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Term Paper on Algae
Term Paper Contents:
- Term Paper on the Origin of Algae and Evolution among them
- Term Paper on the Range of Structure in Algae
- Term Paper on the Reproduction in Algae
- Term Paper on the Origin and Evolution of Sex in Algae
- Term Paper on the Life Cycles in Algae
Term Paper # 1. Origin of Algae and Evolution among them:
The origin of the various groups of present-day algae is supposed to have taken place from some unicellular non-flagellate aquatic protista. From this hypothetical ancestor there appeared the diverse groups polyphylectically. The oldest-known fossil records of algae that could have been traced so far, disclose the presence of some members of the Myxophyceae on the Proterozoic rocks.
Cytologically also, excepting the bacteria, the class Myxophyceae, represents the most primitive group of living organisms, as no one member of this group possesses a well-organized nucleus, nor is there any trace of any other mode of reproduction other than the vegetative.
With these facts in view, the Myxophyceae may be considered to have come first of all the algal lines. Some workers like Fritsch (1935), however, regard the Myxophyceae as the most reduced and consequently, the most advanced group among the algae.
Whatever might be the most primitive algal member, the class Chlorophyceae appears to have been derived from some Chlamydomonas-like flagellate unicellular ancestor. Each class of algae is characterized by the constancy in position of its flagella in zoospores or gametes. From this hypothetical ancestral stock a progressive evolution took place in four different directions.
One of these lines of development terminated in plants like Volvox having a motile colony. In another direction the colonies were non-motile, as in Hydrodictyon and Pediastrum. The third series gave rise to plant bodies with tubular or siphonaceous thalli, as may be found in Vaucheria.
As a result of evolution in the last direction, there appeared multicellular filamentous forms, which were at first un- branched, as in Ulothrix, Spirogyra, Zygnema, and Oedogonium, and subsequently the branched ones, as in Cladophora, Chaetophora, Draparnaldia, Draparnaldiopsis, Stigeoclonium, Bryopsis, and others. It is believed that from some members of this filamentous line of Chlorophyceae the bryophytes originated.
Because of the fact that the Phaeophyceae possess motile reproductive cells, it is thought that the group has arisen from some flagellate unicellular brown algae, in a line parallel with the Chlorophyceae. It is of interest to note, however, that though the class has attained a very high degree of specialization in the body organization, it has not apparently been able to give rise to any higher category of plants.
The class Rhodophyceae is rather unique among the algae in several respects, and resembles no other algal class excepting the Myxophyceae. Members of both these two classes lack any type of motile flagellate cells. Further, the nature of pigments is also allied in them, for phycocyanin which is universally present in the Myxophyceae, is also found in some Rhodophyceae, and the phycoerythrin-like red pigment is also present in some blue-green algae.
The red algae also represent a very ancient algal line, and its fossil records can be traced as far back as to the Ordovician. This group appears to have originated independently from some non-flagellate unicellular form. Some workers believe that the Ascomycetes, among the fungi, have originated from the Rhodophyceae.
Term Paper # 2. Range of Structure in Algae:
The vegetative bodies in Algae in different classes show a gradual progression from simple to more complex forms and the whole range of structure may be grouped under the following heads:
i. Motile Unicellular Type:
The thallus is unicellular, more or less spherical or pear-shaped in form and flagellate. The flagella, two in number, are attached to the narrowed anterior end of the plant body. The nucleus occupies the middle region of the protoplast and the chromatophores, when present, either lie along the sides or occupy the posterior region of the cell.
In certain cases, special rigid envelope with an aperture for the emergence of the flagella and separated from the cell proper by space can be seen. These forms are called encapsuled forms, (e.g. Chrysococcus). Sometimes the thalli are colourless due to absence of chromatophores.
ii. Motile Colonial Type:
In this group are to be found those forms in which the individual flagellate cells become aggregated together within a mucous envelope to form a spherical grouping or colony. The cells are all alike in form, complete in themselves and retain their motile character in the vegetative condition. The movement of the entire colony is brought about by the simultaneous activity of their flagella, (e.g. Volvox).
iii. Palmelloid Type:
In this type, during the vegetative phase, the individual cells are without flagella so that the motility disappears and the daughter cells produced by vegetative divisions remain embedded within a common gelatinous envelope, formed by gelatinization of the parent cell wall.
