Tuatara: Volume 23, Issue 3, May 1979
THE GREEN ALGAE — CRADLE OF THE HIGHER PLANTS
THE GREEN ALGAE — CRADLE OF THE HIGHER PLANTS
Since photosynthesis first became a co-ordinated process, evolutionary development in the autotrophic world has come a long way — from the procaryotic blue-green algae to the eucaryotic angiosperms. The organisms making up this colossal range of evolution share many physiological phenomena, one being photosynthesis based on water as a hydrogen donor. This kind of photosynthesis is characterised by the presence of chlorophyll ‘a’ plus some accessory pigment. In the case of the procaryotic blue-green algae the necessary pigment is the phycobilin, phycocyanin: for all photosynthetic eucaryotes the pigment accessory to chlorophyll ‘a’ is another chlorophyll. In fact the photosynthetic eucaryotes can be subdivided in terms of their accessory chlorophyll:
algae — chlorophyll ‘b’, ‘c’, ‘d’ or 'e’
trachaeophytes — chlorophyll ‘b’ only
While other things have changed during evolution in the Tracheophytes, at least one thing is assumed to have remained constant: all members from what we reckon as primitive to what we think of as advanced have chlorophylls ‘a’ and ‘b’.
Being an incredibly old process in an evolutionary sense, photosynthesis (or specific features of it such as the nature of the accessory pigment) can be used as an indicator of basic homology and of possible interrelationships between different groups of plants and even of supposed evolutionary bridges. Since all Tracheophytes possess chlorophylls ‘a’ and ‘b’ and algae are the only possible progenitors of land plants, it is pointless to consider in an ancestral role any algae that do not possess chlorophyll ‘b’ as an accessory photosynthetic pigment. The only plants earlier than the Tracheophytes to have this combination of chlorophylls are the Chlorophyta, Charophyta and the Euglenophyta. We can dismiss the Euglenophyta without further ado because of their curious morphology and storage carbohydrate, as well as their inability to form a multicellular thallus and their lack of both a true plant-type cell wall and sexual reproduction.page 118
- barrel-pores, which in some species can to some extent open and close like stomata;
- well developed air-chambers with photosynthetic filaments
- the presence of archegoniophores and antheridiophores (both of which are highly specialised structures);
- two kinds of rhizoids — smooth and peg-walled.
So maybe when we are thinking of the bryophytes as the place among the land plants to which we connect our evolutionary bridge, we should not keep Marchantia too much in the forefront: rather, we should think of something a little less advanced — e.g. Riccardia, Pellia.
Others would prefer to consider that the bridge should be connected to the simplest known vascular plants — the Psilophytales. Such folk think that at least some of the bryophytes may have been derived from this group, for it is known that even those psilophytalean examples which have survived the rigours of fossilisation were not quite so far removed from the hornworts (Anthocerotales) or maybe even the mosses and liverworts as one might at first suppose. In other words Bryophytes could have been derived from the Psilophytales and as such might be regarded more properly as a red herring in the present conception of the continuum of plant evolution. Pickett-Heaps 11 remarks that ‘many of the bryophytes appear to be reduced forms and perhaps in evolutionary cul-de-sacs’. But even though bryologists and pteridologists cannot give any clear-cut indication of what constituted the first thoroughgoing land plant, they all agree that the ultimate origin of the Tracheophyta seems to lie within the Chlorophyta. He who subscribes to a monophyletic origin of the land plants sees this ‘hypothetical ancestral form (or forms) somewhere between the Chlorophyta and the most primitive known extinct Psilopsida but he knows it to be as yet a “missing link” unsupported by concrete fossil evidence.’ 15
Irrespective of what were in fact the bridging organisms between the Green Algae and the most primitive land plants, it cannot be denied and one must never lose sight of the fact that at no time page 119 earlier or later in the evolution of the Plant Kingdom was such a momentous evolutionary transition set in motion as when those early members of the Plant Kingdom began to move from an aqueous to a terrestrial environment. This transition could not have been achieved without imposing on the already existing genetic material additional requirements to meet these changes over and above the sharing of similar photosynthetic pigment and storage compounds, such as the presence of oogamy, the necessity of sterile walls around sex organs and the like. Plants were moving from a physically supportive liquid environment to a non-supportive aerial one. This required the evolution over time of new methods of support, of ways of maintaining access to water or developing resistance to desiccation, of new methods for acquiring nutrients. Not only this, plants were moving from an environment where ultra-violet light was not so damaging to certain compounds critical in cellular processes to one where ultra-violet light was much more destructive. How could transitional plants have coped? What would have been present for evolutionary pressure to select from in the form of latent or low-level gene expression, or features resulting from the gene drift?
