Life Cycle of Seed Plants
Seed plants are heterosporous- they have 2 different spore sizes: megaspores and microspores.
The generalized life cycle of plants has been modified (below) to illustrate plants which have separate male and female gametophytes (megagametophyte and microgametophyte) produced by different sized spores (megaspores and microspores).
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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 elongate structure (pollen tube) that grows to 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....