Non-vascular plants lack the internal means for transporting water. Considered by botanists as the simplest form of land-dwelling plants, non-vascular plants do not reproduce by flowers or seeds. They usually grow only 1 to 2 cm in height due to lack of vascular systems for support. Most non-vascular plants grow in damp, shady places due to the inability to draw water from the soil.
Plants involved in the non-vascular life cycle are either of the sporophyte or gametophyte generation. Sporophytes feature two chromosome sets and are considered diploids. Gametophytes feature one chromosome set and are haploid.
Alternation of Generations
Non-vascular plants reproduce through the alternation of generations. One entire generation will be sporophytes, with the next generation being gametophytes. This alternation forms the two stages of the life cycle.
According to the Essential Science for Teachers website, when you look at a mature non-vascular plant, you are looking at the sporophyte generation. Because this generation carries two sets of chromosomes, it is able to produce spores featuring one set of chromosomes. The spores develop into the gametophyte generation, which forms a thin stem and root-like fibers.
The gametophyte generation produces sex cells, both sperm and eggs, through reproductive structures at the top of the plant. The sex cells carry the same single chromosomes carried by the gametophyte. Fertilization occurs between the sex cells, bringing the two sets of chromosomes together and forming a double-chromosome fertilized egg. This egg then develops into a sporophyte, carrying two sets of chromosomes.
The gametophyte stage is the dominant stage of non-vascular life reproduction, necessary for survival. The sporophyte stage is shorter lived.
Sperm Production and Fertilization
The gametophyte produces sex cells within structures called the gametangia. The male sperm is referred to as antheridium, while the femaile egg is the archegonium. Water provides the transportation of sperm to the egg for fertilization in most non-vascular plant species.
Formation of Spores
Once fertilized, the resulting zygote develops into a sporophyte within the archeogonium egg. A long stem grows from the egg, eventually forming a sporagium at the end. The single-chromosome spores develop within the sporagium. Once full, the sporagium breaks open, scattering the spores. Germination then occurs by mitotic division and form structures called protonema, which eventually develop into gametophytes.