Because of these environmental differences it would have been imperative, for instance, that proection against desiccation be acquired by male and female sex organs; that a zygote develop a temporary parasitic association on its gametophyte for nutrients and water—a consequence of parturition unnecessary in a truly aquatic alga, in which a zygote is bathed in its nutrient solution. It would have been necessary for an archegonium while acquiring its protective structure to retain some pathway for sperm penetration. The organisation of such a structure — permitting access to the ovum for sperm while minimising desiccation of the ovum and resulting zygote — would be the evolutionary result of two conflicting demands that must have required some time for a suitable compromise to have been achieved. One cannot say how many features like these were imposed by selection pressure during this transition or how many were resident already in the algal predecessors as sequelae of mutation, minimum selection pressure or maybe just gene drift.
One could not expect all these adaptive features (and maybe more) to have risen spontaneously in response to the progression from water to land; but rather these would have arisen over time. Their ultimate appearance, however, could have been accelerated immeasurably had there already been in the algal genomes latent genes, for instance, controlling biosynthetic pathways which under dry-land conditions would have resulted in the full expression of these characters, e.g. the heavier production of cellulose, the appearance of lignin, the further development of compounds acting as U-V screens. Although these features are not essential in algae, their presence or merely their partial expression would seem propitious in assessing possible algal progenitors. page 120page 121
Considering the Chlorophyta as our general starting level, we face the problem of deciding which order, family or other taxonomic unit contains today organisms whose ancestors may have been among the ‘possibles’ as progenitors of higher plants. Present day Chlorophyta display an incredible range of forms — some of which because of their oddities can be dismissed out-of-hand as likely forerunners of higher plants. We can thus eliminate siphonaceous forms — e.g. the Caulerpales, Codiales, Dasycladales: the multinucleate forms, which would include the Cladophorales, Siphonocladales, Sphaeropleales. Other orders could be excluded for varying reasons: the Zygnematales, because of lack of flagellation of sperms; the Oedogoniales, because of lack of vegetative cell division to give a thallus of indeterminate cell number. So what is left? The Volvocales, Charales, Ulvales, Chaetophorales and Ulotrichales. Having arrived at this point, how do we go about further elimination? What other features might be of some significance in narrowing down the choice? In the past, theories have mainly been based on reproductive similarities and homologies while keeping in the background the uniform pattern of pigments. But the time has come to regard as more appropriate an approach which reflects the concept of the simultaneous transmission of a group of unrelated but critical features from progenitor plants through those following in the evolutionary sequence. Such features would have had to include not just attributes of form and function (such as similarities of pigment. storage products and reproductive features) but these plus features critical to emergence and existence in the completely new medium of dry land — such as similarities in cell-wall materials, biochemical systems such as U-V screen materials, lignin-progenitor synthesis — and others.
We can approach the analytical problem of selecting the progenitor probables' among the Green Algae by grouping under three main headings features which at this point in time one could imagine should have been of evolutionary significance and if present would have been desirable were they found to be congenerous between algae and the first kind of Tracheophytes.
Before analysing the fitness of remaining algal candidates to satisfy the required features set down under these headings, we will pick on one cytological feature which now appears very important and assists us greatly in narrowing down the field of ‘probables’.
