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.
Because of their stationary life style, fungi reproduce by forming spores, cells that are specialised for dispersal to new locations by air, water or animals. Flagellated zoospores are produced by simple aquatic fungi. Most fungi are terrestrial, growing as hyphae and producing thickâwalled nonmotile spores. Asexual spores include sporangiospores and conidia, whereas sexual spores include zygospores, ascospores and basidiospores. Ascospores and basidiospores often are forcibly ejected into the air.