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There are six orders in Xanthophyceae classes, i.e.:
1. Heterococcales and 2. Rhizochlo- ridales, 3. Heterocapsales, 4. Heterotrichales, 5. Heterococcales and 6. Heterosiphonales.
1. Heterochioridales
(i) The plant body is flagellated, naked and unicellular.
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(ii) Each cell contains two or more discoid or band-shaped chromatophores and one or more contractile vacuoles in the protoplast.
(iii) Reproduction takes place by cell division and in one genus statospores are formed.
(iv) There are 9 genera and about 15 species; 4 genera are marine and rest are fresh water in habit.
(v) Important genus-Chloromeson.
2. Rhizochloridales:
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(i) The plant body is amoeboid in pseudopodia. Sometimes several individuals may become jointed to one another by cytoplasmic bridges, mostly naked or partially surrounded by the lorica that may be attached to substratum by stalk.
(ii) Reproduction takes place by cell division, zoospores, aplanospores or statospores.
(iii) The protoplast contains one to many chromatophores. They may be uninucleate or multinucleate.
(iv) There are 7 genera and about 10 species. Majority of them are fresh water.
(v) Important genus-Chlorachnion.
3. Heterocapsales:
(i) The plant body is palmelloid, i.e., gelatinous colonies are found which may be amorphous or dendroid.
(ii) The cells have the capacity to return directly in the motile condition.
(iii) Reproduction takes place by means of fragments, akinetes and zoospores. The zoospores may divide into new zoospores.
(iv) There are 8 genera and about 90 species. All are fresh water in habit.
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(v) Important gemis-Gloeochloris.
4. Heterotrichales:
(i) The plant body is simple or branched filament.
(ii) Reproduction takes place by means of fragementation, akinetes, zoospores, aplanospores and hypnospores.
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(iii) Physiological isogamy is found in one genus, i.e., Tribonema.
(iv) There are 8 genera and about 35 species. All are fresh water in habit.
(v) Important genus-Tribonema.
5. Heterococcales:
(i) They are nonfilamentous and incapable of returning directly to a motile form.
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(ii) The cell may be uninucleate or multinucleate. In majority of cases, the wall consists of two halves.
(iii) The plant body consists of coccoid or colonial (gelatinous) forms.
(iv) Reproduction takes place by means of zoospores and aplanospores. The aplanospores are autospores. They have no capacity of vegetative cell division.
(v) There are 45 genera, and about 270 species. All are fresh water in habit.
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(vi) Important genus-Botrydiopsis.
6. Heterosiphonales:
(i) All the members are multinucleate, siphonaceous and unicellular in structure.
(ii) Asexual reproduction takes place by means of zoospores, aplanospores and hypnospores. These bodies are coenocytic.
(iii) Sexual reproduction is isogamous.
(iv) There are 3 or 4 fresh water genera.
(v) Important genera-Botrydium and Vaucheria.
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Here the genus Vaucheria of order Heterosiphonales has been described in detail.