Implicit in the existence of the very first primaeval kind of cell must also have been the capacity for cell division and the partition of DNA, although how this latter occurred would be anybody's guess — but this must have happened. Cell division would have to be an incredibly older cellular activity than nuclear division with its attendant spindle formation and complicated sequence of chromosome division and separation, since organised nuclei did not arise till after the appearance of the Blue-Green Algae. By the time the Green Algae appeared, however, cell division had become associated with nuclear division to such an extent that generally one would not occur without the other with the exception of multinucleate and coenocytic types. Nuclear and cell division must have undergone evolutionary change over time. Because of their antiquity maybe we should look at these processes first to see if variations are found; and if they exist, then we could have pointers of inestimable value due to their page 123 fundamental nature and longevity on the evolutionary time-scale.
At this juncture it might be as well to briefly review what happens during the latter part of mitosis. Mitotic activity encompasses two phenomena:
|1.||the duplication of the chromosomes and the moving apart of the resulting duplicates;|
|2.||the isolating of the reconstituted daughter nuclei and cytoplasm etc. into compartments by means of a formation of a new cell wall.|
This latter process is referred to as cytokinesis and of late has been found to show at least one form of variation which appears to be of phylogenetic importance. To appreciate the differences, let us recall the essentials of normal higher plant cytokinesis.
|1.||After anaphase when the chromosomes have moved towards the poles and the nuclei are reconstituting, there remain the continuous fibres (called the interzonal fibres) which still stretch between the poles of the spindle;|
|2.||in the equatorial plane of the spindle, microtubules form in an alignment parallel with the interzonal fibres. Golgi bodies (dictyosomes) aggregate more or less at the equatorial extremities of these fibres, and vesicles begin to appear;|
|3.||the association of microtubules. Golgi bodies and vesicles constitutes a more dense plate-like region in the equatorial plane of the spindle. This association of structures, in the form of this denser area, is referred to as the phragmoplast.|
|4.||the vesicles increase in number and coalesce to form larger vesicles until there is a band of them in the plate-like region across the spindle equator;|
|5.||this vesicular continuum gives rise to the middle lamella or cell plate on to which the cell wal lis later deposited — thus completing cell division;|
|6.||during certain phases of mitosis the spindle is open — meaning that the nuclear membrane disappears completely and is absent for some of the stages of the mitotic cycle — i.e. post-prophase through to late anaphase-early telophase.|
This is the form of cytokinesis found in the higher plants from the Bryophytes upwards. But what is found among the antecedents of the Bryophytes — those Green Algae regarded as being the evolutionary progenitors of the land plants? Electron microscopy has now revealed some variations of cytokinesis which appear to be of phylogenetic importance; and associated with these deviations there appear to be other cytological features which also seem significant.
|1.||the widely separated telophase daughter nuclei come together for cytokinesis and may even lie closely appressed to each other. In the Oedogoniales and Chlorococcales the interzonal spindle disperses altogether at telophase;|
|2.||between these nuclei one finds a complex array of microtubules all oriented at right angles to the mitotic spindle and therefore in the plane of impending cytokinesis.|
|3.||This array of microtubules has been called the phycoplast to distinguish it from the phragmoplast.|
This phycoplast system has been found in the Volvocales and Tetrasporales, Chlorococcales, Oedogoniales and some of the Ulotrichales. Also, in the Oedogoniales the spindle is closed and in the Chlorococcales the nuclear envelope has only polar interruptions at metaphase. In Ulothrix fimbriata the metaphase spindle is surrounded by a vesiculate nuclear envelope, and in Stigeoclonium helveticum (Chaetophorales) it is closed. In the Zygnematales the spindle tends to be open and in most cases examined a septum develops as a furrow which is not associated with microtubules. However, in Spirogyra, a phragmoplast-like proliferation of microtubules and vesicles occurs late in the development of the furrow when its inward growth encounters the longitudinally-oriented microtubules of the persistent interzonal spindle.
In other members of the order Ulotrichales such as Klebsormidium flaccidum and K. subtilissimum, the spindle is fully open.3 There is much spindle elongation during anaphase, the daughter nuclei remain far apart during cytokinesis, there are persistent interzonal spindle microtubules. Cytokinesis is accomplished by a furrow but no microtubules are involved in the development of this furrow, i.e. cytokinesis is by a phragmoplast. In the Charales, mitosis and cytokinesis are similar to those of vascular plants — the mitotic spindle is open and a phragmoplast proliferates from a persistent interzonal spindle.9 The same applies to Stichococcus10 and to Coleochaete7.
