第25章種子をつくらない植物

The defining characteristic of the hornworts (Anthocerotophyta) is the narrow, pipe-like sporophyte. Hornworts have colonized a variety of habitats on land, although they are never far from a source of moisture. The short, blue-green gametophyte is the dominant phase of the life cycle of a hornwort. The sporophytes emerge from the parent gametophyte and continue to grow throughout the life of the plant (Figure 25.12).

Stomata (air pores that can be opened and closed) appear in the hornworts and are abundant on the sporophyte. Photosynthetic cells in the thallus each contain a single chloroplast. Meristem cells at the base of the plant keep dividing and adding to the height of the sporophyte. This growth pattern is unique to the hornworts. Many hornworts establish symbiotic relationships with cyanobacteria that fix nitrogen from the environment.

The lifecycle of hornworts (Figure 25.13) follows the general pattern of alternation of generations. The gametophytes grow as flat thalli on the soil with embedded male and female gametangia. Flagellated sperm swim to the archegonia and fertilize eggs. The zygote develops into a long and slender sporophyte that eventually splits open down the side, releasing spores. Thin branched cells called pseudoelaters surround the spores and help propel them farther in the environment. The haploid spores germinate and give rise to the next generation of gametophytes.

The mosses are the most numerous of the non-vascular plants. More than 10,000 species of mosses have been catalogued. Their habitats vary from the tundra, where they are the main vegetation, to the understory of tropical forests. In the tundra, the mosses’ shallow rhizoids allow them to fasten to a substrate without penetrating the frozen soil. Mosses slow down erosion, store moisture and soil nutrients, and provide shelter for small animals as well as food for larger herbivores, such as the musk ox. Mosses are very sensitive to air pollution and are used to monitor air quality. They are also sensitive to copper salts, so these salts are a common ingredient of compounds marketed to eliminate mosses from lawns.

Mosses form diminutive gametophytes, which are the dominant phase of the lifecycle. Green, flat structures with a simple midrib—resembling true leaves, but lacking stomata and vascular tissue—are attached in a spiral to a central stalk. Mosses have stomata only on the sporophyte. Water and nutrients are absorbed directly through the leaflike structures of the gametophyte. Some mosses have small branches. A primitive conductive system that carries water and nutrients runs up the gametophyte's stalk, but does not extend into the leaves. Additionally, mosses are anchored to the substrate—whether it is soil, rock, or roof tiles—by multicellular rhizoids, precursors of roots. They originate from the base of the gametophyte, but are not the major route for the absorption of water and minerals. The lack of a true root system explains why it is so easy to rip moss mats from a tree trunk. The mosses therefore occupy a threshold position between other bryophytes and the vascular plants.

The moss lifecycle follows the pattern of alternation of generations as shown in Figure 25.14. The most familiar structure is the haploid gametophyte, which germinates from a haploid spore and forms first a protonema—usually, a tangle of single-celled filaments that hug the ground. Cells akin to an apical meristem actively divide and give rise to a gametophore, consisting of a photosynthetic stem and foliage-like structures. Male and female gametangia develop at the tip of separate gametophores. The antheridia (male organs) produce many sperm, whereas the archegonia (the female organs) each form a single egg at the base (venter) of a flask-shaped structure. The archegonium produces attractant substances and at fertilization, the sperm swims down the neck to the venter and unites with the egg inside the archegonium. The zygote, protected by the archegonium, divides and grows into a sporophyte, still attached by its foot to the gametophyte.

The moss sporophyte is dependent on the gametophyte for nutrients. The slender seta (plural, setae), as seen in Figure 25.15, contains tubular cells that transfer nutrients from the base of the sporophyte (the foot) to the sporangium or capsule.

Spore mother cells in the sporangium undergo meiosis to produce haploid spores. The sporophyte has several features that protect the developing spores and aid in their dispersal. The calyptra, derived from the walls of the archegonium, covers the sporangium. A structure called the operculum is at the tip of the spore capsule. The calyptra and operculum fall off when the spores are ready for dispersal. The peristome, tissue around the mouth of the capsule, is made of triangular, close-fitting units like little “teeth.” The peristome opens and closes, depending on moisture levels, and periodically releases spores.