This ‘Palmelloid stage’ may be temporary, since sooner or later, the cells may develop flagella and resume active movement, (e.g. Chlamydomonas). It may also be of a permanent nature characterized by the loss of motility during the vegetative phase. In this case the reproductive cells are flagellate and motile, (e.g. Tetraspora).
iv. Dendroid Type:
This type of thallus is similar to that of the palmelloid type but the mucilage is produced locally, usually at the base of the cell, (e.g. Prasinocladus).
v. Coccoid Type:
In this group the thalli are characterized by motionless unicellular individuals which become motile only during reproduction, (e.g. Chlorococcus). The power of dividing vegetatively is lost in this case and the reproduction takes place by the formation of zoospores.
vi. Filamentous Type:
The thalli are characterized by the type of cell division, called vegetative division. During the process, a parent protoplast ordinarily divides within the original cell wall and the two units of the protoplast are simply separated by the formation of a strip of membrane (septum) which is jointed laterally to the cell wall of the parent cell. Repeated vegetative division in one plane results in a filamentous thallus, (e.g. Ulothrix, Spirogyra, Oedogonium, etc.).
If the septa are formed along two planes and at right angles to one another, a flattened leaf-like thallus develops, (e.g. Ulva). The branching habit in a filamentous thallus is obtained due to formation of lateral outgrowths of more or less numerous cells and in which septa-formation takes place in transverse plane as in the main axis, (e.g. Cladaphora and many other Chlorophyceae).
vii. Heterotrichous Type:
This type represents the most highly evolved filamentous plant body.
The thallus is characterized by having two distinct well-differentiated parts, namely:
(a) A prostrate creeping system of the branching filaments which are attached to the sub-stratum, and
(b) An erect system consisting of one or more branched filaments that trail out in water, (e.g. Chaetophora, Draparnaldiopsis, Bryopsis, Batrachospermum, etc.).
viii. Siphonaceous Type:
This group is characterized by considerable extense of the thalli without any septation. Thus, a large multinucleate coenocytic structure originate, (e.g. Vaucheria, Botrydium, etc.).
ix. Complex Type:
In this group the thalli range from more or less compact pseudo-parenchymatous bodies to true parenchymatous bodies of well-differentiated macroscopic forms as evidenced in Phaeophyceae and Rhodophyceae.
Evolutionary Tendencies:
Assuming flagellated origin of algae, it is conceivable that motile unicellular organisation is most primitive in nature. By aggregation of numerous unicells in a common envelop, motile colonial type has been originated. In another line of evolution by loss of motility, palmelloid type and coccoid type develop which gave rise to dendroid type by basal gelatinous encapsulation.
In another line from the unicellular form nuclear division without the formation of cell wall originated the siphonaceous form. From the unicellular type following nuclear division and formation of septa the filamentous form has been originated. On the basis of distribution of labour heterotrichous form with prostrate and projecting system appeared which ultimately led to the more complex thallus organization.
Term Paper # 3. Reproduction in Algae:
The various methods of reproduction in Algae can be grouped under the following categories:
i. Vegetative:
In unicellular forms, it takes place by cell divisions and the daughter cells, thus produced, function as new individuals. In colonial forms, two individual colonies may-arise by mere splitting of the mature colony, (e.g. Volvocales).
In filamentous types, vegetative reproduction takes place by fragmentation of the threads and each broken part gives rise to a new individual. Vegetative reproduction is found in various classes of Algae and is the characteristic feature of the Myxophyceae where complex specialized structures are to be found.
ii. Asexual:
This is the commonest method of reproduction in which spores, one (e.g. Oedogonium) to several hundred, (e.g. Cladophora), are formed within specialized cells, called sporangia which may not be morphologically different from, the vegetative cells. The spores may be naked, motile and with two to four flagella (zoospores), or may be non-motile (aplanospores).
The aplanospores are modified zoospores which have lost their power of movement and each is surrounded by a definite wall which is distinct from the parent cell wall. An aplanospore with greatly thickened wall is a hypnospore. The zoospores resemble morphologically the unicellular members of the Volvocales and from the standpoint of phylogeny they show reversion to primitive ancestral flagellate condition.
iii. Sexual:
This type of reproduction is characterized by the fusion of two gametes and its different methods are as follows:
a. Isogamy:
In this case two gametes of the same morphological identity, flagellate or non-flagellate, fuse to form zygote. Majority of isogamous Algae are dioecious, since the fusing gametes come from different individuals of opposite strains.
b. Anisogamy:
It is a case of modified isogamy and the fusing flagellate gametes show slight difference in their behaviour, one being more passive than the other, (e.g. Chlamydomonadaceae). In some cases the fusing gametes are unequal in size. This is known as heterogamy.
c. Oogamy:
This type of reproduction represents the highest stage in the series. Here, a large motionless, non-flagellate female gamete, called oosphere or egg, is fertilized by a much smaller flagellate, active male gamete, called antherozoid (e.g. Volvocales, Ectocarpales). Both types of gametes may be produced on the same (monoecism) or different individuals (dioecism).
