Structure of Rhynia and Williamsonia

Rhynia:-

1. Systematics:-

Division:- Psilophyta

Class:- Psilophytopsida

Order:- Psilophytales

Family:- Rhyniaceae

Genus:- Rhynia 

2. Introduction:- 

> The fossils of the genus Rhynia were discovered by Kidston and Lang in 1917,1921 from the Rhynie locality (chert) of Aberdeenshire in Scotland. (in middle Devonian Era)

> According to Parihar (1996) the chert deposites at Rhynie, originally thought to be of middle Devonian are now believed to be lower Devonian (Late Siegenian or Emsian epoch – about 390 to 374 million years ago. 

> The specimens are so well preserved that they give detailed information about the form and structure of this very primitive vascular plants. 

> Rhynia is a single- species genus of Devonian Vascular plants. 

3. Study of Sporophyte:- 

> Kidston and Lang (1951), thought and supported by estimony that in those times the plants grew in swampy marshes near the volcanoes. 

> The genus is named after the locality and possess two species: 

i. Rhynia gwynne vaughani 

ii. Rhynia major 

> R. major & R. gwynne vaughani were herbaceous plants. 

> R. major was larger than R. gwynne vaughani.

> The plant was differentiated into horizontally cree- ping rhizome & an upright branched shoot without - leaves. There were rootless.

> The aerial stem of R. major were around 50 cms high and 1.5 mm to 6 mm in diameter , whereas, the corresponding structures of R. gwynne vaughani were 20 cms in height & 1 to 3 mm in diameter. 

>  The aerial branches were cylindrical, naked, leafless, dichotomously forked & tapering at their apices. 

> The aerial branches end in tapering vegetative apices or bore pear – shaped terminal sporangia. 

Plant body of Rhynia gwynne vaughani

Plant body of Rhynia major 

> Internal structure:- The internal organization (anato-my) of the rhizome as well as of the aerial stem was practically similar in both species of Rhynia except that some of the slenderest twigs of R. gwynne vaughani were destitute of any vascular structure. In the anatomy of the aerial stem are as followed:

a. Epidermis:- 

- It was the outermost single- layered envelope covered by a thick layer of cuticle. 

- The epidermis of the aerial shoot was interrupted by stomata. 

- Each stomata have two guard cells, therefore, the rhizome didn’t contain any stomata. 

- The rhizome possessed unicellular rhizoids on it’s surface. 

b. Cortex:- The epidermis was followed by a well organised and make broad zone at cortex. The cortex is differentiated into two zones: 

i. Outer cortex:- 

- The outer cortex consisted of 1-4 cells layer of compactly arranged polygonal paranchy-matus cells without any inter-cellular spaces. 

- This region perhaps represents hypodermis. 

ii. Inner cortex:-

- Inner to the hypodermis, there was broad zone of inner cortex.

- The inner cortex was composed of spherical parenchymatous cells with large intercellular spaces which maintained continuity with the outer atmosphere through the stomata present in the epidermis. 

- This indicates that the inner cortex was the chief photosynthetic region. 

c. Stele / Central cylinder:-

-  A protostele was present in the central part of the axis as well as rhizome.

- The xylem was surrounded by the phloem. 

- The xylem was composed of only tracheids with annular or spiral thickening. 

- The phloem was represented by 4 - 5 layers of thin walled elongated cells with oblique end walls. 

Williamsonia:-

1. Occurrence of Williamsonia:-

> Williamsonia belongs to family Williamsoniaceae of Bennettitales. It has been reported from Upper Triassic period but was more abundant in Jurassic. This was earlier discovered under the name Zamia gigas by Williamson (1870) but has now been named as Williamsonia.

> Professor Birbal Sahni (1932) described W. sewardiana from Rajmahal Hills of Bihar (India). Professor AC. Seward, a well-known palaeobotanist, described W.scotti. Gupta (1943) discovered Williamsonia sahnii from Rajmahal Hills and named if after Professor Birbal Sahni.

> Other reported species from Rajmahal Hills are Williamsonia indica, W. microps and W. santalensis. Bucklandia indica, described from Rajmahal Hills, is now considered to be the stem of Williamsonia sewardiana.

2. External Features of Williamsonia:-

> Williamsonia resembled Cycas in appearance, and its best known species is W. sewardiana. A reconstruction of this species was published by Sahni (1932). The leaves of W. sewardiana were like that of Ptilophyllum. The plant had an upright, branched and stout stem covered by persistent leaf bases.

> A terminal crown of pinnately compound leaves was present. For the stem genus Bucklandia, Sharma (1991) opined that features of leaf bases such as their shape, size and arrangement pattern are of taxonomic significance.

> He observed that leaves in Williamsoniaceae show syndetocheilic stomata with rachis possessing collateral endarch vascular bundles arranged in a double U-manner. A distinct constriction was present at the base of lateral shoots.