The biochemical and physical duet of the mother and the fetus in the formation of the placenta is one of the most carefully orchestrated phenomena in fetal development. It represents the cooperation of two distinct individuals to form a single structure that protects one and enables the genes of the other to live on.
This cycle is known as alternation of generations. The spores of seed plants, however, are produced internally and the megaspores, formed within the ovules and the microspores are involved in the formation of more complex structures that form the dispersal units, the seeds and pollen grains.
Spores and Sporangium. Ferns differ from flowering plants and trees because they do not produce seeds. Instead, ferns reproduce using spores. The sporangium is the structure responsible for producing and releasing spores. Sporangium form in many different ways, sometimes appearing as brown fruitdots on the undersides of leaves. Others grow as slender stalks up from the plant. When the sporangia burst, the wind carries spores to new ground, where they form reproductive structures that, if fertilized, will grow into a new fern.