So, of the group of orders remaining after the first relegation, we can now eliminate all those algae with a phycoplast. This leaves us with the ‘highly probables’ — the Charales, a couple of members traditionally classified in the Ulotrichales (Klebsormidium and Stichococcus) and one at least of the Chaetophorales (Coleochaete). Although the Zygnematales would still be in the running because they have a kind of phragmoplast,4 11 their lack of flagellation and conjugative method of sexual reproduction appear to disqualify them from further consideration as being on a direct line to higher plants.
Having got so far, how do these evolutionary elite stand up to scrutiny when the other criteria are applied?
Of the remaining contenders mentioned immediately above, those known to possess cellulose are the Charales and Spirogyra. The books do not definitely say whether Klebsormidium or Coleochaete have page 125 cellulose in their cell walls; but in many Green Algae cellulose is known to be the innermost component of the membrane surrounding the protoplast.
One assumes the pigment complement is uniform throughout these algae. It has been tacitly accepted that the carotenes and xanthophylls are also uniform, although the algae in question should be examined to make sure the picture is correct. We have recently found that contrary to what Strain13 has written lutein does not appear to be present in the siphonaceous algae such as Caulerpa, Codium and Bryopsis: its place is taken by the orange pigment siphonein. But at present this order provides the only known exception to the general pattern of carotenes and xanthophylls in the Green Algae.
Starch formation is a characteristic of Green Algae, oil being formed especially in resting stages but sometimes in conjunction with starch.
To date, lignin has not been found in any algal nor fungal group. But percursors of lignin in the form of flavonoid compounds have been found — but only in the Charales. Draparnaldia does not possess these precursor compounds. Although true cellulose is present in cell walls of numerous green algae — as it is in Chara and Nitella, it has been suggested14 that the encrustation of the cell wall by silica in the Charales may prevent the proper orientation of the lignin precursors on the cellulose — it being thought that acylation of the cellulose may be a precursor to lignification. This acylation is absent in other algae, primitive bryophytes and some aquatic angiosperms. The ability to form flavone compounds implies the ability to form at least one or more of the precursors of lignin.14 The compounds found are glycoflavones similar to vicenin and lucenin.
Another feature about glyco-flavones which could have had adaptive value for Charales concerns the property of these compounds to absorb light in the ultra-violet end of the spectrum. Glyco-flavones show high absorption in the ranges of 257 → 270 and 335 → 349 nm.14 Because of this they could act as good U-V screens — an important feature in the progression of plants from an aqueous to a terrestrial habitat. Once the ancestors of terrestrial plants began to emerge from their watery habitat, they would have needed something to screen out the U-V and prevent its damaging the cells' DNA by dimerisation of the thymine units — which shows a maximum at 260 nm.14 It would also be necessary for the survival of such plants to screen against the possible photo-destruction of the coenzymes NAD or NADP which process has a U-V absorption peak at about 350 nm.14 It has been said14 that some algae have developed the habit of forming a calcareous coating on the outside of the cells as protection against the mutagenic effects of U-V radiation. According to Banks (quoted in 14) this has however become restrictive in the evolutionary sense and prevented the formation of multicellular thalli.
