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.
Homospory characterizes plants that produce spores that are all the same size (as in bryophytes and most seedless vascular plants). Homospory can result in bisexual gametophytes (as in ferns, pg 611) or unisexual gametophytes (male or female) as in mosses - see pg 607 in the text. Heterospory characterizes plants that produce spores of different sizes: megaspores and microspores which give rise to separate female and male gametophytes, respectively. Contrast sperm delivery in seedless plants with sperm delivery in seed plants.