Moss

Mosses share numerous features with the two other classes of bryophytes. They are more complex than algae yet simpler than the higher vascular plants. Like higher plants, bryophytes use chlorophyll-a, chlorophyll-b, and carotenoids as photosynthetic pigments. Their food reserves are stored as starch. Cellulose is found in their cell walls (cell-produced rigid structures that are external to the plasma membrane) and they form cell plates (structures made of membranes representing the site of newly created cells) during cell division. Their spores, units capable of maturation, develop as a tetrad (a group of four cells) by meiosis, divisions of the cell nucleus that halve the number of chromosomes.

Two features of bryophytes tend to restrict them to moist environments, such as bogs and woodlands. First, unlike vascular plants, bryophytes lack a system with xylem and phloem for efficient transport of water and food. Second, the male sperm cells of bryophytes must swim through water to reach the female egg cells.

Bryophytes also differ from higher plants in that the sporophyte, the spore-producing diploid tissue, is nutritionally dependent on the dominant haploid gametophyte. In higher plants the gametophyte is dependent on the dominant sporophyte.

The life cycle of a generalized moss. Illustration by Hans & Cassidy. Courtesy of Gale Group.