Algae

Thallus Organization in Chlorophyta

Being a major algal division, the members of Chlorophyceae show a great diversity in thallus organization starting from primitive unicellular motile forms to parenchymatous forms. Heterotrichous filamentous organization seen in Chaetophorales is considered the advanced habit in this family. The range of thallus organization in this family is summarized as follows.

Motile Unicellular Thallus

The simplest form of motile unicellular thallus organization in Chlorophyta is seen in Chlamydomonas. It has a spear-shaped or elliptical unicell with a single nucleus, a cup-shaped chloroplast, a pyrenoid, an eye spot, contractile vacuoles, and two anteriorly inserted flagella.

Palmella Stage

The Palmella stage results from the failure of the daughter protoplast formed by division in the unicellular forms to resume motility. It stays within a common gelatinous matrix in a group of four. Stabilization of this stage in evolution resulted in the production of forms like Tetraspora.

Coenobium

Coenobium is a motile colony with a definite number of cells arranged in a specific manner. The cells may be interconnected by protoplasmic strands (Volvox). In the colonial forms, there is a gradual increase in the number of cells and therefore the size of the colony as well.

Algae	Number of cells in the colony
Pascheriella	Flattened horse-shoe-shaped colony with 16/32 cells
Chlamydobotrys	8 cells in two tiers
Gonium	Flat colony with 4/8/16 cells
Pandorina	Spherical colony with 8/16 cells
Platydorina	Flattened horse-shoe shaped colony with 16/32 cells
Eudorina	Spherical 16/32/64 cells
Pleodourina	Spherical 32/64 cells
Volvox	Spherical 500/50000 cells

Coccoid Form

Several unicellular green algae lead a sedentary life. Most of them are Chlamydomonas-like but without motility (with the disappearance of contractile vacuoles, eyespot, and flagella). A variety of shapes could be seen in the coccoid forms.

Spherical	Chlorococcum
Ellipsoidal	Oocystis
Lanceolate	Characium
reniform	Nephrocyticum
Acicular	Ankistrodesmus
Triangular	Tetraedron
Echinate	Lagerheimia
Club-shaped	Characiosiphon
Coenobium	Cleastrum, Scenedesmus, Sorastrum, Pediastrum, Hydrodictyon

Filamentous forms

Filamentous condition is advanced over unicellular and colonial forms. According to Blackmann and Smith, the filamentous condition is derived from palmellate ancestors like Palmodictyon.

However, Fritsch considers that filaments evolved from unicells through division in vegetative cells. Vegetative division leads to the formation of filaments.

It is the basis for the structure of high plants too. The simplest filament is seen in Ulothrix. Uniseriate unbranched filaments are seen in Ulothrix, Microspora, and Oedogonium.

If septa arise in more than one plane, we obtain a flattened leaf-like expansion as seen in Prasiola. Prasiola starts as a simple filament and irregular expansion in one place to get a foliaceous habit.

Longitudinal divisions in such habits lead to the formation of two-layered thallus. In Ulva, the two layers of cells are in close contact with each other while in Enteromorpha, the two layers separate at an early stage to form a tubular thallus.

Advancement over simple filaments is obtained through lateral branching as seen in Cladophora and Oedogonium.

Among the branched filamentous thalli, two types of construction could be seen in some, ie, a single branched filament or an aggregate of many branched filaments (uniaxial and multiaxial constructions). Uniaxial construction is seen in Draparnaldiopsis. Multiaxial construction is seen in tubular siphonous forms such as Codium.

Heterotrichous thallus organization in Chlorophyta

Elaborations of the branched filaments lead to heterotrichy in Chlorophyceae with prostrate and erect systems. However, some algae have selectively suppressed one of the two systems by reduced growth.

Aphanochaete	Erect system reduced to hairs
Protoderma	Projecting system suppressed
Chaetophora	The prostrate system completely suppressed
Draparnaldia	Both systems are equally developed
Fritschiella	Both systems equally developed

Siphonous forms

Protosiphon represents the simplest type of siphonous thallus organization in Chlorophyta. The thallus consists of a green and tiny vesicular portion and a columnar rhizoidal portion.

When Protosiphon is grown under dry conditions, the rhizoidal portion does not develop and resembles the Chlorococcoidan cell. This suggests that Protosiphon probably evolved from some members of Chlorococcales.

Halicystis is a marine form with a vesicular rhizome embedded in the calcareous crusts of red algae.

In Valonia, the vesicular thallus is attached to the substratum by numerous unicellular rhizoids.

In Boergesenia, the thallus consists of a broadly elevated vesicular portion which is attached to the substratum using horizontally growing branched rhizoids.

The rhizoidal portion becomes septate by segregative cell division. Some of the cells of the spectate rhizoidal portion enlarge and grow up as vesicles like the parent.

In this manner, some vesicles are formed upon the parent vesicle. Daughter vesicles also similarly produce vesicles and ultimately, a dense cluster of vesicles constitutes the thallus.

Caulerpa is a coenocytic branched thallus, with creeping rhizomes attached to the substratum using rhizoids. The erect portions grow from the rhizome. These erect fronts have various shapes.

In Codium, the thallus is cylindrical, branched, and coenocytic.