Usually, the oosphere or egg is retained within the female sex organ, the oogonium, where it receives the male gamete. It is, therefore, a receiving cell. The male gametes, produced within the male sex organ, the antheridium, are ultimately discharged and one or more of them enter the oogonium, through an aperture on its wall. In Rhodophyceae, the oogamous reproduction is of a specialized type. The male gametes are non-flagellate, therefore, non-motile and passively carried to the female sex organ by current.
Term Paper # 4. Origin and Evolution of Sex in Algae:
The starting point in the origin of sexuality is denoted by the formation of gametes. The different types of sexual reproduction in algae range from isogamy to oogamy, the culminating point.
Ulothrix, in which modes of formation of gametes and zoospores are identical, has often been taken to illustrate the possibility of the origin of gametes from zoospores, consequently, the origin of sex. Two types of zoospores, large and small, are produced in Ulothrix depending on the number of successive nuclear divisions, and the smaller zoospores are morphologically alike with gametes and also in the method of production.
The difference between the gametes and the zoospores depends mainly, therefore, on the number of nuclear divisions occurring during their development. When the number of divisions is less, the zoospores are formed, but when it is long continued, gametes develop.
It has been suggested that possibly due to accidental fusion of the small zoospores in pairs sexual reproduction has originated. The larger zoospores on germination can only give rise to strong individuals, but the smaller ones on germination can produce only weaklings.
But, if these smaller zoospores combine in pairs, zygotes are produced, and the resources of the two fusing cells are brought together forming a new identity full of increased vitality and vigour. This act of union of two naked protoplasts is a sexual act, and the two fusing cells are, therefore, regarded as sexual cells or gametes. The sexual reproduction originates when zoospores behave as gametes.
It is believed that sexual reproduction in plants takes place in response to unfavourable environmental conditions for vegetative activity. When the conditions are most favourable, the plant usually reproduces vegetatively. When these conditions are on the decline, the plant reproduces sexually and the gametes are produced.
These gametes, by fusing in pairs, produce zygotes, which usually pass into a state of dormancy or resting period, and in this condition the plant perennates. Thus, the formation of zygote is more in response to unfavourable growing conditions for the protection against extermination of the individual than a method for the multiplication of the plant.
The origin of sex in algae is associated with the gradual differentiation of sex. By differentiation of sex is meant the morphological differentiation, i.e., difference in appearance, which involves in the difference in size and activity of the motile organs associated with it. This differentiation of sex leads to such differentiation of gametes that they can be easily recognized as male and female ones. Morphological similarity of pairing gametes is called isogamy, while morphological dissimilarity of the pairing gametes is called heterogamy.
In isogamous plants the terms ‘male’ and ‘female’ are not used, but in heterogamous ones they are commonly applied. The type of isogametes which gradually gives rise to heterogametes is clearly of the active swimming zoospore type, the two copulating gametes being similar in size and activity.
Beginning with this type of gamete, a series of algae can be arranged in which there are increasing differences, step by step, in the pairing gametes and culminating with gametes, morphologically so different in appearance that they can be easily recognized as male and female, and do not suggest identical origin. It is the gradual appearance of differences that makes the two sexes recognizable and not due to gradual differentiation of maleness and femaleness.
In the series of differentiation of sex among algae beginning with isogamy there are types in which one of the pairing flagellate gametes becomes recognizably larger than the other (anisogamy) to the types in which this relative difference in size gradually increases until finally one of the pairing gametes becomes non-flagellate, non-motile, passive and many times larger than that of its small, active and flagellate (oogamy).
The following cases may be chosen to illustrate the gradual differentiation of sex among algae:
Chlamydomonas, Ulothrix, Ectocarpus, etc., illustrate isogamy, in which the pairing gametes are zoospore-like bodies which are flagellate and motile. Spirogyra is also isogamous, and the fusing gametes are non-flagellate but are ordinary protoplasts of vegetative cells. Of the two fusing gametes, one remains passive and the other moves through the conjugation tube and fuses with it.