Swain also points out another possible advantage that flavonoid page 126 compounds might have bestowed on these early amphibious, quasiland plants. Flavonoids possess anti-oxidant properties which might have reduced adventitious photo-oxidation of several sensitive compounds — which process more than likely would have been increased in the higher light intensity of the land environment.14
|1.||a single broad band of closely adjacent microtubules with which the flagellar basal bodies are associated;|
|2.||the flagellar insertion distinctly lateral — those of other genera being anterior.8|
Therefore the flagellar insertion is asymmetrical. Such a condition is found in the spermatozoids of higher land plants,1 and is unlike that found in such algae as Chlamydomonas and other volvocalean genera which have for quite some time been thought of as possible algal ancestors of land plants.11
It now remains to examine the peculiarities of sexual reproduction in Coleochaete and Chara since these are the only two Green Algae of the elite which have advanced oogamy. Let us review Coleochaete first. Oogonia are formed terminally on short lateral branches — usually displaced to one side by a branch arising from the underlying cell. The oogonium is flask-shaped with a swollen basal part containing a chloroplast which projects into a long neck (called a trichogyne) containing a colourless cytoplasm. When mature, the tip of the neck disintegrates and the basal protoplasm rounds off to form a single ovum. The antheridia are borne in clusters at the ends of branches of the projecting system — often on the same main thread of cells that bears the oogonium. The antheridia arise as small colourless outgrowths which become cut off from the parent cell; and each antheridium produces a single colourless spermatozoid oval in shape with two flagella. After fertilisation the neck of the oogonium becomes cut off by a septum and the basal part enlarges. The oogonium becomes overgrown by branching threads which originate from underlying and other adjacent cells. These threads ultimately unite to form a pseudoparenchymatous sheath around the zygote. A thick brown membrane develops around the zygospore, formed partly from the inner membranes of the surrounding threads and partly from the oogonial wall. Subsequently, cells of the threads die and the page 127 zygospore hibernates in its double membrane. On germination, the zygospore divides by a wall perpendicular to the long axis of the original oogonium: two more walls are formed at right angles to form an octet stage. Further division gives rise to 16 or 32 wedge-shaped cells forming a multicelled body — which in the opinion of Wesley16 ‘is the nearest approach to the simplest sporopyhte found in the liverworts’. The cells, however, are haploid — not diploid: so the resemblance to a liverwort sporophyte must be solely one of appearance and cell number. The envelope bursts and each cell gives rise to a swarmer which attaches to a substrate and becomes an ordinary plant.
|1.||the oogonium with its trichogyne-like neck|
|2.||the pseudoparenchymatous sheath around the zygote|
|3.||the formation by meiosis and mitosis of so many cells still within the zygospore envelope.|
Coleochaete is heterotrichous and growth is always apical, taking place by means of a marginal meristem in the discoid types. Branching of the threads is effected either by lateral outgrowth or by dichotomous division of the apical cells.6
The Charales are characterised by having a nodal and internodal arrangement, and growth is in all cases instituted by a dome-shaped apical cell. Although Chara internodal cells are described as multinucleate, descriptions of the process of mitosis imply that it is not in fact mitosis. There is no spindle formation — merely a budding off of nuclear material of some kind or other which does not result from a true and equal partition of the chromosomal material. The use of the term ‘multinucleate’ in this context cannot be correct and should not be used. The antheridium has been described as the most complex in the Plant Kingdom and bears no resemblance to any other structure of comparable function. But the spermatozoid is biflagellate and helical in shape. The oogonium has a large central egg ensheathed by elongated spiral cells which extend beyond the fertile cell to form a crown or corona. This opens to allow the sperm to enter the oogonium and fertilise the ovum. The zygote undergoes reduction division and three of the haploid nuclei distintegrate. The remaining uninucleate cell gives rise to a protonema-like filament which grows out from the enclosed zygote. This protonema also gives rise to a filamentous rhizoid. Both are differentiated into nodes and internodes; and from the second node of the protonema the main axis of the plant arises as a lateral branch.
|1.||both the antheridium and oogonium are enclosed within a layer of sterile cells.|
|2.||The oogonial wall is extended into a corona.page 128|
|3.||The spermatozoid is biflagellate and helical in shape — similar to that of Marchantia.|
|4.||The zygote on germination gives rise to a haploid protonemal structure.|
|5.||In Chara, the oogonial structure (egg + jacket) arises from different cells.6|
The following tabulation sets out a summary of the distribution of desirable features among the algal ‘probables’.
