Bryophyte

Bryophytes include the mosses, liverworts, and hornworts. Bryophytes are the simplest of plants (excluding the algae, which are not considered plants by most botanists). Bryophytes are small, seldom exceeding 6-8 in (15-20 cm) in height, and usually much smaller. They are attached to the substrate (ground, rock, or bark) by rhizoids, which are one or a few-celled, root-like threads that serve only for anchoring and are not capable of absorbing water and nutrients from the substrate. Brypohytes lack vascular tissue (the specialized cells grouped together to pipe water and nutrients to various parts of the body), or in the rare cases when this tissue is present, it is not well differentiated. The leaves of bryophytes are technically not true leaves, because in most species they lack vascular tissue. However, they are functionally equivalent to leaves, containing chlorophylls a and b for photosynthesis. Leaves are usually one-cell thick, except for the midrib, which may be up to 15 cells thick. Bryophytes satisfy their nutritional requirements by absorbing minerals from dust, rainfall, and water running over their surface.

The life cycle of bryophytes is characterized by an alternation of generations, one of which is a multicellular, diploid individual called a sporophyte, having two of each type of chromosome per cell. This stage alternates with multicellular, haploid individual called the gametophyte, with only one of each type of chromosome per cell, as is also the case with animal sperm. Bryophytes are unique among plants in that the dominant, conspicuous generation is the haploid gametophyte. In all other plants, the dominant stage is the diploid sporophyte.

Most reproduction of bryophytes is asexual, occurring by fragmentation of body parts, and by the production of specialized vegetative units called gemmae. Gemmae may be produced as microscopic plates (in the genus Tetraphis), as bulbils in the axils of leaves (in Pohlia), or as microscopic filaments (in Ulota). When sexual reproduction occurs, it always involves a flagellated sperm (produced in a specialized organ called an antheridium) that must swim through water to reach an egg located in a specialized, flask-shaped organ (the archegonium). The antheridia and archegonia are surrounded by a layer of sterile cells, which protects the sex organs from mechanical damage and desiccation.

The union of the sperm and egg results in a diploid zygote, i.e., a new sporophyte. This is nourished by the gametophyte and grows on it in a parasitic fashion, although the sporophytes of some bryophytes photosynthesize and make some contribution to their own growth. Initially, as the young sporophyte grows, the archegonium also enlarges. However, it ultimately fails to keep pace with the growth of the sporophyte and becomes detached from its base, forming a cap-like structure called a calyptra.

True mosses

One of the most distinctive features of true mosses involves the development of their gametophytes. Spores germinate to produce a characteristic mass of algal-like threads, called protonema, which looks like a loose ball of wool. Bud-like structures develop later, and give rise to the familiar leafy gametophyte. Although mosses are considered to be non-vascular plants, many true mosses in fact have a primitive vascular system consisting of a central strand of water-conducting cells called hydroids. Some also have specialized cells around the column of hydroids called leptoids, which function in the transport of carbohydrates, the products of photosynthesis. The stems of true mosses are more-or-less uniformly leafy and erect. Their leaves usually have a midrib, and their sporophytes possess capsules that are borne on stalks that are made of sporophytic tissue. Also, the capsules contain one or two rows of toothlike appendages (peristome) over the opening of the capsule, which are exposed when the lid is shed. This is by far the largest group of mosses.

Peat mosses

This is a small, but extremely important group of mosses, numbering about 350 species. Their stems are branched at nodes, with the nodes closely spaced at the tips, giving the plants a tufted appearance. Their gametophytes develop from the margins of plate-like protonema, in contrast to the filamentous protonema of true mosses. The leaves of peat mosses lack a midrib, and the bulk of the leaf mass is composed of large, translucent cells that are dead. These hyaline cells contain pores, allowing them to readily take up water. Some peat mosses can absorb an amount of water equal to 26 times their dry weight. Narrower, living cells that photosynthesize occur in networks between the hyaline cells. The sporophytes of peat mosses are distinct in that the stalk on which the capsule sits is part of the gametophyte and not the sporophyte itself as in the true mosses. The capsule also characteristically disperses its spores by a minute explosion. At maturity, the globular capsules begin to dry so that the middle portion contracts inward. The contraction produces great internal pressure on the air trapped inside, which eventually increases enough to blow off the lid with an audible pop, shooting the spores into the air. The capsules lack a peristome.

Granite mosses

This is the smallest group of mosses containing only about 100 species. Granite mosses are small, dark, tufted plants that grow on exposed rocks in alpine and arctic regions. Their leafy gametophytes arise from a lobed structure, rather than from a filamentous protonema. Their sporophytes generally are stalks that are derived from the gametophyte, as in the peat mosses. Their tiny capsules typically have four vertical sutures that split at maturity to release the spores. This method of spore dispersal is unique among the mosses.