Hornworts
Hornworts are a group of small, non-vascular plants that belong to the division Anthocerotophyta. They are called “hornworts” due to the elongated, horn-like structure of their sporophyte, which is the spore-bearing stage of their life cycle.
Here is a detailed overview of the characteristics and classification of hornworts:
Characteristics:
- Hornworts lack true roots, stems, and leaves, and instead have a flattened, thallus-like structure that is typically less than 2 cm in diameter. [1]
- The thallus contains cavities that house colonies of symbiotic cyanobacteria, usually Nostoc species, which provide the hornwort with fixed nitrogen. [1][2]
- Hornworts have a single chloroplast per cell, except for some species in the genera Megaceros, Nothoceros, and Anthoceros, which have multiple chloroplasts. Many hornworts also possess a pyrenoid, a specialized structure that enhances their photosynthetic efficiency. [2]
- The sporophyte of a hornwort grows from the gametophyte (the dominant stage of the life cycle) and can reach up to 5 cm in height. It has a persistent basal meristem, unlike the apical or intercalary growth seen in mosses and liverworts. [2]
- Hornwort spores are relatively large, measuring 30-80 μm in diameter, and have a distinctive Y-shaped tri-radiate ridge on the proximal surface. [2]
Classification:
- Hornworts were previously classified together with mosses and liverworts in the division Bryophyta, but more recent molecular evidence has led to them being placed in their own distinct division, Anthocerotophyta. [1][2]
- The hornworts are a relatively small group, with around 300 named species distributed across 6 genera, the largest being Anthoceros. [1][2]
- The classification of hornworts remains somewhat controversial, and the exact number of species is still debated among biologists. [2]
In summary, hornworts are a unique group of early-diverging land plants with a number of distinctive characteristics, including their symbiotic relationship with cyanobacteria, their specialized chloroplast structure, and their unusual sporophyte development. [1][2]