|Presence of Chlorophylls ‘a’ and ‘b’||YES||YES||YES||YES|
|Similar Xanthophyll pattern to higher plants||ASS/Y||ASS/Y||ASS/Y||ASS/Y|
|Lignin precursors present||?||?||?||YES|
|Flavonoids present as U-V screens, etc.||?||?||?||YES|
|Phragmoplast and open spindle||YES||YES||YES||YES|
|Lateral flagellar insertion in motile cells||YES||N.A.||YES||YES|
|Multilayered structure assoc. with flagella||YES||N.A.||YES||YES|
|Sterile layers around sex organs||NO||N.A.||FEM. ONLY||YES|
|Helical shape of sperm||N.A.||—||NO||YES|
|ASS/Y = assumed ‘yes’||N.A. = not applicable|
Of the present-day ‘probables’, Coleochaete and the Charales seem to be the leading contenders. Each of these lacks certain features which would divert it from the mainstream of evolution; but which would have been the better equipped as a potential progenitor?page 129
…Coleochaete is classified amongst the Chaetophorales. It is unfortunate we do not have information about the presence or absence of flavonoids in this alga; but if it is anything like its taxonomic bedmate, Draparnaldia, it will lack these compounds.14 If this is the case, we can relegate Coleochaete to a penultimate position in the line of preference, since the possession of these chemicals would have been of great value in response to selection pressure in the sorting out of subjects most suitable for their new and exacting environment. We also presume its cell wall possesses cellulose. Its antheridium has no sterile jacket around it; the sperm is oval although the flagella are inserted asymmetrically. The zygote of Coleochaete on reduction division gives the usual tetrad but none of these dies. Instead, mitosis occurs to form a multicellular structure of 16-32 cells each of which ultimately gives rise to a separate swarmer — not the slightest resemblance to a filamentous protonema.
So it appears that Chara and its cohorts possess the characteristics which put them in the vanguard of the probable contenders, and from our standpoint today they are the most ‘on line’ as algal progenitors of the tracheophytes. But they, too, possess disqualifying features. One would expect any ancestral land plant to have developed the capacity for cell division in three planes to accommodate a structure such as the archegonium; but no organism in our short list of ‘probables’ practises cell division in this way — although it may be argued that the oogonia both of Chara and Coleochaete demonstrate a cell division which might be regarded as pseudo-three-dimensional. The overall morphology of the Charales precludes their existence in situations where water may be minimal or absent for short periods: one cannot imagine their having any resistance even to mild desiccation. And who could imagine an alga with cells the length of Chara's progressing on to dry land, even if its cells were encrusted with silica? There is no equivalent in the freshwater algae of a multicellular 2N sporophyte with uninucleate cells — no candidate shows even the slightest hint of such a structure; yet this type of sporophyte is a universal feature in all lower tracheophytes. The freshwater Cladophora glomerata has a multicellular 2N sporophyte; but here the cells are multinucleate. In Chara the zygote on germination undergoes reduction division to give the usual tetrad — but three nuclei disintegrate. This would hardly represent the acme of efficiency in exposing genes to environmental selection for survival of the fittest. Admittedly, the remaining uninucleate cell develops to form a protonema-like structure the like of which is not unknown in the mosses. Meiosis occurs very early in the development of the zygote and nowhere is there the possibility for the institution of a diploid generation. All our known contenders have not managed to escape the rut of haploidy and a haplontic life-cycle.
One could imagine an archegonial neck to have evolved from the corona of Chara were it not for the fact that in an archgonium not page 130 only are there the neck cells but there is also a line of cells down the middle of the archegonium which disintegrate to allow access to the ovum. No single cell in the corona of Chara exists which could be regarded as ancestral to the canal cells. Further, in Chara the oogonial structure of egg + jacket arises from different cells: in Marchantia, the egg and sterile layer of cells which constitute the archegonium arise from the same cell. The antheridium of Chara has a sterile jacket of cells; this is not so in Coleochaete. In the latter, the sperm is oval but the flagella are inserted asymmetrically; this is not the case in Chara — whose sperm is like that of Marchantia, helical as well as having flagella inserted laterally.
