The whole range of the somatic structure of algae may be divided into following important type!
1. The motile type.
2. Palmelloid and dendroid types.
3. Coccoid forms.
4. Filamentous habit.
5. Siphonous habit.
6. Advanced type.
1. The motile type:
The simplest type of body is motile and unicellular. The shape varies from species to species. These bodies are flagellate. The number and type of flagella also vary. The body may be naked or with a cell wall. There are many variations in the motile type. These variations may be, amoeboid, encapsulated colourless forms, colonial forms and double individual; e.g., Volvox, Eudorina, Chlamydomonas, etc.
2. Palmelloid and dendroid types:
For a time being palmella stage prevails in the species of Chlamydomonas. But this is a permanent feature in the case of Tetraspora; here the plant bodies are surrounded by mucilaginous covering. The dendroid type is the variant of the palmelloid type in which mucilage is produced at the base of the cell, and the dendroid colonies are resulted; e.g., Prasinocladus of Chlorophyceae.
3. Coccoid habit:
In coccoid forms the flagella have been lost and the plant body becomes rounded. This rounded body has no power of division and cannot reproduce vegetatively. Coccoid habit is very common in order mollusc shell Chlorococcales, e.g., Chlorococcum humicola.
4. Filamentous habit:
According to Blackman and Smith, the filaments originate from motile unicellular forms. According to these authors, the filamentous habit has been originated from motile unicellular bodies by the loss of motility and the restriction of vegetative division in one plane only. In the beginning, the cells were united by mucilaginous discs (false septation) but later the transverse septa of adjacent cells decayed and union between the cells of the filament took place.
The view was supported by Dr. F.E. Fritsch. The filamentous forms possess several variations. The filaments may be simple, e.g., Ultothrix, Spirogyra, heterotrichous, e.g., Stigeoclonium; foliaceous, e.g., Ulvcr, tubular, e.g., Enteromorpha; discoid, e.g., Coleochaete. The filaments of Ulothrix and Spirogyra are simple and unbranched. The filaments of Stigeoclonium are heterotrichous, i.e., the plant body consists of two systems, one erect and the other prostrate.
The prostrate system is comparatively smaller and possesses many erect branches. Ulva is a good example of foliaceous variant. The divisions take place in all the planes and a foliaceous structure is developed. The thallus is two celled thick. In tubular variant, the thallus is a hollow tube with a wall one cell in thickness, e.g., Enteromorpha. The discoid type has been evolved from heterotrichous filament. Coleochaete scutata is the well known example of this type. The cells of
Coleochaete are joined end to end in branching filaments and a discoid habit is attained. The other species Coleochaete pulvinata possesses free branches.
5. Siphonous habit:
The plant body consists of branching filaments having many nuclei and no partition walls. The cross .walls appear only at the time of the formation of reproductive bodies. The well known examples of this type are – Vaucheria, Codium, Caulerpa, etc., of order Siphonales.
6. Advanced type:
The advanced types may be uniaxial filamentous type, multiaxial filamentous type and parenchymatous type. In uniaxial filamentous type, the single main axial thread has close branch systems to form more or less compact pseudoparenchymatous thallus, e.g., Dumontia of Rhodophyceae. In the multiaxial filamentous type, there is a number of axial threads and the branches and all of them form a compact cortex, e.g., Scinaia furcellata of Rhodophyceae. Many advanced types are true parenchymatous, e.g., Laminaria, Fucus, Sargassum of Phaeophyceae.