Range of thallus organisation in bryophyta
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Answer:
The range of thallus variation is different in different bryophyte species. EXPLANATION: It may be a single layer of cells in some species whereas it may be a complex organisation of multiple layers of cells in some bryophytes. The branching of a bryophyte may be regularly frond like, digitate, or completely irregular.
Range of Thallus Organization
Seaweed thalli vary from a few millimeters to ∼100 m, from unicellular to multicellular organization. Cell size ranges from a few micrometers to one or more centimeters long, eg, Valonia. These large cells are restricted to green seaweeds, where a single cell can contain several nuclei and organelles (siphonous and siphonocladous condition) in order to ensure the production of proteins are sufficient to sustain cellular function and rapid growth of the seaweed as a whole, eg, Codium (Fig. 3.1E) and Acetabularia commonly known as “Mermaid’s wine glass” (Fig. 3.1H). The different types of thallus organization in seaweeds are:
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Filamentous: This type of habit is formed through vegetative divisions that occur mainly in a transverse plane. In a filament the row of cells is joined end to end in a transverse plane through the middle lamella. Most of the filamentous seaweeds are branched and heterotrichous∗ (eg, Chaetomorpha, Ceramium, Polysiphonia, and Ectocarpous) (Fig. 3.1).
∗A highly evolved heterotrichous thallus consists of two systems: a prostrate creeping base and an erect branched upright system. During its development, a system of branched creeping filaments is produced first and functions mainly as the holdfast. These creeping system filaments will further give rise to a system of erect and branched filaments.
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Cylindrical: The thallus forms through vegetative divisions that occur mainly in a transverse plane but in a circular manner. As in normal filamentous forms, the rows of cells are joined end to end in a transverse plane through the middle lamella in cylindrical forms. Most of the cylindrical forms are also branched (eg, Kappaphycus).
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Siphonous and siphonocladous: Enlargement and elaboration of the thallus proceeds in the absence of septa. In these seaweeds, nuclear divisions are not followed by cytokinesis (free nuclear division) and the result is a coenocytic, multinucleated thallus and a siphonous organization. Siphonous organization ranges from saccate (eg, Botrydium) to uniaxial (eg, Vaucheria, Bryopsis) and multiaxial (eg, Codium) forms. Siphonocladous forms are restricted to green seaweeds in which filaments (branched or unbranched) are composed of multinucleate (semicoenocytic) cells. One of the siphonocladous algae Valonia occurs as a single, spherical vesicle up to 10 cm in diameter.
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Flattened or foliaceous: The thalli develop as the primary filament cells divide in all directions and the essential filamentous structure is lost, eg, Porphyra, Ulva, and Enteromorpha (Fig. 3.1).
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Complex: Flat foliose or tubular thalli are formed by the division of cells in two or more planes. It may be as complex as in Sargassum.
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Additional thallus morphologies include: sphere, fan, cup, and ball shaped (Fig. 3.1).
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