In the past when reviewing evolutionary lines like these, we have considered such things as the presence of oogamy, heterotrichy and other morphological factors as the main criteria for divining evolutionary trends. But since all life is dependent on chemical reactions and their products, we must add biochemical and physiological factors (i.e. those never expressed in a morphological fashion) to our list of criteria. It is true this has been done in the past by considering the distribution pattern of photosynthetic pigments and storage products between the algae and higher plants. But now that equally fundamental and evolutionarily-ancient factors are being revealed through the further studies of biochemistry and particularly algal-cell ultra-structure, we must surely draw up new lists of criteria which will incorporate the new as well as the time-honoured. One such fundamental factor also revealed through recent bio-chemical research concerns a reaction in the area of photosynthesis. In the Calvin Cycle, the carbon dioxide-fixing enzyme is ribulose diphosphate carboxylase which under normal circumstances catalyses the addition of carbon dioxide to ribulose diphosphate. This same enzyme can however in the presence of oxygen degrade ribulose diphosphate to phosphoglycerate and phosphoglycolate. The former is easy to dispose of metabolically, the latter not quite so easy. Plants seem to have selected two ways of handling phosphoglycolate — through the action of the enzymes glycolate dehydrogenase or glycolate oxidase. Algae with a phycoplast use glycolate dehydrogenase for this purpose; whereas algae with a phragmoplast use glycolate oxidase. Both of these enzymes oxidise glycolate but by different pathways. Differences in the oxidation of glycolate may point to fundamental differences in the glycolate pathway or other photorespiration-linked processes or both.5 Associated also with this basic difference it is found that some at least of the ‘phycoplast’ algae show a very low carbon dioxide compensation point, whereas a ‘phragmoplast’ alga, Nitella shows a compensation point much closer to that of higher plants.5 The full significance of these phycoplast- and phragmoplast-associated phenomena at present escapes us; but the coincidence exists. And again, at the risk of being repetitious, one must point out that the progress of plants from water to dry land must have been achieved page 131 only as a result of the genetic expression of numerous unrelated but critical factors (biochemical, cytological and ultra-structural) acting in concert, and not just because of evolutionary progression expressed in morphology or reproduction. Never in the history of the plant kingdom has the possibility of so much been passed on ultimately to so many from the starting point of so few gene pools for selection.
Lack of information about both ends of the evolutionary bridge is such that one imagines the twain shall never meet — not even in hypothesis: and it is fatuous to indulge in further lines of argument and theoretical excursion which all too quickly can become tortuous and intricate intellectual meander. One might admit this exercise was doomed to failure from the start due to a high improbability that something conclusive would emerge; but its pursuit can surely be justified if only because, knowing this transition occurred, one must try to determine how it could have proceeded.
Evolution has in the past been thought of more in terms of a morphological continuum; but this is only a small part of the story. At rock bottom evolution incorporates a genetic continuum expressed through biochemical, physiological and ultra-structural features as well as morphological and reproductive, all subject to the testing of selection pressure — just as occurs today. As our knowledge in these areas improves, we may be better able to project into the past and apply with increased perspicacity and probability criteria which more than likely were important in evolutionary progressions, trusting of course that those features we see in today's algae are true relicts of the past.
So, as a result of continuing advances in our knowledge of biochemistry and ultrastructure, we should be in a position to define with increased accuracy current taxonomic units amongst the algae which manifest a greater probability of having those kinds of characters we would expect in algal antecedents to the higher plants.
I wish to thank Prof. H. C. Bold of the University of Texas, Austin, for criticising an early draft of this article; and also members of the Botany Department of Victoria University for their most constructive comments and suggestions.
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16. Wesley, O. C., 1928: Asexual reproduction in Coleochaete. Bot. Gaz. 86: 1-31.
In common with other scientific periodicals Tuatara is having difficulty in keeping up with increasing costs. One New Zealand journal, for example, recently increased its subscription from $6 to $30 per annum!
Beginning with the next volume our subscription will remain the same at $5, but the number of issues per volume will be reduced from three to two.
Sufficient articles and reviews for Volume 24, Part 1, have now been received and the issue should appear within a few months.