Nonvascular plants are primarily defined by their lack of vascular tissue through which they can conduct water and nutrients, requiring them to remain small and grow close to a water source. However, the distinct life cycle of the nonvascular plants also differs them markedly from their vascular cousins and shows that the plant kingdom has its evolutionary roots in water, not on land.
Most familiar plants belong to the broad classification of vascular plants, a grouping that includes everything from grasses and wildflowers to massive sequoias. However, beneath your feet--and often beneath your notice as well--are a primitive group of plants called the bryophytes. Bryophytes include mosses, liverworts and hornworts and are defined by their lack of vascular tissue and unusual life cycle.
Alternation of Generations
All plants exhibit an alternation of generations: One form contains a full set of chromosomes and a second form has only a half-set present. In most plants, the dominant generation--the form consistently observed in the environment that provides for the nutritional needs of the plant--is the sporophyte, where all cells have a full set of chromosomes. In nonvascular plants, however, the gametophyte generation dominates and cells have only half the expected number of chromosomes. When the sporophyte generation forms, it depends on the gametophyte for subsistence.
A new nonvascular plant begins with a spore, carried on the wind and dropped to the earth. If the location contains adequate moisture, the spore will begin to grow. It develops into a network of filaments, some anchoring the new plant to the ground and others developing into tiny leaves. As Dr. Barbara Crandall-Stotler of Southern Illinois University explains, one spore develops into multiple filaments, producing several plants. This generation--the gametophyte--supports the life of the plant through photosynthesis. The tiny leaves also absorb water and minerals from rainwater.
Once growth has established, bryophytes develop reproductive structures. Individual plants are male or female and develop structures that produce sperm or egg cells. Rainwater splashes the sperm cells to female plants, and the resulting fusion creates a cell with a full set of chromosomes: the sporophyte. This cell grows into a tall structure; and at the tip, cells split in half to form spores, each containing half the chromosomal material. The wind catches the spores, and the cycle begins again.
The ancestors of the bryophytes occupied water, explaining why bryophytes, as the earliest land plants, rely so heavily on water for the completion of their life cycle. As plants continued to evolve, not only did vascular tissue eliminate the need for constant contact with water but generational dominance shifted to the sporophyte. The gametophyte generation, seen whenever you observe tiny moss plants clinging to the roots of a tree, became secondary, dependent on the sporophyte and, eventually, in the seed plants, easily overlooked.