Hornworts inhabit moist to wet soils and tolerate brief periods of inundation. But the tiny plants rot quickly if kept too wet. The photographed plants were on a mound of sandy soil in a seepage slope (rainwater percolates downward then hits a soil horizon where it seeps outward from a hillside).
Hornworts have an alternation of generations; one generation, the gametophyte, is haploid with a single set of chromosomes. It appears as a leafy thallus lying close to the soil. The leafy gametophyte is the light green plant near the center of the photograph. The gametophyte gives rise to gametes, the sex cells (sperm and eggs). The sperm cells are produced in a structure called the antheridium. They are flagellated and must swim on a film of water to the archegonium where an egg is produced.
The sporophyte is the result of the union of the sex cells. It is diploid with two sets of chromosomes. The sporophyte grows out of the archegonium. When the sporophyte is mature, its tip called the sporangium splits lengthwise and releases haploid spores that float on the wind. If the spores land on the proper substrate, they will grow into the leafy gametophytes.
Numerous sporophytes appear like stalks growing out of the gametophytes. In the photograph, the two generations growing together are seen on either side of the light green gametophyte generation. The cause of the color variation between the photographed hornworts is unknown, but may have to do with available nutrients.
("Chapter 15: Bryophytes", Biology of Plants, Raven, Evert, and Eichhorn, 5th Edition, Worth Publishers, 1992)
