In this article we will discuss about:- 1. History of Sphenophylum 2. Anatomy of Sphenophylum 3. Reproduction 4. Phylogeny.
History of Sphenophylum:
Sphenophyllales represent a small and compact group of sphenopsida. They have left no surviving representatives. The geological history of Sphenophyllales dates back to a time earlier than carboniferous period. Some of the genera like Sphenophyllum appeared during Devonian reaching maximum development during carboniferous period. A few of these Sphenophyllales survived upto Triassic.
The principal genus of the order is Sphenophyllum. The compressed fossils of the plant body have been found extensively scattered in coal bearing formations belonging to the carboniferous period. The plant body of Sphenophyllum was small and herbaceous. There is a high probability that the plant had a climbing habit. While, some of the morphological features indicate an aquatic habitat, anatomical features however, point out to a terrestrial habitat.
The plant body consisted of a main stem, which was slender and jointed, rarely exceeding 5 mm in diameter. In its externals the stem recalls what is seen in Equisetum. The stem was ribbed which did not alternate at nodes.
Leaves which are so typical of Sphenophyllum were present in clusters at the nodal regions. At each whorl the number of leaves ranged from 6 to 9. Occasionally the number went upto 18. The leaves were cuneate and the apex variously modified. In S. emarginatum, the leaf apex had rounded teeth, while they were pointed in S. cunifolium and S. majus. The leaves had veins with dichotomous branching, each branch terminating in an apical tooth.
Anatomy of Sphenophylum:
Anatomically the stem showed the occurrence of secondary growth. In some of the details of stem anatomy, Sphenophyllum resembled Calamites. The Central pith was completely absent. Instead, the central region was occupied by a triradiate exarch xylem mass. Secondary xylem formed a thick sheath surrounding the primary xylem. Cortical region exhibited a corky tissue indicating the activity of phellogen. The phellogen probably arose deep in the primary cortex.
The plant body was anchored to the soil with the help of adventitious roots that arose at the nodal regions. Anatomically the roots resembled the stems, except that the former had a two angled xylem. Secondary growth was also seen in roots.
Reproduction in Sphenophylum:
Sphenophyllum reproduced by means of sporangia borne on sporophylls. The sporophylls aggregated to form long and slender cones terminating the stem apex. The cones which are often given the name Bowmanites had a central axis on which were arranged many whorls of sporophylls. The sporophylls of each whorl fused at their base to form a saucer like structure.
The sporophylls had two parts namely an adaxial fertile part and an abaxial sterile part. The sporangia were borne on the fertile part either singly or in pairs. In some cases the fertile lobe had two or three branches each having one to two sporangia.
The number and arrangement of sporangia on the lobe are taxonomic features used in the categorization of the species. The sporangia had a stalk which was highly reduced. Most of the species of Sphenophyllum were homosporous. Some species could have been heterosporous.
Phylogeny of Sphenophylum:
The sphenophyllales constituted a characteristic and compact group of plants in the paleozoic area. It is very unfortunate from the phylogenetic point of view that there are no extant representatives of this group. In spite of this, however, their affinity to Equisetales is beyond doubt.
In the habit of the plant body (ridged stem, whorled leaves and strobili) Sphenophyllales clearly recall the present day Equisetum. In the stem anatomy, however, Sphenophyllales differ from Equisetales, in having secondary growth. In this respect they are similar to Calamitales.
One of the unique features of sphenophyllales is the division of the sporophyll into sterile and fertile appendages.
In many of the features-morphological, anatomical and reproductive, sphenophyllales, represent a very highly specialised group which is uncommon to an early era like palaeozoic.
The classification of sphenophyllales however does not pose a major difficulty. All the features clearly point out to the inclusion of Sphenophyllales among other articulates.