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The Life Cycle of Hornworts

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

You've heard of ferns, tulips, oak trees, but hornworts? Hornworts are a plant that belong to the same group as mosses and liverworts, and they have a complex lifecycle. This lesson explores that life cycle.

Hornwort Defined

Let's compile a list that includes things with horns. You've got bulls, orchestras, Vikings and a plant called a hornwort. Upon closer inspection, however, it appears hornworts don't have horns at all, or they do they?

Things with horns
things with horns

Hornworts are plants that belong in the same group as mosses and liverworts. They are found in damp or humid areas around the world, produce spores and are flowerless and seedless. Hornworts also do not have true roots, leaves or stems.

Hornwort growing on a tree
hornwort on tree

They get the common name 'hornwort' because their sporophytes looks like horns. Sporo-what? Don't worry, we'll delve into that definition, among others, when we go over the life cycle of the hornwort.

Life Cycle

The life cycle of a hortwort is fairly complex (and quite different from your life cycle), so we will need to break it down a bit.

Spore to Gametophyte

A hornwort start as a haploid spore, which just means a spore that contains one set of chromosomes. Sometimes you will see a haploid represented by an n. You can contrast that with diploid organisms that contain two sets of chromosomes (one from each parent). Most of your cells, in case you are wondering, are diploid, and diploid is often represented by 2n.

The colored sticks represent chromosomes
haploid diploid

The plant that grows from this haploid spore is called a gametophyte. The gametophyte hornwort is green or blue-green and the entire plant looks like a leafy clump. The gametophyte undergoes photosynthesis (meaning it makes its own food from sunlight, water and carbon dioxide).

Once this plant reaches adult size, it will produce the plant's sex organs. Depending upon the species, the male and female sexual organs can be produced on the same gametophyte plant, and other times they can be produced on two separate plants.

Sex Organs and Fertilization

The female sex organs of the hornwort are called archegonia (plural) and the male sex organs are called antheridia (also plural). The archegonia have eggs and the antheridia have sperm (which, fun fact, has two tails). If you've studied plants before, the term 'anther' may sound familiar. It is the male part of a flowering plant, so I always remember antheridia is the male part of a hornwort by recalling anther is the male reproductive structure in flowers.

The sperm leaves the antheridia and swims to the archegonia, where fertilization takes place and a zygote forms. A zygote is a diploid cell produced when haploid sperm and egg unite. Hornworts like wet areas, and this helps the sperm swim to its egg-destination.

Sporophyte

The zygote eventually becomes the sporophyte, which is the part of the plant that will produce haploid spores.

A hornwort with immature sporophytes
labeled hornwort

The sporophyte is the reason a hornwort is called a hornwort (remember it kind of looks like a horn)? The sporophyte is attached to the gametophyte and depends on the gametophyte for food. The sporophyte contains spores, which will be released and will eventually grow into more gametophyte plants. To remember this, think spores come from the sporophyte.

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