In this article we will discuss about:- 1. Distribution of Phylloglossum 2. Anatomy of Phylloglossum 3. Vegetative Multiplication 4. Spore Production 5. Phylogeny.
Distribution of Phylloglossum:
The genus Phylloglossum unlike Lycopodium has a very limited distribution. In fact its entire distribution is confined to a few places in Australia, New Zealand and Tasmania. The genus is monotypic having a single species P. drummondii.
The plant body of Phylloglossum is very small when compared to that of Lycopodium. It so much resembles the protocorm found in some species of Lycopodium, that it is very often called the permanent protocorm of Lycopodium.
The plant is perennial in its habit. It has an underground tuber which gives rise to roots and leaves. The plant body may reach a height of 10 cm. To help in perenneation the tuber persists underground, while all the other parts die during unfavourable season.
At the favourable season every year the tuber produces a short stalk, bearing quill like leaves. The axis at its base also produces one to three un-branched roots. The tip of the axis bears the leaves. The axis may also produce a short leafless branch which terminates in a young tuber (Fig. 53).
Anatomy of Phylloglossum:
Anatomically the stem shows a siphonostelic organisation. The vascular cylinder which traverses the entire length of the stem disappears in the tuber. The siphonosteles has two groups of phloem (Amphiphloic) (Fig.54).
Vegetative Multiplication in Phylloglossum:
Perenneation of the plant body is achieved by underground tuber which persists in the soil long after the aerial parts wither away. The surviving tuber produces the leaves at the return of favourable season. Vegetative multiplication is also brought about by young tubers produced at the tip of a short leafless branch. Occasionally the tuber bearing branch may have leaves.
Spore Production in Phylloglossum:
Phylloglossum reproduces by means of spores. The spores which are homophonous are produced in sporangia. The sporangia are borne on sporophylls which in turn aggregate to form cones or strobili. The cones are long and slender terminating the stem apex in a fertile plant.
Each cone has a few sporophylls arranged on the cone axis in whorls of three. Each sporophyll bears a sporangium which is nearly axillary in position. In its essentials, the cone resembles that of Lycopodium. Each sporangium is a small globose or ovoid structure. It has a short stalk.
Enclosed in the wall of the sporangium and nearly filling the cavity are a large number of spore mother cells. These undergo reduction division to produce tetrads of haploid spores. The spores are arranged in tetrahedral fashion and are yellow in colour. All the spores are of the same type.
At maturity the sporangium breaks open releasing the spores. The spores are wind dispersed and develop into gametophytes on the availability of a suitable substratum. Unfortunately very few details are available regarding spore germination and the subsequent development of gametophyte, embryo etc.
According to the scanty information available, development of gametophyte and embryogeny are basically similar to what is seen in Lycopodium. The gametophytes as per the reports are very minute growing on soil rich in organic matter.
They are found on the surface of soil and are green in colour. This is indicative of their nutritional independence. Photosynthetic gametophytes of Phylloglossum resemble those found in a few species of Lycopodium.
Phylogeny of Phylloglossum:
Even though Phylloglossum has a highly reduced plant body its affinity to Lycopodium has never been contested. In its eligular leaves and a similar cone organisation Phylloglossum is closer to Lycopodium than to any other genus. The resemblance to the protocorm of some species of Lycopodium is indeed so close, that very often it is referred to as a ‘permanent embryonic form of Lycopodium’.
Lepidodendrales:
Among the extinct early vascular plants, the Lycopods are outstanding for various reasons. In their arborescent habit, compound leaves and a mega sporangium which almost reached a seed habit the lycopsids are truly a morphologist’s joy.
Among the ancient lycopods, the lepidodendrales are the most significant not only because of their excellent preservation but also because of the structural diversity that they exhibit. The lepidodendrales formed the predominant flora during the early palaeozoic.
Most of the genera were tree like with a bushy foliage true to their popular name the “giant club mosses”. The stem had secondary tissues produced as a result of the activity of the cambium. The leaves were ligulate, microphyllous and deciduous.
The root system was of the type known as stigmarian root system, it consisted of four horizontal branches (Fig. 55) radiating from the base of the stem. These in turn branched dichotomously, ultimately forming the rootlets.
In their reproductive features the lepidodendrales showed advancement to some extent over the extant lycopods.
The order consists of four families namely Lepidodendraceae, Lepidocarpaceae, Borthodendraceae and Sigillariaccae.