In this article we will discuss about the evolutionary trends in the development of plant body in the chlorophyta.
i. Evolution of Sexuality in the Chlorophyta:
The Chlorophyta represent an evolutionary series in which the gametic union became most firmly established even in the most primitive forms and persists among the highest members of the series.
The evolution in sexuality has taken place from isogamy to oogamy through anisogamy. To start with, the fusing gametes are identical in size and shape but with gradual evolution become vastly differentiated from one another so that one becomes large, passive, non-motile and ultimately remains enclosed in a gametangium; while the other becomes small, active and motile.
Isogamy is evident in a large number of Chlorophyta. Special mention may be made of Chlamydomonas moewusii, where isogamy is between two flagellate isogametes. In the Conjugates, however, there is isogamy between aplanogametes, but the motility of the gametes is determined by physiological characters.
In Zygnema and Mougeotia both the fusing gametes are equally active and fusion of isogametes takes place in the conjugation tube. But in Spirogyra, though the gametes are isogametes morphologically, they are distinguishable by their motility. Of the two fusing gametes, in some species, one remains passive and the other becomes actively amoeboid and moves to the passive gamete and fuses with it.
This is a case where there is deviation from the typical isogamy.
Here the gametes are isogametes morphologically but their behaviour is not similar, hence they are physiologically dissimilar. The passive one, from the physiological standpoint, may be identified as female and the active as male, such a deviation from typical isogamy indicates a tendency which leads to anisogamy where the gametes are morphologically different.
This is transitional stage between isogamy and true anisogamy.
Evolutionary trend may be traced from true anisogamy as follows:
(i) Both the fusing gametes are anisogamous, flagellate, and active—exhibited by Chlamydomonas braunii;
(ii) Both the fusing gametes are flagellate and active, but the larger one is sluggish and just before fertilization it loses its flagella, and becomes non-motile, as in Aphano- chaete repens, which clearly indicates how anisogamy has been derived from isogamy;
(iii) Of the fusing gametes, the larger one is non-flagellate and the smaller flagellate, such a condition is encountered in Chlamydomonas coccifera.
Oogamy is the most advanced method of sexual reproduction, where a large non-motile passive female gamete (egg) is fertilized by a small actively motile male gamete (antherozoid). Both male and female gametes are borne in male and female gametangia respectively.
In majority of oogamous Chlorophyta the egg is retained in the oogonium and fertilization takes place there in the oogonium.
But within the oogamous forms of Chlorophyta a less-advanced condition is exhibited particularly by Chlorogonium oogamum, where the female gametangium produces a single large egg which is liberated out from the female gametangium immediately after maturity and is fertilized by an elongated antherozoid. This is a case where fertilization takes place outside the female gametangium.
A most advanced type of oogamy is found in Coleochaete, Chara and Nitella where the female gamete is retained in the female reproductive organ and is fertilized by an antherozoid. The structure of female reproductive organ in these algae is very much advanced.
In Chara and Nitella the female reproductive organ has a special protective device, but being unicellular, fails to rise much in the evolutionary scale. Again in Coleochaete, Chara and Nitella there is elaborate post-fertilization stage. Since meiosis takes place immediately after the formation of oospore this elaborate post-fertilization stage is not of much evolutionary importance.
ii. Evolutionary Trend in the Development of Plant Body in the Chlorophyta:
Members of the Chlorophyta exhibit a wide degree of variation in their external form, ranging from very simple unicellular motile and non-motile on the one hand to a highly developed complex structure with clear distribution of labour on the other.
Considering all these various forms, trend of evolution can be traced broadly along four lines with a starting point from the Chlamydomonad type:
(i) the motile coenpbial line, (Gonium-Eudorina, Pandorina- Volvox),
(ii) the non-motile net-like line (Chlotella- Scenedesmus-Pediastrum-Hydrodictyon),
(iii) the coenocytic line, (Caulerpa)
(iv) the filamentous line (Ulothrix, Oedogonium).
The filamentous line has again given rise to:
(i) simple parenchymatous form, (Ulva),
(ii) a line terminating in the Conjugates, and
(iii) the elaborately developed heterotrichous line.
The heterotrichous line has further developed in two directions with the suppression of one form or the other.
(i) a line having the tendency to develop thalloid structure at the cost of the aerial portion which turns out to be very rudimentary, (Coleochaete, Fritschiella)
(ii) a very elaborately branched aerial portion with very insignificant prostrate portion (Draparnaldia, Draparnaldiopsis, Chara, Nitella).
All these forms are schematically represented in Figure 92.
The evolutionary trend indicated in Figure 92 is broadly based on the observation of E. F. Blackman, who in 1900 traced the evolution in the construction of plant body in the Chlorophyta from one-celled flagellated Chlamydomonad type of condition.
He pointed out that evolution took place along the following lines, designated by him as tendencies:
(i) The volvocine line or tendency in which the flagellated vegetative cells divide to produce daughter cells which organize a motile colony.
(ii) The tetrasporine line or tendency in which motility is lost, except in reproductive stages; non-motile cells are produced as a result of vegetative division.
(iii) The chlorococcine or endosporine line or tendency in which just like the tetrasporine line, motility is lost, except in reproductive stages, but plant body has lost the capacity to divide vegetatively.
(iv) The rhizopodal line or tendency in which the trend of evolution is toward a naked amoeboid type of development.
