The Xanthophyta or Heterokontae are commonly known as yellow-green algae include only one class Xanthophyceae. This division has close relationship with the Ghlorophyta comprising both marine and fresh-water forms. Certain species grow on drying mud, on trunk of trees, on damp walls, and similar other habitat.
There are varied forms of vegetative body ranging from unicellular coccoid, siphonaceous to filamentous condition.
Each vegetative cell has a wall composed chiefly of pectic substances which, depending on species, may be impregnated with silica. The cell contains one to several discoid chromatophores. Except in the siphonaceous forms each vegetative cell is uninucleate. Due to the presence of excess of carotinoids, the colour of chromatophores is yellow-green.
Chlorophyll a and chlorophyll e are present. But chlorophyll b is absent. The chromatophores lack pyrenoids. The usual food reserve is oil. One of the outstanding features of the Xanthophyta is the presence of motile cells bearing two flagella of unequal length.
Vegetative reproduction is by accidental breaking of the vegetative body. Asexual reproduction in most species is by zoospores with a few exceptional cases where aplanospores are formed for asexual reproduction. The zoospores are characterized for having two unequal flagella borne at the anterior end or are multi-flagellate. Sexual reproduction is isogamous and oogamous.
Like the Chlorophyta three major evolutionary lines can be traced among the Xanthophyta.
I. One line may have developed from an unicellular motile ancestry giving rise to non-motile unicells which may be solitary or colonial.
II. Another tendency is to produce a tubular, or siphonaceous form.
III. While the third one is leading to the formation of multicellular filamentous type.
Members of this division of algae were at one time included in the Chlorophyta. Because of their unequal flagella they were classed in the subgroup ‘Heterokontae’ and the others with equal flagella were classified as ‘Isokontae’.
As an increasing number of species were examined, the separate division Xanthophyta was created for the following characters they possess:
1. There is frequently an excess of carotinoids over chlorophylls; moreover, chlorophyll e appears to be restricted to this division.
2. The two flagella are normally of unequal length, being tinsel type and whiplash type.
3. The food storage products are oil and fat, but never starch.
4. A cell wall is frequently absent and when present it generally has a higher content of pectic material than the walls of Chlorophyta.
One order under the single class Xanthophyceae is: Vaucheriales (Heterosiphonales).
Order: Vaucheriales (Heterosiphonales):
Unicellular multinucleate—coenocytic plant body; asexual reproduction by bi- or multi-flagellate zoospores; sexual reproduction iso- or oogamous.
Plant body of unicellular multinucleate vesicular, globose or dichotomously branched aerial portion and a colourless rhizoidal portion; discoid chromatophores; asexual reproduction by biflagellate zoospores, aplanospores or resting spores; sexual reproduction isogamous.
Coenocytic branched tubular thallus; asexual reproduction by multiflagellate zoospores—flagella being distributed throughout the entire body of the zoospore, or by aplanospores; sexual reproduction oogamous.
Order Vaucheriales (Heterosiphonales):
Plants are coenocytic, balloon-shaped to filamentous, with differentiation of rhizoidal and assimilative portions; chromotophores peripheral, small, sometimes with pyrenoid-like structures. Food reserve is oil. Asexual reproduction is by zoospores or aplanospores and sexual reproduction iso-, aniso-, or oogamous.
This family comprises of algae whose plant body is characterized by unicellular multinucleate vesicular, globose or dichotomously branched aerial portion and a colourless rhizoidal portion. Asexual reproduction is by biflagellate zoospores, aplanospores or resting spores and sexual reproduction is isogamous.
It is a vesicular, coenocytic terrestrial alga growing extensively rooted either on drying damp muddy banks of streams and pools or on bare damp soil forming a dense green coating.
The plant body is differentiated into a pear-shaped or lobed aerial portion, also known as assimilator, may be one to two millimetres in diameter, and a branched, root-like underground rhizoidal portion'(Fig. 94A & B). The shape of the aerial portion is considerably influenced by environmental conditions.
The aerial vesicular or lobed portion is covered with a relatively tough wall chiefly of cellulose, within which there is a thin, peripheral layer of cytoplasm containing many nuclei; lenticular or fusiform plastids; oil droplets; and leucosin granules.
The rhizoidal portion may be profusely or sparsely branched and has dense or vacuolated cytoplasm with many nuclei without plastids. When exposed to drought the protoplasmic contents of the vesicle migrate into the rhizoids and divide to form thick-walled globose to ellipsoid coenocysts also known as cysts, which either germinate directly or produce zoospores (Fig. 94C).
