrients from the stored food supply until it can carry out photosynthesis on its own.

The Life Cycle of a Gymnosperm How does fertilization take place in gymnosperms in the absence of water?

The Life Cycle of a Gymnosperm How does fertilization take place in gymnosperms in the absence of water?   In gymnosperms, the direct transfer of pollen to the female cone allows fertilization to take place without the need for open water.

The Life Cycle of a Gymnosperm The word gymnosperm means “naked seed.” Gymnosperms produce seeds that are exposed on the scales within cones.

Pollen Seeds and Seed Cones Reproduction in conifers takes place in cones, which are produced by the mature sporophyte plant.

Pollen Seeds and Seed Cones Pollen cones produce pollen grains, which make up the entire male gametophyte stage.

Pollen Seeds and Seed Cones One of the haploid nuclei in the pollen grain will divide later to produce two sperm nuclei.

Pollen Seeds and Seed Cones Seed cones produce female gametophytes and are generally much larger than pollen cones.

Pollen Seeds and Seed Cones   Near the base of each scale are two ovules, where the female gametophytes develop.

Pollen Seeds and Seed Cones Within the ovules, meiosis occurs producing haploid cells that grow and divide into female gametophytes. Each gametophyte contains a few large egg cells.

Pollination and Fertilization In the spring, pollen cones release pollen grains that are carried away by the wind, as shown in the figure.   Some pollen grains are caught in a sticky secretion on the scales of female cones and are pulled inside towards the ovule. This direct transfer of pollen to the female cone allows fertilization to take place without the need for open 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 female gametophyte, one sperm nucleus disintegrates; the other fertilizes the egg.

Development Inside Seeds Fertilization produces a diploid zygote, which grows into an embryo.

Development Inside Seeds A seed is an encased embryo that is ready to be scattered by the wind and grow into a plant.