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Rhynia – an Extinct Vascular Cryptogram

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Rhynia – an Extinct Vascular Cryptogram Dr. Arindam Kumar Assistant Professor Dept. of Botany JMDPL Mahila College Madhubani Rhynia – an extinct vascular cryptogram Systematic Position Division. :Psilophyta Class. :Psilophytopsida Order. :Psilophytales Family. :Rhyniaceae Genus. :Rhynia. Species :R. gwynne-vaughani : R. major The Division Psilophyta includes living as well extinct representative. They have simple vasculature with xylem lacking vessels and fibres. The tracheids may be spiral,annular or scalariform or even pitted. The division includes two classes. 1. Class: Psilophytopsida -It includes all the extinct members . 2. Class: Psilotopsida- It is represented by living genera. Order Psilophytales includes extinct plants that were discovered among the rock of the Early Devonian period of the Palaeozoic age.The class Psilophytopsida are characterized by 1.Simple plant body distinguished into a rhizome and an aerial region. 2.Rhizoids arise from rhizome and develope in tufts. 3.Absence of roots. 4.The stem branch dichotomously and are naked or have reduced leaves. 5.Vasculature is a protostele;pith is absent. 6.Sporangia are terminal;may occur solitary or in groups or in simple racemes.The are not associated with sporophylls. 7.The wall of sporangia is thick and several layered thick. Dr. Arindam Kumar Assistant Professor Dept. of Botany JMDPL Mahila College Madhubani 8.Sporangia are homosporous and spores are arranged in tetrad. The genus Rhynia was discovered in 1913 by Kidston and Lang from Rhynie chert. The genus Rhynia is named after the locality and possesses two species.,Rhynia gwynne-vaughani and Rhynia major. Rhynia gwynne -vaughani is a small herbaceous plant and small in size measuring about 18cm high. The plant body consists of aerial cylindrical slender stem arising from basal rhizome. Rhizome bears tufts of rhizoids on the underside. Roots are lacking. The rhizoids perform the dual functions - absorption and anchorage . The aerial stem is dichotomously branched and tapers gradually towards it's apex.It bears s number of hemispherical outgrowths.These outgrowths have often been regarded as rudimentary leaves.The aerial shoots may end in pointed tips or they may bear oval sporangia.Stomata are present all over the surface of the aerial shoots.Adventitious branches also arose from the aerial shoots.According to Mercker (1955,1959),the rhizomatous part of the plant represent the gametophyte and the upper part is the sporophyte.Mercker given two evidences in his support. 1.The presence of some flask shaped cavities in subterranean creeping axis.These cavities have been regarded as disintegrated sex organs. 2.Presence of groups of four cells having opening in the centre resembles the surface view of embedded archegonia. Anatomy of the stem. The outermost layer is epidermis,which is interrupted at places by the presence of stomata.Epidermis is cutinized.Cortex is next to the epidermis and differentiated in two layers ,outer and inner cortex.The cells of the outer cortex are larger than the epidermis.The cells of inner cortex have smaller size and intercellular spaces.The inner cortex was most probably the seat of photosynthesis. Dr. Arindam Kumar Assistant Professor Dept. of Botany JMDPL Mahila College Madhubani The centre of the stem is occupied by a vascular strand with very simple organization.Xylem is made up of xylem tracheids having annular or spiral thickenings.It has been observed in certain that the tracheids occupying the centre are smaller in size than the outer surrounding them.This reflected the endarch differentiation of the xylem.The cells surrounding the xylem tissue are thin walled and without any intercellular spaces.The cells are elongated with oblique end walls.They appear to be steler in nature and very much similar to phloem.There are no traces of sieve plates or pores in the cells of phloem.Structure of phloem differs from the structure of phloem of the living Pteridophytes. In the rhizome ,there is no distinction between the outer and inner cortex.The xylem strand consists of fewer tracheids. There is no apical cell .The growing point consists of numerous dividing cells. Sporangia. Sporangia are club shaped or cylindrical structures borne terminally on the fine aerial branches .Wall of sporangium is three layered thick - an outer epidermal layer,middle layer is three layered thick and an inner layer.The outer layer is heavily cutinized.Large number of spores are arranged in tetrads.Arrangement of spores in tetrads are very much similar to modern ferns.The spores are homosporous .The development of sporangium was eusporangiate type ,it developed from a group of superficial cells that divided periclinally to form an outer primary wall layer and an inner layer of primary sporogenous cells. Rhynia major Rhynia major is a leafless plant. The plant body is distinctly differentiated into rhizome and an aerial portion. Rhizome is subterranean and bears tufts of rhizoids. Aerial portion branches dichotomously .Aerial portion lack any adventitious branches . Sporangia develop at the ends of the branches. Dr. Arindam Kumar Assistant Professor Dept. of Botany JMDPL Mahila College Madhubani Discussion: The Rhyniaceae shows very simple structure and the have been placed by different botanists in three of the great sub- kingdoms of plant. Kidston and Lang assigned them to the Pteridophytes because the spore bearing plant was a vascular plants. Dr. Newell Arber referred the genus to Thallophytes on the ground of its undifferentiated thalloid stem. The family Rhyniaceae has also been included under Bryophytes due to thallus like plant body. The family Rhyniaceae bear resemblances with the Psilotales which include living genera. The aerial stem of Rhynia and Psilotum, living genera, are in the same fashion. They are dichotomously branched and lack leaves. There are no roots. The rhizome is underground and bears tufts of unseptate rhizoids. The fructification of Rhyniaceae suggest a close relationship with the Bryophytes. Primitive structures in Rhynia 1.Plant body is differentiated into rhizome and aerial stem. Rhizome bears tufts of rhizoids. Aerial portion may bear adventitious branches .Plant body lacks root and leaves. 2.Aerial portion is dichotomously branched. Dichotomous branching is a primitive character. 3.Sporangium is unilocular, homosporous, and terminal in position. All these conditions are primitive characters. 4.Aerial portion is photosynthetic, leaf is absent. Dr. Arindam Kumar Assistant Professor Dept. of Botany JMDPL Mahila College Madhubani 5.Internal structure of aerial stem is a protostele, protostele surrounded by metaxylem. There is distinct epidermis ;cortex divided in two zones - outer and inner .Outer cortex do not have intercellular spaces. Pith is absent. Protostele is a primitive character. 6.Sporangium is unilocular, terminal and homosporous. Sporangium is showing primitive character. Phylogenetic importance. Recent views believe in the origin of both Bryophytes and Pteridophytes from a common stock and most likely from Algae. Bryophytes do not have vascular system. Hence Bryophytes cannot be considered as progenitor of higher plants. Bryophytes and Pteridophytes form parallel branches of the evolutionary tree stemming from a common green algal ancestor. It is believed that such an ancestor belonged most probably to the heterotrichous Chaetophoales. Very simple plant body of Rhynia with vascular system suggests that vascular plants might have originated from Rhynia like ancestor. Dr. Arindam Kumar Assistant Professor Dept. of Botany JMDPL Mahila College Madhubani .
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