In Botrydium tuberosum the protoplast of aerial vesicle migrates into the rhizoidal portion and collects at the ends of the rhizoids which become inflated and swell to develop into rounded thick-walled tubers or cysts (Fig. 94D & E). The alga is incapable of vegetative division.
The common method of reproduction is by the formation of zoospores which are usually produced when the vesicle is flooded or submerged under water. The protoplasm divides into innumerable uninucleate protoplasts. These protoplasts become metamorphosed into pear-shaped zoospores which bear two anteriorly placed unequal flagella.
The method of liberation of zoospores from the mother cell is not definitely known, but it appears that it is accomplished by a gelatinization of apical portion in the vesicular wall (Fig. 94F).
Occasional formation of many biflagellate isogametes or anisogametes which fuse in pairs being liberated from the same vesicle to form zygote indicates that B. granulatum is homothallic (Fig. 94G to I). The zygote germinates immediately into new individual without undergoing any resting period when meiosis takes place (Fig. 95). Formation of parthenospore has also been reported in some species.
Some Indian species of Genus Botrydium:
Botrydium divisum Iyeng.; B. granulatum. Gerv.; B. tuberosum Iyeng.
Special features of Genus Botrydium:
1. Coenocytic balloon-shaped plant body bearing rhizoidal structures.
2. Photosynthetic reserves are oils and leucosin granules.
3. Asexual reproduction by zoospores bearing two unequal flagella.
4. Asexual reproduction also takes place by resting spores.
5. Sexual reproduction isogamous or anisogamous.
6. Gametes biflagellate with unequal flagella.
Plants are filamentous, irregularly or dichotomously branched coenocytic. Filaments are without regular wall formation except to segregate the reproductive organs. Asexual reproduction is by zoospores produced singly in sporangia formed at the tips of branches.
The zoospores are multiflagellate, the flagella in pairs over the whole surface. Sexual reproduction is oogamous. Plants are dioecious or monoecious. The antheridia are cylindrical, generally curved, producing many small biflagellate anterozoids. The oogonia are single or in groups, producing single egg, which is fertilized in place and develops thick walls to form an oospore.
Genus Vaucheria of Family Vaucheriaceae:
This is an alga which has both terrestrial as well as aquatic species. The terrestrial species are often seen to form green felt on the surface of damp soil and on mud of drying pools. Whereas, the aquatic species occur in shallow fresh-water streams, while there are a few marine species.
The thallus consists of a long, tubular, sparingly branched filament, which, in spite of its size, consists of one cell only as it lacks cross- walls. Cross-walls, however, occur only in the event of injury or in connection with the formation of reproductive organs. Attachment of the filament to the substratum is by a hapteron-like structure provided with colourless branched outgrowths known as rhizoids (Fig. 96A).
The filament wall is thin and is composed of an inner cellulose layer and an outer of pectose.
The central portion of the filament is occupied by a large vacuole; the cytoplasm is in a thin, peripheral layer continuous along its entire length. Embedded in the cytoplasmic layer are numerous, small disk-shaped-or elliptical chloroplasts containing a greater proportion of xanthophyll with no pyrenoids, many nuclei, and many oil globules, oil being the reserve food. Chlorophyll b is absent.
But chlorophylle has been detected in the zoospores.
This undivided multinucleate, single-celled filament is known as a coenocyte which may attain a length of several inches. The elongation of the filament is accompanied by repeated division of nuclei without the formation of cross-walls. The filament and its branches grow by terminal elongation.
Vaucheria reproduces both asexually and sexually. The asexual method of reproduction is by means of large multiflagellate zoospores which are formed singly in zoosporangia, cut off at the ends of the tubular thallus by cross-walls. The branch tip’ bulges out forming a potential zoosporangium (Fig. 96B).
A large number of nuclei and chloroplasts along with cytoplasm stream into swollen tip which is cut off by a cross-wall differentiating the zoosporangium from the rest of the filament (Fig. 96G).
The protoplasmic vacuole diminishes in size as a result of which the protoplasm appears deep green in colour. The entire protoplast of the zoosporangium contracts to form an oval mass. The nuclei arrange themselves in the outer layer of the cytoplasm, and the chloroplasts in the central region. Opposite each nucleus arises a pair of flagella.
The mature zoospore escapes through an opening formed by the gelatinization of the zoosporangial wall (Fig. 96D & E). Since this zoospore bears numerous chloroplasts and nuclei, in addition to the many flagella, it is considered a compound zoospore and is also known as a coenozoospore.
After a short period of motility, followed by a subsequent loss of the flagella it germinates and forms one to several tubular outgrowths that may continue to grow indefinitely to produce an adult plant (Fig. 96F & G). The protoplast of the sporangium may also divide to form many small immobile thin-walled aplanospores.
