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.
Fern leaves grow from a rhizome. Fern leaves are fan-shaped and also play an important role in reproduction. The production of spores occurs in structures called sporangia. These structures are found on the surface of the fronds, usually on the underside of the leaf. When the sporangia or spore casings dry out, the dispersal of spores occurs into the atmosphere. Certain fern species date back nearly 400 million years. There are about 12,000 fern species.