Pollen and seeds are produced only by seed plants, taking on some of the roles previously played by sperm and spores. To understand these new structures and how they relate to the life cycles we have already discussed, we consider four major steps that distinguish the life cycle of seed plants.

The first step is the formation of two types of spore, each produced in separate sporangia. One type develops into a male gametophyte and the other into a female gametophyte. Male gametophytes produce only male gametes, and female gametophytes produce only female gametes. Although this condition is not unique to seed plants (it also occurs in a small number of spore-dispersing vascular plants), it had to have evolved first or the rest of the modifications that result in the life cycle exhibited by seed plants could not have evolved.

The second step is unique to seed plants. Seed plant spores are not dispersed. Instead, gametophytes develop within their sporangium and thus remain attached to the sporophyte. The significance of this is most apparent in the development of the female gametophyte. By developing in place, the female gametophyte can draw resources from the sporophyte to produce female gametes (eggs) and to store energy and raw materials that the embryo can make use of following dispersal. Within each sporangium, only a single haploid spore develops into a female gametophyte, and this gametophyte continues growing until it fills the sporangium. The haploid female gametophyte is protected by one or more layers of sporophyte tissue that surround the sporangium wall. This entire structure, consisting of female gametophyte, sporangium and protective outer layers, is the ovule. Ovules, if fertilized, develop into seeds.

The male gametophyte also develops while still attached to the sporophyte. Many cells within each sporangium undergo meiosis, forming large numbers of haploid spores. Within each of these spores, a male gametophyte develops, producing a handful of cells by mitosis that all fit within the wall of the original spore. Pollen consists of the multicellular male gametophyte and a surrounding outer wall containing sporopollenin.

The third step begins when pollen is shed. Pollination is the transport of pollen, either in the air or by an animal, from the sporangium where it was produced to a location near an ovule. Because the male gametophyte produces male gametes, pollination eliminates the need to release swimming sperm into the environment. However, pollination alone is not sufficient to ensure fertilization. The male gametophyte must germinate and grow through the tissues of the ovule to deliver the male gametes to the egg. For this reason, pollen should not be thought of as flying sperm. The growth of the male gametophyte plays an essential role in bringing the male and female gametes together.

The fourth and final step is the maturation of a fertilized ovule into a seed. A seed consists of an embryo, stored resources, and an outer, protective coat. Seeds increase the probability that offspring will survive because their larger size allows the transport of more resources that can be used to fuel the early growth of the next generation.