Systematics, Reproduction and Phylogeny of Horneophyton

Horneophyton:-

1. Systematics:-

Division:- Psilophyta

Class:- Psilophytopsida

Order:- Psilophytales

Family:- Rhyniaceae

Genus:- Horneophyton 

2. Introduction:- 

> Geographical occurrence = Lower Devonion period 

> Geographical distribution = Mostly reported from Rhynia chert bed of Scotland. 

> Horneophyton lignieri was described for the first time by Kidston and Lang (1920) from Rhynie chert bed as Hornea lignieri. 

> According to ICBN, the original generic name Hornea was previously occupied by a flowering plant.

3. Study of sprophyte:- 

> The plant was approximately 6-8 inches (15-20 cm) high. 

> The underground portion of the plant consists of a series of bulbous, corm-like structures bearingseries of bulbous, corm-like structures bearing numerous unicellular rhizoids. 

> The basal corm lacks any evidence of vascularization and is made up of dark, thick- walled parenchymatous cells.

> Internal structure:- Internally the aerial axes have protostele with central placedcentral placed protoxylem is surrounded externally by phloem.

4. Reproduction:-

> The sporangia are borne externally at the apex some of the aerial branches. 

> Each sporangium isEach sporangium is dichotomously branched and may have 2-4 lobes of varying length. 

> The sporangial lobes are ellipsoidal cylindrical in shape with truncated distal end. 

> Dehiscence of sporangia took place through an apical pore. 

> Extending into the sporangial cavity is a column of sterile tissue around which a continuous layer of sporogenous tissue, composed of trilete isospores, overarched a central columella. 

> The sporangium of Horneophyton is unique.

> The sporangium of Horneophyton is unique among all vascular plants, because it consists of a branched fertile unit resulted from dichotomies of the stem apex. 

> Moreover, each fertile lobe was produced by its own apex and remained meristematic for an extended period of time.

> Origin of its sporangium suppose to be derrived from the sporangium of Anthoceros.

5. Phylogeny:- 

> Phylogeny:- The history of the evolution of a species or group, especially in reference to lines of descent and relationships among broad groups of organisms.

> A possible phylogeny for Horneophyton is shown below (based on Crane et al. for the polysporangiophytes and Qiu et al. for the bryophytes.

> With vascular tissue but bryophyte-like vascular tissue and sporangia, the organism has been considered a missing link between the hornworts and the vascular plants or tracheophytes (which molecular data suggest are sister groups).

> Features suggesting a relationship with the hornworts include the general form of its sporangia; its corm also resembles the foot of some hornworts. 

> The free living nature of its sporophytes, and the fact that they branching repeatedly, are marked differences which force it into the stem group of tracheophytes (along with Aglaophyton).