5.1: Hornworts

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May 4, 2022
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- 5.2: Liverworts

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Hornworts, Phylum Anthocerotophyta

A hornwort showing both sporophyte and gametophyte generations — Figure $\PageIndex{1}$ : Hornworts growing in a cluster. The gametophytes are the leafy green material growing closer to the surface. The sporophytes are the emergent structures. The appearance of the sporophytes is what gives the hornworts their name. Photo by Maria Morrow, CC-BY-NC.

Diagram of a hornwort life cycle — Figure $\PageIndex{2}$ : The *Phaeoceros* life cycle. The thalloid gametophyte is a flat flap of green, photosynthetic tissue with rhizoids extending from the underside. Within the thallus, an embedded antheridium produces sperm with two flagella and an embedded archegonium produces a single egg. When a sperm fertilizes an egg, a sporophyte composed of a single thin sporangium grows from the archegonium. At the base of the sporangium is a foot. A collumella extends up the center of the sporangium with long pseudoelaters aiding in spore dispersal. Notably, there are stomata on the sporangium. Diagram by derivative work: Smith609 (talk)Hornwort_life_cicle_svg_diagram.svg: Mariana Ruiz user:LadyofHats, CC BY 3.0, via Wikimedia Commons.

Gametophyte Morphology

Hornwort gametophytes are exclusively thalloid, often with compartments of mutualistic cyanobacteria from the genus Nostoc. Cells within the gametophyte are monoplastidic, containing one large chloroplast in each cell. On the exterior of the thallus, simple pores allow for gas exchange (no guard cells, meaning pores are permanently open).

A piece of a thallus viewed under a microscope. The cells of the thallus each have a single, large plastid. — Figure $\PageIndex{3}$ : This image of part of a *Phaeoceros proskaueri* thallus shows the cells of the gametophyte each containing a single large plastid (monoplastidic). Photo by Amanda Heinrich, CC-BY-NC.

Close up of monoplastidic cells, each with one large plastid — Figure $\PageIndex{4}$ : Antheridia on the male gametophyte of Phaeoceros. They are formed in a depression (crypt) within the thallus. Note the monoplastidic condition of the cells. Photo by George Shepherd, CC BY-NC-SA.

Sporophyte Morphology

Hornwort sporophytes are comprised of a linear sporangium that lacks a seta. It grows from a basal meristem, meaning the cells at the apex are the oldest. When you see the sporangia mature, this becomes more obvious, as the tip dries out first and dehisces (splits apart) to release the spores. Unlike the gametophyte, the sporophyte possesses true stomata for gas exchange.

A cross section through a hornwort sporangium. — Figure $\PageIndex{5}$ : A cross section through an *Anthoceros* sporophyte. Spore tetrads (A) fill the sporangium and surround the central columnella (B). The second photo shows the spore tetrads at a higher magnification. Together, the four spores make a sphere. When separated, they have one rounded side and several flat sides Photos by George Shepherd, CC BY-NC-SA with labels added by Maria Morrow.

clusters of ornamented spores inside a hornwort sporangium — Figure $\PageIndex{5}$ : A cross section through an *Anthoceros* sporophyte. Spore tetrads (A) fill the sporangium and surround the central columnella (B). The second photo shows the spore tetrads at a higher magnification. Together, the four spores make a sphere. When separated, they have one rounded side and several flat sides Photos by George Shepherd, CC BY-NC-SA with labels added by Maria Morrow.

A view through a microscope of the tip of a sporophyte, many dark globose structures (spores) are visible. — Figure $\PageIndex{6}$ : The tip of a sporophyte dehiscing (splitting down the center, here). There are many large spores visible inside the sporangium. Photo by Amanda Heinrich, CC-BY-NC.

six large, brown, globose structures covered in a network of small spines. — Figure $\PageIndex{7}$ : The large, ornamented spores of *Anthoceros agrestis*. Photo by HermannSchachner, CC0, via Wikimedia Commons.

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Hornworts, Phylum Anthocerotophyta

Gametophyte Morphology

Sporophyte Morphology

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