In this article we will discuss about:- 1. Salient Features of Anthocerotae 2. Systematic Position of Anthocerotae 3. Affinity 4. Evolutionary Significance.
Salient Features of Anthocerotae:
1. Plants (gametophytes) are thallose with lobed margins and dichotomous branching. The thallus is dorsiventral and there is no internal differentiation of tissues. Rhizoids are present on its ventral surface but without scales or mucilagenous hairs.
2. Large intercellular spaces on the ventral surface of the thallus are filled up with mucilage which open to the external surface by means of narrow slit-like openings and these spaces often contain endophytic algae, like Nostoc.
3. Generally, a single large chloroplast is present in contact with or more or less inventing the nucleus in each cell of the thallus. Each chloroplast has a prominent pyrenoid.
4. All Anthocerotae are chiefly homothallic. Sex organs are deeply embedded in the dorsal surface of the thallus and are not borne on special branches as in many other bryophytes.
5. Antheridia arise in clusters from the hypodermal cells and these remain within a mucilage filled antheridial chamber. The sperms are biflagellate.
6. Archegonia arise singly and are embedded into the thallus and are in direct contact with the vegetative cells, lateral to them, with neck projected upwards.
7. A meristematic tissue is present at the base of the capsule, by the activity of which the capsule has a continued apical growth. Ripe spores are towards the apex and young spore mature cells are below.
8. In the outer layers of the jacket of the capsule there are intercellular spaces and these are in communication with the exterior through stomata. Cells in the parenchymatous portion of the capsule contain chloroplasts.
9. Archesporial tissue is differentiated into alternate blocks of elaters and spore tetrads in the sporophyte.
10. A sterile central axis, known as columella, is present in the sporophyte.
Systematic Position of Anthocerotae:
Among the systematic hepatilogists, Endlicher was the first to recognise Anthoceratae as a distinct group within the Hepatics. Then Verdoon in his classification placed it at the end of the Hepatics, as the second subclass, co-ordinate in rank with the rest of the Hepatics included in the first subclass Hepaticales. This position was also maintained in the classifications of Buch. But, in the classification of Evans it was placed in the fourth order under the Hepatics.
Anthoceros and its related genera have so many characteristic features, viz. indeterminately growing green sporophyte, with its broken up sporogenous tissue, photosynthetic equipment and central columella, which readily distinguish them from the rest of the bryophytes. Hence, Smith placed them under the class Anthocerotae, equal in rank with other Bryophytes (Hepaticae and Musci).
In the modern system of classification Anthocerotae has been placed between Hepaticae and Musci. Bold (1956), however does not believe that the liverworts and related to mosses, and as such he has put them its two divisions, Hepatophyta (containing liverworts, and hornworts) and Bryophyta (containing only the mosses).
Affinity of Anthocerotae:
(a) Affinity with Hepaticae:
The gametophyte of Anthocerotae is very simple and similar to the Hepaticae, though in many respects it differs from the group. It also contains special characters of its own which are not found in any other group of Bryophyta.
The dorsiventral thallose nature of the gametophyte, production of rhizoids on the ventral surfaces, the nature of the sex organs and gametes, etc., are characters allied to Hepatics. The characters by which it differs from Hepatics are embedded sex organs, internal homogeneous cell structure, antheridia developing from hypodermal cells on the dorsal surface of the thallus, etc.
The sporophyte of Anthoceros, on the other hand, is much more advanced than that of hepatics.
The chief points of different between the two are as follows:
1. The base of the capsule is meristematic, which continually adds new cells giving rise to the respective tissues of the growing capsule.
2. The sporangium is differentiated into fact and capsule without seta.
3. Embryo, developed from the fertilized egg, is an octant.
4. The endothecium gives rise to the sterile columella, and the amphithecium gives rise to sporogenous tissue and sterile filamentous pseudoelaters.
(b) Affinity with Musci:
The gametophyte of Musci is much advanced than that of Anthocerotae, where the mature of the plant is rather foliose than thallose, which is developed from filamentous protonema. The sex organs instead of being embedded are produced near about tips of the lateral leafy branches or on the main stems.
The sporophyte of Musci is differentiated into capsule, seta and foot-like Anthocerotae, it contains chloroplasts and columella inside the capsule and stomata on its epidermis, and is partially dependent on the gametophyte for nutrition.
Evolutionary Significance of Anthocerotae:
Among this group there is a remarkable association of a primitive gametophyte with a highly developed sporophyte. It is evident that from Riccia to Anthoceros there is a progressive sterilization of sporogenous tissues, the sterilized tissue forming the vegetative structures.
These noteworthy features found in the group are:
1. The beginning of independent sporophyte with the development of green tissue with stomata. In A. fusiformis (the Californian sp.) the capsule attains a length of 6 inches in exceptionally favourable conditions for growth and there was evidence that the surface of the foot, if exposed by the disorganization of the gametophyte, probably absorbed water from the soil.
2. Beginning of localised sporangia by breaking up of continuous sporogenous layer into smaller separate blocks each separated from the other by sterile tissue.
3. Establishment of a sterile axis by transforming the sporogenous tissue from the central portion (endothecium) to the exterior region of the capsule (amphithecium). The chance suggests the beginning of a region for the development of the vascular tissue and superficial sporangia.
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