Hicklingia
Encyclopedia
Hicklingia is a genus of extinct plants of the Middle Devonian (around ). Compressed specimens were first described in 1923 from the Old Red Sandstone
of Scotland
. Initially the genus was placed in the "rhyniophytes", but this group is defined as having terminal sporangia
(spore-forming organs), and later work showed that the sporangia of Hicklingia were lateral rather than strictly terminal, so that it is now regarded as having affinities with the zosterophylls.
had a tufted growth habit, with narrow leafless stems (axes) up to 17 cm high which branched dichotomously. Sporangia were borne on short stalks (up to 3 mm), on all sides of the stem and also terminally. There are oval scars on specimens where the stalks are presumed to have broken off. The lateral sporangia were closely adpressed to the stem. The effect is of a 'spike' of sporangia which terminates some stems. The sporangia opened via slits, but these did not have the thickened borders which are a feature of some Zosterophyllum species. The vascular system
of the stem was not observed. Spore
s are up to 50 µm in diameter and trilete. The gametophyte
is not known.
published in 2004 by Crane et al. in which Hicklingia is placed as a sister to all the other lycophytes
(living and extinct clubmosses and relatives).
Old Red Sandstone
The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...
of Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
. Initially the genus was placed in the "rhyniophytes", but this group is defined as having terminal sporangia
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...
(spore-forming organs), and later work showed that the sporangia of Hicklingia were lateral rather than strictly terminal, so that it is now regarded as having affinities with the zosterophylls.
Description
The sporophyteSporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...
had a tufted growth habit, with narrow leafless stems (axes) up to 17 cm high which branched dichotomously. Sporangia were borne on short stalks (up to 3 mm), on all sides of the stem and also terminally. There are oval scars on specimens where the stalks are presumed to have broken off. The lateral sporangia were closely adpressed to the stem. The effect is of a 'spike' of sporangia which terminates some stems. The sporangia opened via slits, but these did not have the thickened borders which are a feature of some Zosterophyllum species. The vascular system
Vascular tissue
Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...
of the stem was not observed. Spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s are up to 50 µm in diameter and trilete. The gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....
is not known.
Phylogeny
The affinity with zosterophylls is recognized in the cladogramCladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...
published in 2004 by Crane et al. in which Hicklingia is placed as a sister to all the other lycophytes
Lycopodiophyta
The Division Lycopodiophyta is a tracheophyte subdivision of the Kingdom Plantae. It is the oldest extant vascular plant division at around 410 million years old, and includes some of the most "primitive" extant species...
(living and extinct clubmosses and relatives).
External links
- Cladogram from
