SYSTEMATIC BIOLOGY					THE VIRIDIPLANTAE
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PHYLUM ANTHOCEROTOPHYTA

INTRODUCTION TO THE ANTHOCEROTOPHYTA

Anthocerotophyta (an-tha-se-ro-TA-fa-ta) is made of three Greek roots that mean flower (anthos -άνθος); horn (keras -κέρας); and plant (phyto -φυτό).  The reference is to the horn-like sporangium or fruiting body.  The phylum name is formalized from a common genus, Anthoceros.

Three genera, Anthoceros, Phaeoceros, and Notothylas, are common in the northern temperate zone.  In the gametophyte stage, they resemble the thallose hepatics; however, when grown in uncrowded conditions, the gametophytes tend to be almost circular.  The thallus structure of hornworts is simple, much simpler than Marchantia.  Typically, they have an epidermis and air chambers.  Some taxa enter into a symbiotic relationship with Nostoc, which enters the air chambers through cracks in the ventral surface. In general, the gametophyte has a rather greasy appearance.  Another unique feature of this phylum is the occurrence of massive plastids opposed to the small lenticular ones of other embryophytes

They never have gametophores; rather, archegonia and antheridia occur on the upper surface of the thallus.  The archegonia and antheridia, however, are not well differentiated from the thallus.  For example the archegonium is reduced to neck canal cells embedded in undifferentiated thallus tissue. The situation is similar for the antheridia.  Despite the apparent simplicity of the gametangia, the sporophyte is quite complex (Figures A-C).  The sporophyte emerges by the growth of cells from an intercalary meristem at the top of the foot.  The developing capsule appears horn-like with relatively little fertile (or sporogenous) tissue.  The sporophyte capsule also has photosynthetic tissue and stomata.   The capsule is somewhat indeterminate with spores at different developmental stages and a continual dehiscence along its longitudinal slits.  Spores are shed with the aid of elaters.   [See the Anatomy and Life History of the Hornworts].

SYNOPTIC DESCRIPTION OF THE ANTHOCEROTOPHYTA

SYSTEMATICS OF THE ANTHOCEROTOPHYTA

The taxonomy of the hornworts has been fairly standard.  It has almost always held a rank equal to that of the Mosses and liverworts (e.g. Scagel et al. 1982; and Bold et al. 1987).  Although this group seems to reflect an early divergence of the archegoniate embryophytes, the analysis of Marin and Melkonian (1999) suggests that the hornworts are sisters to the leafy liverworts.  If so, the synapomorphy that unites them may be longitudinal lines of dehiscence.   Shaw and Renzaglia (2004), Stotler and Crandall-Stotler (2005), and Frey and Stech (2005) demonstrate that the taxonomy of the hornworts is complex for such a small group.  Leiosporoceros is a sister group to the rest of the hornworts with one or two clades.  The systems of Stotler and Crandall-Stotler (2005) and Frey and Stech (2005) differ in the ordinal structure of the Anthocerotopsida.  Stotler and Crandall-Stotler (2005) have two orders (as given below) while Frey and Stech (2005) reorganize the the class into three orders.

HIERARCHICAL CLASSIFICATION OF THE ANTHOCEROTOPHYTA

This page is maintained by Jack R. Holt.  Last revised 03/11/2008.