In Chapter 29, we saw that bryophytes—liverworts, mosses, and hornworts—make up the earliest branches on the phylogenetic tree of land plants (see Fig. 30.1). They also share many reproductive traits. Here, we use Polytrichum commune, a moss common in forests and the edges of fields, to illustrate how movement onto land was accompanied by the evolution of two multicellular generations: one specialized for fertilization and the other for dispersal.

Like Chara and Coleochaete, Polytrichum has a photosynthetic body—the familiar green tufts observed on the forest floor—that is made of haploid cells and that forms gametes by mitosis. And like Chara and Coleochaete, Polytrichum and other bryophytes release swimming sperm into the environment. For fertilization to occur, the sperm must swim through films of water that coat moist surfaces to reach eggs retained within reproductive organs.

Their reliance on external water for fertilization helps explain why bryophytes produce their gametes near the ground, where they are most likely to encounter a continuous film of water. It also helps explain why bryophytes tend to be small, as sperm are able to swim only a couple of centimeters. Many bryophytes release their sperm only when agitated by raindrops. Raindrops signal the presence of surface water, and their impact can splash sperm farther than they could swim on their own.

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It is after the fusion of egg and sperm gives rise to a diploid zygote that the life cycles of plants and their green algae relatives diverge. In plants, the zygote does not undergo meiosis, and it does not disperse. Instead, the zygote is retained within the female reproductive organ, where it divides repeatedly by mitosis to produce a new multicellular plant—this one made of diploid cells (Fig. 30.4). Some cells within this diploid plant eventually undergo meiosis, giving rise to haploid spores, cells that disperse and give rise to new haploid individuals. Because the diploid multicellular plant produces spores, it is called the sporophyte, and this generation is called the sporophyte generation; because the haploid multicellular plant produces gametes, it is called the gametophyte, and that generation is called the gametophyte generation. The resulting life cycle, in which a haploid gametophyte generation and a diploid sporophyte generation follow one after the other, is called alternation of generations, and it describes the basic life cycle of all land plants.

Because land plants are descended from green algae with only haploid gamete-producing individuals, we can infer that the diploid sporophyte of land plants is an evolutionary novelty on this branch of the phylogenetic tree. How might this new generation have made it possible for plants to colonize land?

To answer this question, let’s look at the sporophyte of Polytrichum. Late in the growing season, the green tufts of this moss sprout an extension—a small brown capsule at the end of a cylindrical stalk a few centimeters high (Fig. 30.5a). This is the sporophyte, which originated from a fertilized egg. Because the fertilized egg was retained within the female reproductive organ, the sporophyte grows directly out of the gametophyte’s body. Thus, the Polytrichum specimen pictured in Fig. 30.5a shows both a gametophyte (green) and several sporophytes (brown). The gametophyte is photosynthetically self-sufficient, but the sporophytes must obtain water and nutrients needed for their growth from the gametophyte.

The significance of the multicellular sporophyte is that it enhances the ability of plants to disperse on land. The capsule at the top of the Polytrichum sporophyte is a sporangium, a structure in which many thousands of diploid cells undergo meiosis, producing huge numbers of haploid spores. In contrast, fertilization in Chara and Coleochaete is followed by meiosis in only a single cell (the zygote).

Spores are small and can be carried for thousands of kilometers by the wind. But their tiny size puts spores at risk of being washed from the air by raindrops. The sporangia of many bryophytes release their spores only when the air is dry. In Polytrichum, the sporangium has small openings from which it releases spores the way a salt shaker releases salt. When the air is wet, the spores form clumps that are too big to fit through these opening. More common is the presence of a ring of teethlike projections that curl over the top of the sporangia. These bend backward when air humidity is low, flinging the spores into the air (Fig. 30.5b).

What protects the spores from drying out or from exposure to damaging ultraviolet radiation as they travel through the air? Earlier, we noted that the zygotes of Chara and Coleochaete secrete a wall that protects the zygote within from desiccation. The wall contains sporopollenin, a complex mixture of polymers that is remarkably resistant to environmental stresses such as ultraviolet radiation and desiccation. In mosses and other land plants, it is the spore, not the zygote, that secretes sporopollenin. This suggests that early land plants retained a capacity present in their algal ancestors—the ability to synthesize sporopollenin—but the timing of gene expression changed in these plants.

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Following dispersal, most spores germinate within a few days, but some can survive within the spore wall for months or even years. When conditions for growth are favorable, the spore wall ruptures, and the haploid cell inside can then develop into a multicellular and free-living gametophyte.