Anthocerophyta-1

Their Growth Form = Thallose
The Gametophyte Thallus can be rounded and may be described as Orbicular. They have Marginal Meristems which promote this morphology. Meristematic activity is spread around the Margin of the Thallus. This is distinct from Apical Growth which develops from terminal Apical Meristems. They are Dorsiventral (have Top & Bottom) & the Ventral side produces Rhizoids. Stomata have been reported but are not common for the Gametophyte! The Sporophyte typically has Stomata. Stomata represent one of the crucial adaptations of terrestrial plants. The basic structure of stomata is identical throughout Vascular Plants.		Orbicular Thalli Anthoceros with Frilly Gametophytes grown in Culture
Air Chambers are present and Mucilage cavities are also observed. The mucilage may help to conserve water.		Mucilage Cavities
SEM of the Ventral side of Thallus with Openings to the Mucilage Cavities (Look for the small Dark Spots near the "Midrib".)
Thalli can contain Endosymbiotic, Nitrogen-Fixing Cyanobacteria! This is an important adaptation! The nitrogen derived from the Cyanobacterium has obvious value for the host. Nitrogen is one of the most limiting elements in terrestrial environments. Consequently, plants with N-fixing bacteria can colonize poor sites more readily than other plants. This might allow them to survive in the midst of plants which would otherwise out-compete them for space and sunlight. in terrestrial environments. Consequently, plants with N-fixing bacteria can colonize poor sites more readily than other plants. This might allow them to survive in the midst of plants which would otherwise out-compete them for space and sunlight.
The Cyanobacteria appear as dark spots when the Thallus is illuminated from below. Cyanobacteria produce a lot of chlorophyll when they are in a dark environment. Hornworts often inhabit dark places and since the Cyanobacteria are inside their host it is darker yet. The concentrated chlorophyll makes them appear almost black.

*The images above show cross sections through an Anthoceros* Thallus containing Cyanobacteria**.
I see these plants on newly disturbed sites in the moist forests of Oahu. They could be very important for soil stabilization & enrichment for areas disturbed by pigs or abiotic insults. Their ecological role in disturbance biology in Hawaii deserves some attention.

Unique Chloroplasts are seen in this Taxon.
They have extremely Large Chloroplasts and Each Cell may only have One Chloroplast. However, some cells have several small chloroplasts. Their Chloroplasts have a Pyrenoid. This is common in Chlorophyta BUT not in other land plants. The Pyrenoid appears to be involved in Starch Synthesis. It could be interesting to compare the Ultrastructure and the Molecular Biology of Anthoceros plastids with those of Green Algae.

Large Chloroplasts are evident in Epidermal Cells as viewed from Above	Cross Section showing the Large Epidermal Chloroplasts

Diversity & Distribution
This is a small group with only 4 - 5 Genera. *The Genus to remember is Anthoceros.* They are Widespread and occur in the Temperate & Tropical zones. They generally grow in wet habitats.	Anthoceros with Prominent, Immature Sporophytes. It should be obvious why these are called "Hornworts".

Their Growth Form = Thallose