Seed Plants

Seed plants are heterosporous- they have 2 different spore sizes: megaspores and microspores.
The generalized life cycle of plants has been modified to illustrate plants which have separate male and female gametophytes produced by different sized spores.

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 pollen grain is transferred to the vicinity of the pollen grain 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.