Q.23. What is Rhynia ? Describe structure and reproduction. (1998)
Related Questions-
Q. Explain the morphology of Rhynia. (2006, 13)
Q. Describe the reproductive organs of Rhynia. (2006, 08, 13)
Q. Describe morphology, anatomy and reproductive organs of Rhynia. (2005, 11)
Q. Give classification of Rhynia. (2010)
Q. Describe the structure of aerial stem of Rhynia. (2007, 09)
Q. Mention the characteristic features of Rhynia which are not found in other Pteridophytes. (2005, 07)
Q. Explain morphology of fossil pteridophyte studied by you. (2005)
Q. Explain external morphology of sporophyte of the Pteridophyte which bears rhizoids. (2004)
Ans. Systematic Position: -
Division - Pteridophyta
Class - Psilophytopsida
Order - Psilophytales
Familiy - Rhyniaceae
Genus - Rhynia
Habit: - Rhynia is a fossil genus known only from a single middle Devonian deposit in northern Scotland. The two known species of Rhynia are R. major and R. gwynne-vaugnanii. There is evidence that these plants were growing in petty habitats, where atmosphere contained sulphurous vapours and solid in which they grow was saturated with acid water from hot springs.
In 1913 in Rhynia region of scotland from works of middle devonian period some impressions of fossils plants were met Prof. Kidston and Prof. Long studied them in 1917 and named the genus Rhynia due to its occurence from Rhynia.
Unique features of Rhynia: -
1. Lacks of leaflet 2. Occurrence of spore tetrads 3. Absence of root or occurrence of rhizoids.
Morphology: - The plants were herbaceous and rootless. The plants body consisted of a horizontal rhizome whose branches turned gradually or abruptly upward and become leafless aerial shoots. Rhizoids were borne in patches on the underside of the rhizome. The aerial branches were dichotomously forked, cylindrical and tapered gradually. The elongated and oval sporangia were borne at the tips of ultimate dichotomies of the aerial branches. Since leaves were not seen, hence it may be referred that aerial stems were green and photosynthetic. In R. gwynne vaughanii several adventitious branches were present on the aerial stems as well as rhizomes. It was assumed that vegetative reproduction occured through them.
Internal Structure: - The aerial branches and rhizome of Rhynia was differentiated into epidermis, cortex and stele. The endodermis and pericycle in between cortex and stele was absent. Both have protophloem and pith was absent.
Anatomy of Aerial Stem: - It has following three layers:
1. Epidermis: - It was the outermost single-layered covered by a thick cuticle. The epidermis was interrupted by stomata through which exchange of gases took place for photosynthesis.
2. Cortex: - Below the epidermis there remain present cortex which was differentiated into outer cortex and an inner cortex e.g., The outer cortex consisted of 1-4 layers of thick cells without an intercellular spaces. The inner cortex was composed of spherical cells with intercellular spaces which maintained continuity with stomata. Hence, the inner cortex was the chief photosynthetic region.
3. Stele: - A protostele was present in which xylem was surrounded by phloem. Xylem was made of tracheids having angular and spiral thickenings.
Phloem was made of 4-5 thin walled cells. They did not contain sieve plates.
Reproduction: -
The sporangia were cylindrical and borne single on the tip of some aerial branches. They were large, oval or cylindrical structures with pointed ends. The sporangia had walls several cell layers thick in which the cells of the outermost layer were thick walled and had a heavy cuticle. The sporangium was without any specialised mechanism of dehiscence. The inner cells of the jacket probably functioned as tapetum. The sporangia contained numerous spores with cutinised walls. The spores were apprently all alike and were arranged in tetrads. The presence of tetrads in some specimens suggests that meiosis occured in Rhynia and that the plant body therefore, belonged to the sporophytic generation.
The gametophyte: -
The spores - The spores were cutinised and usually were borne in tetrads.
Nothing is known about the gametophyte of Rhynia because the gametophyte of this fossil plant has never been discovered. Lyon (1957) reported some germinating spores from Rhyine Chert, which show multicellular structures, developing at the ends of germ tubes. These may represent the gametophytic generation. Meker (1959) has suggested that the underground part of Rhynia might possible be the gametophytes. According to Pant (1961), R. gwynne-vaughanii may be the gametophyte of the larger R. major.
Q.24. Differentiate between Rhynia gwynne vaughanii and Rhynia major.
Ans. Difference between Rhynia gwynne vaughanii and Rhynia major -
