This article throws light upon the three sub-classes of Class Filicineae. The sub-classes are: 1. Subclass I Primofilices Archaeopteris 2. Subclass II Eusporangiatae 3. Subclass III Leptosporangiatae.
Subclass I Primofilices Archaeopteris:
Archaeopteris is one of the most ancient plants, known only from the Devonian and that also from the Upper Devonian. The genus is based on leaf compressions. Recently, the leaves have been found attached to the Callixylon type of stem. The reconstruction by Beck (1962) shows that the plant was a tree of ex-current habit with an apical crown of branches much as living pines.
The leaves are large and possess a rachis bearing almost opposite pairs of pinnae and a pair of stipules at the base of the rachis. A pinna, in its turn, bears a number of pinnules which are arranged in an alternate fashion. The pinnules on a pinna may be either all sterile or all fertile, or there may be an admixture of the two. The sterile pinnules are flat and oval or wedge-shaped with repeatedly dichotomous venation, which extend up to the entire or incised apices.
The sporangia are borne in a psilophyte-like fashion on the tips of branched or unbranched pedicels, which are borne adaxially on a fertile pinnule. The spores in one species can be differentiated into micro- or mega-spores and both kinds of sporangia are situated on the same pinnule.
In size, the megaspores are about ten times in diameter than the microspores. It is interesting to note, however, that both the micro-and mega-sporangia are approximately of the same length, but the latter are about double the former in breadth. A single microsporangium contains more than one hundred microspores, while a mega-sporangium contains only eight or sixteen megaspores.
Recent studies by Beck (1960, 62) show that Archaeopteris was neither a primitive fern nor a fern ancestor. The anatomy of the stem and the morphology of the leaf suggest that the plant was a ‘progymnosperm’.
Subclass II Eusporangiatae:
The Order Ophioglossales includes a single family Ophioglossaceae, popularly known as ‘Adder’s tongue ferns’. It consists of three living genera only, of which Ophioglossum and Botrychium are cosmopolitan in their distribution, while the monotypic genus Helminthostachys is endemic to the Indo-Malayan region. This Order is regarded as a very primitive one.
Subclass III Leptosporangiatae:
The members of the Filicales or ‘true ferns’ are world-wide in their distribution. Like the eusporangiate ones, they also inhabit warm, moist and shady places, and the majority of them are mesophytes or hydrophytes, although xerophytic types are by no means uncommon.
These are usually perennial plants. The Order is characterized by homospory. The sorus may be simple (all sporangia developing simultaneously), gradate (sporangia developing in a basepetal succession), or mixed (sporangia developing in an irregular sequence).
There are a number of Families included under the Order with about 300 genera and 9,100 species. Geological records show that the members of this Order are comparatively younger than other groups of ferns and date back from the Permian and in all probability reached their climax in the Lower Cretaceous.
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