2. Lesson Overview
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.
4. Lesson Overview
Seed Plants
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.
5. Lesson Overview
Seed Plants
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.
6. Lesson Overview
Seed Plants
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.
7. Lesson Overview
Seed Plants
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.
8. Lesson Overview
Seed Plants
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.
9. Lesson Overview
Seed Plants
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.
10. Lesson Overview
Seed Plants
The Life Cycle of a Gymnosperm
How does fertilization take place in gymnosperms in the absence of water?
11. Lesson Overview
Seed Plants
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.
12. Lesson Overview
Seed Plants
The Life Cycle of a Gymnosperm
The word gymnosperm means “naked seed.” Gymnosperms produce seeds that
are exposed on the scales within cones.
13. Lesson Overview
Seed Plants
Pollen Seeds and Seed Cones
Reproduction in conifers takes place in cones, which are produced by the mature
sporophyte plant.
14. Lesson Overview
Seed Plants
Pollen Seeds and Seed Cones
Pollen cones produce pollen grains, which make up the entire male gametophyte
stage.
15. Lesson Overview
Seed Plants
Pollen Seeds and Seed Cones
One of the haploid nuclei in the pollen grain will divide later to produce two
sperm nuclei.
16. Lesson Overview
Seed Plants
Pollen Seeds and Seed Cones
Seed cones produce female gametophytes and are generally much larger than
pollen cones.
17. Lesson Overview
Seed Plants
Pollen Seeds and Seed Cones
Near the base of each scale are two ovules, where the female gametophytes
develop.
18. Lesson Overview
Seed Plants
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.
19. Lesson Overview
Seed Plants
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.
20. Lesson Overview
Seed Plants
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.
