Anthocerotophyta, the Hornworts

Of the three extant divisions of bryophytes, the hornworts (Anthocerotophyta) are thought to be the closest relative to vascular plants. Like the other two bryphyte divisions (the mosses and liverworts), the gametophyte is the dominant generation in hornworts. However, the protonema formed from the germinated spore resemble those of ferns and the locations of the anteridia (in groups in a chamber) and the archegonia (embedded inthe thallus) resemble those of the clubmosses (1).

Phaeoceros laevis

Hornworts consist of a gametophyte thallus and a sporophyte thallus. The thallus differs from many other bryophytes in having only one chloroplast per cell, not possessing oil bodies and possessing a pyrenoid (1). This is a body that serves as a nucleus for starch storage. The thallus often has a bluish tinge to it due to the presence of the cyanobacteria Nostoc embedded within the tissues which serves to fix atmospheric nitrogen which can be transferred to the sporophyte (2). This can in turn transfer fixed carbon to the thallus. Its is from the large, elongated sporophyte that hornworts get their common name. Its size is due to the fact that it will continue to grow throughout its life. Growth is from a near basal meristem, unlike most other plants which grow from the top, a feature that is unique to hornworts.

Phaeoceros laevis

Only three species of hornwort occur in Ireland (3) a number that is not all that surprising when one considers that there are only c. 300 species present worldwide (4). These are the Field Hornwort (Anthoceros agrestis), the Dotted Hornwort (Anthoceros punctatus) and the Smooth Hornwort (Phaeoceros laevis).

References:

1. Glime, 2007.  Bryophyte Ecology.  Volume 1.  pp. 57-62  (online at http://www.bryoecol.mtu.edu/)
2. Villarreal and Renzaglia, 2006. American Journal of Botany 93 pp. 693-705
3. http://www.botanicgardens.ie/herb/census/lists.htm
4. Taylor et al., 2009. Paleobotany (Second Edition) pp. 161-177