Chlorophyta

Flagellar apparatus of most green algae are characterized by a cruciate root system. A cruciate root system consists of four roots spreading out more or less evenly from the basal bodies, and with opposite roots usually possessing identical numbers of microtubules. As in most Chlorophyta two of the roots are two-stranded, the general arrangement of microtubular root follows the X-2-X-2 system, with X varying from three to eight microtubules. The root system will be described for each class of this division, selecting when possible a genus representing the morphological cell type within the class.

The diversity of flagellar apparatuses of Prasinophyceae is unique among the Chlorophyta. The root system is cruciate with a 4-2-4-2 system in the tetraflagellate Pyramimonas and Tetraselmis, a 4-2-0-0 system in Mantoniella squamata, due to the reduction of one of the two flagella, and to its lack of roots, a 4-3-0-(7-12) system in Nephroselmis, and a 4-6-4-6 system in Mesostigma.

When four microtubular roots are present, as in Pyramimonas, they extend up the sides of the flagellar pit towards the anterior of the cell, where they join the microtubules of the cytoskeleton, which radiate from the flagellar region below the membrane. An elaborate system of non-striated fibers connects the basal bodies, which are associated with one or more rhizoplasts. These contractile structures extend from the proximal ends of the basal bodies down to the chloroplast, where they branch over the chloroplast surface and get in contact with it. Some tetraflagellate members of this class possess a synistosome, a fibrous band longitudinally striated connecting two of the basal bodies, and an asymmetrical structure termed lateral fibrous band, which forms an arc on one side of the four basal bodies. Proximal connective fibers may be present between the basal bodies.


The microtubular root system of flagellate Chlorophyceae has the X-2-X-2 pattern except in the stephanokont reproductive cells of Oedogoniales. The roots diverge from the basal bodies and run beneath the cell membrane towards the posterior of the cell. Fibrous roots are generally present and associated with the two-membered microtubular roots. Rhizoplasts extend from the basal bodies to the nucleus. The basal bodies are connected by a robust upper striated connective and two lower striated connectives (Figure 2.57). In the stephanokont zoospore of Oedogonium sp. the flagellar bases are connected by a transversely striated fibrous band running around the top of the zoospore above the flagella. Three-membered microtubular roots perpendicular to the basal bodies depart from them and extend towards the posterior of the cell. Other striated components are present in close association with the microtubular roots.


The microtubular rootlets of the flagellate reproductive cells of Ulvophyceae follow the X-2- X-2 arrangement of most green algae, with X = 4 in Ulva and Enteromorpha, and X = 5 in Ulothrix. Striated bands connect the rootlets to the basal bodies, which are connected anteriorly by an upper connective; in some genera, additional striated bands between the basal bodies and striated component associated with the two-membered root can be detected.

Members of the Cladophorophyceae generally conform to the X-2-X-2 system, with X = 3 in Chaetomorpha and X = 4 in Cladophora. The microtubular rootlet system of the biflagellate cells extend posteriorly nearly parallel to one another and to the basal body from which they arise. In the X-membered rootlets, an electron-dense strut or wing connects one of the uppermost microtubules to the subtending singlet, and the entire rootlet is usually subtended by a massive, more or less striated structure. Also the two-membered rootlets may be accompanied by such structures. An upper transversely striated connective links the basal bodies.

The flagellar apparatus of the reproductive cells of the Bryopsidophyceae is anchored in the cell by four microtubular roots following the usual X-2-X-2 pattern. Each of the microtubules in the rootlet may be subtended by an electron-dense wing. The basal bodies are connected anteriorly by a non-striated upper connective, with a typically pronounced arched appearance. Transversely striated bands connecting the rootlets to the basal bodies are also present. Species exist in this class, which produce stephanokont zoospores with more than 30 flagella. In these cells, the basal bodies are connected by a non-striated fibrous upper ring, which can be considered the result of the fusion of many non-striated upper connectives. The proximal ends of the basal bodies are partially enclosed in a second lower ring of amorphous material. Four- and six-membered microtubular roots depart from between the basal bodies.

No flagellate reproductive cells are present in the Zygnematophyceae.

The microtubular root system of the Trentepohliophyceae motile cells (tetraflagellate zoospores and biflagellate gametes) do not follow the X-2-X-2 pattern, but show a 6-4-6-4 arrangement in Trentepohlia sp. The dorsiventrally compressed form of the flagellate cells forces the basal bodies and the root system components in a flattened arrangement. Basal bodies are aligned perpendicular to the long axis of the cell, with a parallel or antiparallel arrangement, and the microtubular rootlets, especially in the gametes, extend posteriorly nearly parallel to one another and to the basal body from which they arise. Two of the rootlets (the four-membered rootlets in Trentepohlia sp.) are associated with a complex columnar structure resembling the multilayered structure of the Charophyceae.

Biflagellate cells of Klebsormidiophyceae such as Chaetosphaeridium sp. and Coleochaete sp. are characterized by a unilateral construction, in which the two equal flagella emerge on one side of the cell, below the apex. A transversely striated connective links the basal bodies. The system anchoring the flagella in the cell consists of a single lateral root of about 60 microtubules, which extend from the basal bodies along the cell side down to the posterior. At the level of the basal bodies this broad root enters a multilayered structure in which the microtubules are located between two laminate plates.

In the biflagellate male gametes of the Charophyceae the basal bodies are connected to each other by a conspicuous fibrous linkage; the root system consists of a main broad band of microtubules and a small secondary root.

The root system and the associated structures of the biflagellate motile cells of Dasycladophyceae is scarcely distinguishable from those of the Cladophorophyceae. The X-2-X-2 pattern is present, with the basal bodies and microtubular rootlets showing a flattened arrangement. A striated distal fiber connects the proximal ends of the basal bodies, from which two prominent rhizoplasts depart. A strongly suggested reading on this topic is the review by Moenstrup (1982).