In Chapter 29, we saw that the evolution of vascular tissues allowed plants to grow tall. Xylem and phloem are present only in the sporophyte generation because spore dispersal is enhanced by height whereas gametophytes must remain small and close to the ground to increase the chances of fertilization. A major change in the plant life cycle is that in vascular plants the sporophyte is the dominant generation both because the sporophyte is physically larger than the gametophyte and because its photosynthetic production is much higher than that of the gametophyte.

The first two groups of vascular plants—the lycophytes and the ferns and horsetails—depend on swimming sperm for fertilization and disperse by spores that are released into the air (see Fig. 30.1). In these ways, their life cycle is similar to that of the bryophytes. However, the spore-dispersing vascular plants are distinct from bryophytes in that both the gametophyte and the sporophyte generation are free-living, meaning that each is able to supply its own nourishment.

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To illustrate this, let’s examine the life cycle of the bracken fern Pteridium aquilinum. We begin with the largest component of the life cycle—the leafy diploid sporophyte generation (Fig. 30.6). If you examine a bracken leaf closely, you may find tiny brown packets along its lower margin. These are sporangia, and each contains diploid cells that undergo meiosis to generate haploid spores. The spores become covered by a thick wall containing sporopollenin, making them well suited for dispersal through the air.

As the sporangia dry out, they break open and the spores are released. Most spores travel only short distances, but a few may end up far from their parent. Bracken ferns can be found on every continent except Antarctica, as well as on isolated oceanic islands, testimony to the ability of spores to travel long distances.

Spores that land in a favorable location can germinate and grow to form the haploid gametophyte generation. You have probably never seen a fern gametophyte—or never noticed one. Typically of most ferns, the bracken gametophyte is less than 2 cm long and only one to a few cells thick (Fig. 30.6). The small size of the fern gametophyte has led many to assume that the haploid generation is a weak link in the fern life cycle. In fact, because fern gametophytes tolerate drying out, they often live longer and withstand stressful environmental conditions better than the much larger sporophytes. As discussed in Chapter 29, desiccation tolerance is important for plants that lack roots, which include both bryophytes and free-living gametophytes.

Fern gametophytes typically produce either male or female gametes. The male gametes swim to the egg on a nearby gametophyte through a film of water. The union of a male and a female gamete forms a diploid zygote, which develops in place and is supported by the gametophyte as it begins to grow. Eventually, the developing sporophyte forms leaves and roots that allow it to become a physiologically independent, diploid plant.

Ferns release swimming sperm and thus are able to reproduce only when conditions are wet. That is true of the other spore-dispersing plants (lycophytes, horsetails, and bryophytes) as well. What if a plant could eliminate the requirement for swimming sperm? Seed plants did just that.

Quick Check 2 In what ways is fern reproduction similar to moss reproduction? In what ways is fern reproduction different from moss reproduction?