20 Reading: Seed Plants
Gymnosperms
The four phyla of gymnosperms are cycads, ginkgo, gnetophytes, and conifers.
Gymnosperms have naked seeds. The seeds of angiosperms are contained within a fruit.
Gymnosperm Diversity
We will examine conifers in some detail during this lab class but will use photographs on the Internet to study the other three divisions. Click on the links below to view photographs of them.
Cycads
Cycads are cone-bearing palm-like plants found mainly in tropical and subtropical regions today. They were very numerous in the Mesozoic Era.
Ginkgo
There is only one species of Ginkgo left. It survived due to the Chinese planting them along roadsides. Click here for information and photographs.
Gnetophytes
Welwitschia and Ephedra (information and photographs)
Conifers
Conifers are the largest group of gymnosperms. They include evergreen trees such as pine, cedar, spruce, fir, and redwood trees. Examine the leaves of pine on display.
The leaves of conifers are needle-like and are adapted for dry conditions such as hot summers or freezing winters. Needles lose water slower than broad, flat leaves and therefore do not need to be shed during seasons when water is scarce, so most conifers are evergreen.
Reproduction in Pine
Life Cycle of Seed Plants
Seed plants are heterosporous—they have two different spore sizes: megaspores and microspores.
The generalized life cycle of plants has been modified (below) to illustrate plants that have separate male and female gametophytes (megagametophyte and microgametophyte) produced by different-sized spores (megaspores and microspores).
The evolutionary trend from nonvascular plants to seedless vascular plants to seed plants has been a reduction in the size of the gametophyte. In seed plants, the gametophyte is usually microscopic and is retained within the tissues of the sporophyte.
The megasporangium is surrounded by layers of sporophyte tissue called the integument. The integument and structures within (megasporangium, megaspore) are the ovule.
Microspores germinate within the sporophyte tissue and become pollen grains. The microgametophyte is contained within the tough, protective coat of the pollen grain.
The entire microgametophyte (pollen grain) is transferred to the vicinity of the megagametophyte by a process of pollination. Wind or animals usually accomplish this transfer.
When pollen reaches the female gametophyte, it produces an elongated structure (pollen tube) that grows into the egg cell. Sperm are transferred directly through this tube to the egg. The advantage of this process is that sperm do not have to swim long distances as they do in seedless plants.
Seeds
Seeds contain the sporophyte embryo, food for the embryo, and a protective coat.
The embryo within the seed is dormant; it can survive for long periods without additional food or water. When conditions become favorable, the embryo resumes growth as the seed germinates.
- Draw the life cycle of pine and include the following terms