12.0.1 12.3: Mosses and other non-vascular plants lack vessels for transporting nutrients and water.

The earliest land plants were low-growing for a reason: they had no structures that could transport water and nutrients from the soil upward into the plant. The only way that these substances could move was by diffusing from one cell into an adjacent cell, and so on. Diffusion is a slow process, and plants that rely on it can grow only a few centimeters tall.

Despite the limitations of diffusion, three groups of plants—liverworts, hornworts, and mosses—all known as bryophytes, still use diffusion to move substances through their bodies, rather than having any sort of “circulatory system.” More than 12,000 species of mosses grow in habitats extending from arctic and alpine regions to the tropics. Liverworts and hornworts are small (less than an inch in height), simple plants that grow in moist and shady places and resemble flattened moss. These three types of bryophyte plants are referred to as “non-vascular” because they do not have vessels to transport water and food. Water and nutrients are absorbed into the outermost layer of cells by projections that penetrate a few micrometers into the soil. Because these projections are so short, non-vascular plants must either live in places where the soil is always moist or become dormant when the soil surface dries out (FIGURE 12-7).

To adapt to land, the non-vascular plants had to develop a method of reproduction that protected the plant embryo from drying out and provided it with a source of nutrients. What made this possible was a life cycle of alternating haploid and diploid generations, which is radically different from the life cycle of humans and most other animals. In animals, the haploid gametes, at fertilization, produce a new, diploid cell that becomes a multicellular organism, which, in the adult stage, starts the process all over again. In plants, however, there is an “alternation of generations.”

All plants exhibit the alternation of generations, but the life cycle of a moss provides a useful example of how the alternation of generations works. When you look at a spongy mass of moss (or any other non-vascular plant), what you see is the haploid (or gametophyte) part of the life cycle, rather than the diploid (or sporophyte) part of the life cycle (FIGURE 12-8). For this reason, mosses are described as having a dominant gametophyte. The adult moss plants are haploid plants—that is, all the cells have only one set of chromosomes. There are male and female moss plants that have male and female reproductive structures located among the feathery leaves at the tips of the stems. Water collects here during rainstorms, allowing the sperm to “swim” (or be splashed) from the male structures to fertilize eggs in the female structures. Once the egg is fertilized in the female structure, a diploid zygote is formed and divides to become an embryo.

The embryo is sheltered within the female reproductive structure, which provides water and nutrients for the growing embryo. Eventually, the female structure elongates to such a degree that it breaks in two and forms a capsule extending over the top of the plant like a raised fist. Inside the capsule are haploid spores—single cells, containing DNA, RNA, and a few proteins. When a capsule ruptures, it releases hundreds of spores. The spores that land in moist, sheltered spots grow into new (haploid) male or female moss plants.

Some non-vascular plants are economically or ecologically important. “Peat moss” (Sphagnum moss), found in many parts of the world, is sold as a soil enhancer for gardening. In areas where trees are scarce, peat is dried and burned as fuel. In places with monsoon seasons, such as Malaysia and Indonesia, peat bogs—consisting of partially decayed moss—are important in flood control, because they can absorb enormous amounts of water and release it over a period of months. Mounds of burning peat are also used to dry the barley used in producing Scotch whisky, giving Scotch its distinctive smoky taste (FIGURE 12-9).

Unlikely as it seems, non-vascular plants can grow in deserts, and mosses (along with lichens and cyanobacteria) are important components of the biological crust that holds desert soils in place. The crust cements the soil particles together and allows the soil to resist wind erosion, but it is extremely fragile and very slow to regenerate. When people or cattle walk over the crust, they break it into small pieces that cannot resist wind, and erosion can then strip a meter or more of soil in a few decades, leaving tree trunks supported in mid-air by their roots (FIGURE 12-10).