Hornworts

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 The base of the hornwort plant, called the thallus, has a wrinkled, leaf-like appearance. The sporophytes are a cluster of slender green stalks with brown tips grows from this wrinkled mass.
Hornwort sporophytes. Hornworts grow a tall and slender sporophyte. (credit: modification of work by Jason Hollinger)

OpenStax Biology 2e

The defining characteristic of the hornworts (Anthocerotophyta) is the narrow, pipe-like sporophyte. Hornworts have colonized a variety of habitats on land, although they are never far from a source of moisture. The short, blue-green gametophyte is the dominant phase of the life cycle of a hornwort. The sporophytes emerge from the parent gametophyte and continue to grow throughout the life of the plant.

Stomata (air pores that can be opened and closed) appear in the hornworts and are abundant on the sporophyte. Photosynthetic cells in the thallus each contain a single chloroplast. Meristem cells at the base of the plant keep dividing and adding to the height of the sporophyte. This growth pattern is unique to the hornworts. Many hornworts establish symbiotic relationships with cyanobacteria that fix nitrogen from the environment.

The lifecycle of hornworts follows the general pattern of alternation of generations. The gametophytes grow as flat thalli on the soil with embedded male and female gametangia. Flagellated sperm swim to the archegonia and fertilize eggs. The zygote develops into a long and slender sporophyte that eventually splits open down the side, releasing spores. Thin branched cells called pseudoelaters surround the spores and help propel them farther in the environment. The haploid spores germinate and give rise to the next generation of gametophytes.

In hornworts, the gametophyte is a haploid 1 n leaf-like structure with slender stalks called rhizoids underneath. Male sex organs called antheridia produce sperm, and female sex organs called archegonia produce eggs. Both male and female sex organs form just beneath the surface of the gametophyte, and are exposed to the surface as the organs mature. The sperm swims to the egg or is propelled by water. When the egg is fertilized, the embryo grows into a hollow tube-like structure called a sporophyte. Meiosis inside the sporophyte produces haploid 1 n spores. The spores are ejected from the top of the tube. They grow into new gametophytes, completing the cycle.
Reproductive cycle of hornworts. The alternation of generation in hornworts is shown. (credit: modification of work by “Smith609”/Wikimedia Commons based on original work by Mariana Ruiz Villareal)

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e

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