Evolution along the volvocine line is very limited since size of motile colony and their cells are variable in number and perhaps Volvox is the culminating member of the line. But evolution along the chlorococcine line exhibited by the Chlorococcales, has taken place in different directions such as: unicellular to coenocytic condition, and to colonial habit with zoospores or aplanospores.
Both of these conditions have limited scope of evolution. Again, the tetrasporine line has great scope for evolution. Here the tendency for the vegetative cells to become non-flagellated is a very dominant phase.
This is accompanied by: the tendency to organize filamentous condition in the Ulotrichaceae, heterotrichous condition in the Chaetophorales, and parenchymatous form by gaining the ability to divide both vertically and transversely seen in Ulvaceae.
The other two lines that segregate from the tetrasporine line are, evolution of a coenocytic condition in cells of a filamentous branched condition in the Cladophorales and evolution of a distinctive type of zooid as in the Oedogoniales or of aplanogametes exhibited by the Conjugales.
iii. Evolutionary Tendencies in Chlorophyta towards a Land Habit:
Land plants have advanced over the members of the Chlorophyta along the line of these adaptations:
(i) Increased structural complexity of the plant body.
(ii) Highly developed reeproductive organs with special adaptation to protect the gametes.
(iii) Protection of zygote, embryo formation, and elaborate development of post- fertilization stages.
Though most of the members of the Chlorophyta are different from the plants which have become established on land, yet with a closer analysis it may be indicated that there is definite tendency among the members of the Chlorophyta to migrate towards land as can be exemplified by the following instances.
Terrestrial algae which grow on rocks, bark of trees, damp soil and other similar places show definite adaptations to protect themselves against desiccation. To elucidate this point, there may be cited as examples of various species of Trentepohlia, where the cell wall is stratified and cells develop a special pigment (haematochrome) to serve the purpose of a light screen.
Besides, the delicate apical cell is protected by the pectose cap. Cladophora calcicola, a terrestrial alga which grows on rocks, develops cuticle on the cell wall. Similar adaptations may also be seen in different species of Hormidium, Zygnema, Characium, Pleurococcus, Oliveria and Fritschiella.
As to the structural peculiarities of the plant body, members of the Orders Chaetophorales and Charales exhibit considerable morphological differentiation and show a wide range of construction from heterotrichous to a highly complex aerial portion with a capacity to grow indefinitely.
In heterotrichous forms, the plant body consists of an expanse of dorsiventrally differentiated branched creeping threads spreading over the substratum and affording a firm anchorage, as well as a primary photosynthetic system well exposed to the light.
From this, sooner or later, branched filaments grow out at right angles into the surrounding water, or in sub-aerial form into the air, and take over most of the photosynthetic functions, as well as commonly bearing the reproductive organs. Both prostrate and projecting systems are well developed in Stigeoclonium.
A further advancement is seen in Draparnaldiopsis, in which the aerial portion is composed of long and short cells, the latter alone bearing the laterals of limited and unlimited growth, commonly in whorls resembling more or less nodes and internodes. In Draparnaldiopsis indica, the branches have the tendency to coil around the main axis giving a somewhat corticated appearance.
In Draparnaldia, the aerial portion is strikingly differentiated into long and short branches, while the prostrate system may be rudimentary or sometimes may be represented by multicellular rhizoids. In all the more extreme forms the base shows a tendency to become pseudoparenchymatous, as in Coleochaete.
As long as there is a properly developed upright system the reproductive organs are usually confined to it, as in Coleochaete. In Trentepohlia, however, gametangia tend to arise from the base, whilst the sporangia are found on the upright threads.
The alga Fritschiella tuberosa found on relatively dry soils in India, Nepal, Sudan and Africa has an extremely elaborate specialized heterotrichous habit, possessing a rhizoidal system comparable to the rhizoids of the bryophytes, a prostrate system with the potentiality to develop a parenchymatous structure from which arise projecting short-celled primary and long-celled secondary systems.
Rhizoids serve for anchorage and absorption of nutrients. The prostrate system possesses the function of storage and reproduction. The projecting system is exclusively photosynthetic.
The maximum complexity in the structure of the aerial portion is found in Chara, where there is definite differentiation of nodes and internodes, the nodes bearing branches and leaf-like structures, and the internodes being corticated. Growth is by an apical cell with a definite mechanism for protection. Reproductive organs are borne at the nodes.
The prostrate portion is represented by the rhizoids with oblique septa, resembling members of Bryophyta.
As far as the structure of the reproductive organ is concerned, in Coleochaete the oogonium has a long trichogyne which resembles the archegonial neck of the bryophytes. In Chara, the reproductive organs are invested with a cellular envelope which protects the gametes (male and female) against unfavourable external conditions.
The most remarkable feature in Coleochaete is the development of sterile filamentous structures surrounding the base of the oogonium immediately after fertilization; this is an adaptation to protect the zygote, and may very well be considered as a starting point for the gradual elaborate development of the post-fertilization stage towards further complexity.
In Chara, however, the oogonium is already well protected by a sterile covering which can very well serve the purpose of protecting the zygote. Elaborate post-fertilization stages and embryo development can very well be indicated in Chara. Though the so-called embryo is haploid, yet there is definite provision for food storage and clear separation of the protonemal and rhizoidal initials in the early stage.
Though many green algae represent ‘dead ends’ as far as subsequent evolutionary development is concerned, there is no doubt that they are extremely successful forms, since they are able to exist in a wide range of habitats all over the earth. Furthermore, the great diversity among the green algae would suggest that many forms among these plants would be able to adapt themselves to changing environmental conditions.