This difference in motility is a feature of sperm and egg, therefore, the two pairing protoplasts of Spirogyra, may be regarded as male and female, although they do not differ in size. Zygnema, a relative of Spirogyra, has both of its fusing gametes motile at the time of union. Therefore, the advance to the condition of Spirogyra is the entire loss of motility of one gamete and its retention by the other.
The next step in the evolution of sex is anisogamy, which may be illustrated by forms like some species of Chlamydomonas, Pandorina, Caulerpa, etc., where the fusing gametes, differing only in size, are both flagellate. In Pandorina, both the gametes are motile, but the larger one may be more sluggish than its mate.
In a few species of Chlamydomonas and in Eudorina, a relative of Pandorina and Volvox, one may be motile, the other immobile, and hence they show a very close approach to oogamy in that the larger gametes are not free-swimming and in that the gametes are dissimilar in size and shape.
The maximum differentiation of sex is found among oogamous forms like Volvox. Oedogonium, Vaucheria, Chara, Fucus, etc., in which the gametes are not only differentiated into male and female by their differences in size, but also due to the fact that the larger female gamete becomes a completely passive cell so far as motility is concerned, due to the final disappearances of its swimming appendages (flagella). On the other hand, the male gamete becomes less bulky than its mate and develops swimming mechanism as a secondary feature.
The mode of sexual reproduction in Rhodophyceae, though oogamous, is of a highly specialized nature and illustrates further essential feature of the gametes. In this group distinct male and female organs are present, but the sperms resembling ordinary protoplasts are not even detached from the cell walls. In some cases, they are discharged but without any swimming appendages, and sometimes they are not discharged.
Within the female sex organ no egg is recognizable as different from the ordinary protoplast. It does not separate from the wall of the sex organ and rounds off, so that it is not a typical egg. It is simply the protoplast functioning as the female gamete, because the sperm or its nucleus fuses with it. Thus, in this group, the sexuality is of a distinctive type, there being no eggs and sperms in the ordinary sense, their role being played by two protoplasts not different in appearance from vegetative protoplasts.
Term Paper # 5.
Life Cycles in Algae:
The type of life cycles in algae can be conveniently grouped into two broad categories, namely:
i. An alternation of haploid generations in which a diploid generation is absent, and
ii. An alternation of distinct haploid and diploid generations.
Those forms in which regular alternating haploid and diploid generations are absent can be divided into two groups:
i. The oogonium is haploid and
ii. The oogonium is diploid.
In the former case, the vegetative body of the plant is a gametophyte. The diploid phase, resulting from the sexual union, is restricted only to the zygote. The diploid nucleus of the zygote, at the time of germination, undergoes meiosis producing four haploid nuclei, from which usually four asexual reproductive units are formed, each of which is capable of producing a new haploid plant.
This type of life cycle may be found in the majority of Chlorophyceae and possibly in all Xanthophyceae and Chrysophyceae. In case, where the oogonium is diploid, the vegetative body of the alga is a sporophyte and is evidently in the diploid condition. Meiosis takes place during the formation of gametes which represent only the haploid phase.
These gametes soon fuse in pairs to form diploid zygotes, each of which is capable of giving rise to a new diploid plant. This type of life cycle can be met with in some members of Siphonales and all Ba- cillariophyceae.
The cases where an alternation of haploid and diploid generations takes place can be grouped under the following categories:
a. Isomorphic Alternation:
This type is characterized by the presence of two individuals which are morphologically identical, but one of them is haploid (gametophyte) producing gametes and the other is diploid (sporophyte) producing spores. Meiosis takes place in the diploid generation, during the formation of spores, and each of the spores on germination produces a haploid generation.
Gametes, formed from this haploid generation, unite and give rise to the diploid generation, as found in some Chlorophyceae (e.g. Cladophorales, Ulvaceae), Phaeophyceae (e.g. Dictyotales, Cutleriales) and a number of Rhodophyceae. Some members of the Rhodophyceae are very peculiar in that they are diplobiontic, in which diploid generations alternate with a haploid one.
b. Heteromorphic Alternation:
This type of life cycle is characterized by an alternation of haploid and diploid generations which are morphologically dissimilar. Usually, the diploid plant (sporophyte) is much larger than the small, few-celled, haploid individual (gametophyte), as can be found in some members of Ectocarpales, Laminariales, etc. This type of alternation is quite similar to that found in higher cryptogamic plants (e.g. ferns and their allies).
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