The thallus is made of many cells forming either the colonies or filaments.
In these algae many cells are grouped together to form a colony. The colony may be motile due to the presence of flagella (e.g., Volvox, Gonium, etc.) or may lack flagella and hence remains non-motile (e.g., Pediastrum,Hydrodictyon).
In some of these colonies, the number of cells remain constant throughout the life; they are called coenobium; c.g., Volvox
There are non motile colonies in which the number of cells is not definite; these are called coccoid; e.g., Hydrodictyon.
In these multicellular algae, the cells are arranged in a series, one over the other to form a filament. The filaments may be branched (e.g., Cladophora, Polysiphonia;
In some filamentous algae the thallus has two distinct systems prostrate or creeping that helps in attachment and erect or projecting which is green and hence photosynthetic. Such a thallus is known as heterotrichous. Fritschiella, a hetero- trichous alga, growing in moist soil is considered to be a link between aquatic habit and land habit.’ It is perhaps the seat of origin of land plants.
In many algae, thalli are aseptate and thus nuclei remain scattered through out the tubular thallus. Such a multinucleate thallus is known as coenocytic; e.g., Vaucheria
It is a thallus where the filaments are repeatedly branched and the branches are in close aggregation, giving an appearance of a parenchymatous organization. Such a thallus with a single large central (axial) filament is called uniaxial (e.g, Batrachospennun) and those with several axial filaments is known as multiaxial (e8, Polysiphonia).
In many algae (brown algae) parenchymatous thallus is formed due to extensive cell divisions. These thalli show elaborate internal differentiation; e.g Fucus, Laminaria.