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Characteristic Features and Classification of Stoneworts !
There are 6 genera and 250 species which are living, rest are fossils. The living members of this order are known as ‘stoneworts’.
Features
1. The stoneworts usually occur in still and clear waters in attached condition to the mud of the bottom of the pools. They are found in less oxygenated water and best survive in clear and hard waters.
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Calcium carbonate is deposited on the plant body and the surface of the plant becomes rough.
Image Curtsey: http://www.bio.miami.edu/dana/pix/dodder.gif
2. The thallus is attached to the mud by a rhizoidal system. The plant body is erect and possesses nodes and internodes. Secondary laterals, also called ‘leaves’ arise from the nodes which are of limited growth. The leaves may or may not be differentiated into nodes and internodes.
3. The reproduction takes place by vegetative and sexual methods. Asexual reproduction is altogether absent. Vegetative reproduction takes place by means of special vegetative bodies such as amylum stars, bulbils, secondary protonema, etc. Sexual reproduction is oogamous and takes place by oogonia (nucule) and antheridia (globule).
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4. The zygote nucleus divides reductionally producing 4 haploid nuclei. Out of these 4 haploid nuclei one is functional and rest degenerate. The functional nucleus divides into two cells, the lower cell is rhizoidal and the upper one gives rise to main thallus.
Classification:
According to Dr. F. E. Fritsch, this is last order of Chlorophyceae. There is a single family Characeae. G M. Smith has divided his division Chlorophyta in two classes, i. e., 1. Chlorophyceae and 2. Charophyceae. He includes his single order Charales and family Characeae along with other three families in class Charophyceae. He had included all the living genera in the family Characeae. There are about six genera and 250 species in this family.
There are also Characeae known only as fossils. The remaining three families are all fossil. According to Peck (1946) the Charales are known from as for back as the Devonian. The Characeae are known from as far back as the Upper Carboniferous. Another family the Trochilisceaceae, is known only from the Lower Carboniferous and from the Devonian.