In some terrestrial species, if exposed to greater desiccation, the threads become septate and rows of cysts are formed giving rise to what is termed Gongrosira stage (Fig. 96H). Under favourable conditions these cysts germinate either into new filaments or into small amoeboid structures from which new filaments are produced.
Sexual reproduction is of frequent occurrence in all species of Vaucheria but less frequent in those species growing in flowing water. All species are oogamous. Both fresh-water and terrestrial species are homothallic, while there are quite a few hetero- thallic marine ones. Considering the simple structure of the plant body, the sexual reproduction is of fairly complicated nature.
There is a great variation in the arrangement of sex organs in the different species and even in different individuals of the same species. In homothallic species antheridia and oogonia are generally produced on short branches close together on the filament. The antheridium is a slender, curled hook-like tubular structure which is walled off from the rest of the filament (Fig. 96 I).
The contents of the antheridium consist of cytoplasm, numerous nuclei, and chloroplasts.
During development of sperms, also known as antherozoids or spermatozoids, small portions of cytoplasm surround each nucleus, thus making small pear-shaped bodies which acquire a pair of flagella. At maturity, numerous small male gametes, or antherozoids are formed.
Each antherozoid bears two laterally inserted unequal flagella (one being whiplash, the other tinsel) and consists of a nucleus and a small amount of cytoplasm surrounded by a membrane. When the antherodium is mature, the antherozoids are set free through a terminal pore.
The oogonium is a spherical or oval sessile or short-stalked body with a short, rounded beak which opens to receive the sperms (antherozoids) (Fig. 96 I & J).
The oogonium appears as a globular outgrowth, usually near an antheridium; and is separated from the rest of the filament by a septum. The oogonium consists of cytoplasm, a single rather large nucleus, numerous chloroplasts, and stored food in the form of oil. The contents of the oogonium form a single large spherical egg, or ovum laden with much food.
The cytoplasm opposite to the beak is clear, while in the rest of the ovum chloroplasts are present. When the oogonium is mature, the end of the beak opens slightly and the sperms by chemotaxis, cluster round, swimming in a film of water.
Eventually one passes in and fuses with the nucleus of the egg resulting in the formation of oospore (Fig. 96 J & K). Actual fertilization, the fusion of nuclei may be delayed. Following fertilization the oospore secretes a thick wall and passes through a resting stage, having within it reserve food in the form of oil. The oospore germinates and directly forms a new plant (Fig. 96 L to N).
Gytological behaviour at the reproductive stage has been insufficiently investigated. It is likely, however, that meiosis is zygotic and hence the vegetative filament is haploid (Fig. 97).
Some Indian species of Genus Vaucheria:
Vaucheria aversa Hass.; V. clavata DC.; V. geminata; V. globulifera Westet West; K. orientalis Westet West; V. piloboloides Thur.; V. polysperma; V. sessilis (Vauch) DC.; V. terrestris Randh.; V. uncinata Kutz.
Special features of Genus Vaucheria:
1. Coenocytic filamentous branched plant body attached to the substratum by specialized structure provided with rhizoids.
2. Reserve food fats and oils.
3. Asexual reproduction by means of multi-flagellate zoospores whose flagella are distributed throughout the entire body—coenozoospores.
4. Sexual reproduction oogamous.
5. The oogonium bears single large, uninucleate oosphere, while the antheridium gives rise to numerous small biflagellate sperms bearing unequal flagella.
Position of Vaucheria:
Since long time the genus Vaucheria has been included in the Chlorophyceae. This arrangement was doubted by Bohin (1897), Printz (1927), Fritsch (T935), and Fldmann (1946). In spite of his own dislikeness, Fritsch placed Vaucheria in the family Vaucheriaceae under the order Siphonales belonging to the Chlorophyceae.
The main reasons for excluding Vaucheria from the Chlorophyceae are:
1. The carotinoids are in excess of the Chlorophylls.
2. Chlorophyll b is absent.
3. Chlorophyll e has been found to be present in the zoospores.
4. Two Xanthophylls—siphonein and siphonoxanthin are absent.
5. Starch never accumulates as food reserve.
6. Mercerized cellulose is not present in the cell wall.
7. Antherozoids with laterally inserted unequal flagella—one being whiplash, the other tinsel.
Although its inclusion in the Xanthophyceae appears well established, its elaborate oogamous sexual reproduction is unlike any process in the remainder of the Xanthophyceae, so that it is difficult to establish any affinity between Vaucheria and other genera of the class Xanthophyceae.
Smith (1950) has placed the genus Vaucheria in the Xanthophyceae among the Chrysophyta.