In this article we will discuss about Williamsonia. After reading this article you will learn about: 1. Occurrence of Williamsonia 2. External Features of Williamsonia 3. Reproduction.
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.
External Features of Williamsonia:
Williamsonia (Fig. 6.13) 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 as shown in Fig. 6.13.
Reproduction in Williamsonia:
The fructifications of Williamsonia were large and attained a diameter of about 12 cm. They were borne on a peduncle. Many spirally arranged bracts were present around the base of the floral axis. In W. gigas the cones were present among the crown of leaf bases while in W. sewardiana they were present on the short lateral branches. Williamsonia plants were unisexual.
The female ‘cones’ of Williamsonia gigas and W. sewardiana have been investigated in detail. Instead of ‘strobili’ or ‘cones’, Sporne (1965) has proposed to use the term “flower” in Williamsonia. The structure of female flower of W. gigas is illustrated in Figs. 6.14, 6.15. The conical receptacle was surrounded by many perianth-like bracts. The ovules were stalked.
The apex of the receptacle was naked and sterile. The nucellus was surrounded by a single vascularize integument, which was fused with the nucellus. The nucellus had a well-marked beak and a pollen chamber. In young ovules the micropylar canal was long and narrow.
In mature ovules, the canal widened because of the formation of nucellar plug and disappearance of interlocking cells. In the apical part of endosperm, Sharma (1979) observed 2 or more archegonia.
Out of several known male flowers of Bennettitales some have been described to belong to Williamsonia. Male fructifications have never been found in actual connection with the plant, and are sometimes referred to the genus Weltrichia.
Male flowers consisted of a whorl of microsporophyll’s, which were united to form a more or less cuplike structure. In majority of the investigated species (e.g. Williamsonia whitbiensis) the sporophylls were un-branched (Fig. 6.16) but in some species (e.g. W.spectabilis) they were also pinnately branched (Figs. 6.17, 6.18).
Sitholey and Bose (1953) discovered Williamsonia santalensis from Upper Gondwana (India), and observed that microsporophyll’s in the species were bifid. One of the branches of microsporophyll was fertile while the other was sterile. The fertile part had finger-like structures called synangia Each synangium had two rows of chambers enclosing microsporangia. Sharma (1977,1983, 1991) has confirmed the synangiate nature.
The entire male flower attained a length of about 20 cm., while a single microsporophyll was about 10 cm long. The fertile branch of the bifid sporophvll possessed many purse-like capsules, in each of which there were present many monocolpate pollen grains.