Bryophtes are thought to have been the first true plants, evolving from charophytes almost 500 million years ago. Phylum Bryophyta includes the mosses (Class musci), of which there are more than 10,000 species, making mosses almost twice as diverse as mammals, as well as the liverworts (Class Hepaticae) and hornworts (Class Anthocerotae). Sadly, bryophytes do not receive much attention from us because they are so small and inconspicuous. They have no vascular tissue or wood to give them structural support, nor do they have large leaves or flowers or cones. However, bryophytes still have an ecological significance. They play important roles in minimizing erosion near streams, water and nutrient cycling in tropical forests, and insulating the arctic permafrost.
Ecologically and structurally, mosses are more closely related to lichens than to other members of the plant kingdom. For instance, both mosses and lichens depend on external moisture in order to transport nutrients. Because of this, mosses prefer damp places and have even developed special methods of dealing with long dry periods. Unlike higher plants, mosses do not have specialized organs to transport fluid, confining them to a narrower spectrum of habitats.
As we know, all plants reproduce through alternating generations, and this fact is quite apparent in mosses. The first generation, the gametophyte, forms the leafy green structure we normally think of when talking about mosses. It produces a sperm and an egg (the gametes) which combine under certain conditions to grow into the next generation: the sporophyte (spore- bearing structure). Typically, the moss sporophyte is a capsule growing on the end of a stalk called the seta, as shown in the picture below. The sporophyte contains no chlorophyll of its own: it grows parasitically on its gametophyte mother. As the sporophyte dries out, the capsule release spores which will grow into a new generation of gametophytes, if they germinate.
from left to right: picture of a moss plant, picture of a moss sporophyte on the end of a stalk called a seta, a sporophyte with peristome teeth which are arranged radially around the mouth of the capsule and whose movement aids in the release of spores
Liverworts are primitive plants which function much like mosses and grow in similar habitats. There are two classes of liverworts, which are separated by their physical differences. One class of liverworts, the Jungermanniidea, are leafy like moss, whereas the other class of liverworts, the Marchantiopsida, are leaf- like (thalloid).
The leafy liverworts look very much like mosses and, actually, it is very difficult to tell them apart when only gametophytes are present. The stalk of the sporophyte is translucent to white while its capsule is typically black and egg-shaped. After it matures, the capsule splits open into four equal parts, releasing the spores into the air. The liverwort sporophyte shrivels up and disappears shortly after releasing its spores. Moss sporophtyes, on the other hand, may persist much longer.
picture of a leafy liverwort; note the black capsule of the sporophyte
Thalloid or leaf-like liverworts are much easier to identify than their leafy counterparts due to the wider variety of gametophyte shapes. The gametophyte is typically flat, green and more or less strap-shaped. However, the body may branch out several times to round out the form. When the gametophyte has become fertilized and is ready to produce its sporophyte generation, it may grow a tall green umbrella-shaped structure, as shown in the photo below. The sporophyte grows on the underside of this structure, often completely hidden from view. During the dry season, leaf-like liverworts may shrivel up and completely disappear from view until the rain comes again.
picture of a leaf-like liverwort; note the tall umbrella-shaped structure
Hornworts are quite similar to liverworts but differ in the shape of the sporophytes. Instead of generating spores in a capsule att the end of a stalk, the hornwort generates spores inside a green horn-like stalk. When the spores mature the stalk splits, thereby releasing the spores. In addition, interestingly, unlike other plants which typically have many small chloroplasts per cell, hornwort cells have a single, large chloroplast in each cell. This unique structure gives the body of a hornwort a unique color and translucency.
picture of a hornwort