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Vascular Tissue

In the ferns the vascular tissue branches off and extends into the branches of the frond. As we shall see when we discuss evolutionary trends in vascular tissue, this branching is correlated with the appearance of side branches or leaves along the stem. In the simpler vascular plants these leaf traces simply separate off from the central cylinder or stele of vascular tissues and extend toward the leaf bases. In the ferns and more complex vascular plants, they extend right into the frond branches and into the twigs and the leaves. This branching of a main stem is the functional basis for frond formation. It involves three related processes: overtopping, planation, and webbing. Overtopping refers to the occurrence of lateral extensions, but with further growth of the higher part of the plant. The result is an elongation of the plant because one branch is the topmost. Planation describes the planar distribution of branches; that is, they tend to extend in one plane. The presumed advantage of this is to present a broad surface for the absorption of sunlight. Webbing is the appearance of leaf tissue between the branching stem tissue. This, too, aids in presenting a broad surface for absorbing light. Fern fronds formed from overtopping, planation, and webbing are called megaphylls; ferns are megaphyllous plants.

It is worth looking more closely at the tracheid cells of the xylem. These are the transport cells and, within the vascular plants as a whole, we find four kinds. In ferns themost common kind of tracheid is the scalariform one, but ones with bordered pits are found in certain fern species.Fern root structures are best considered prostrate stems, lying along the ground, from which fine filaments of cells extend into the earth or other substratum, such as rocks or even the bark of trees. Ferns are world wide in distribution, but most typically are restricted to moist areas. One important reason for this restriction is that water--from rain, dew, or other sources--is necessary for fertilization to occur. The male gametes of ferns are flagellated and behave like actively swimming sperm cells. There must be water on the gametophyte surface to allow the sperm to swim from the structures where they are formed to the structures in which the nonmotile eggs are situated.

Most species have sporophytic fronds under 1 m tall, but there are still larger ferns growing in special tropical or semitropical habitats. In the late Paleozoic, there were more tree ferns, but evergreens and especially modern flowering shrubs and trees appear to have competed successfully with the tree ferns and replaced them as our dominant modern large plants.

Non-Vascular Plants

There are two divisions or phyla here: the Bryophyta, or mosses, and the Hepaticae, or liverworts. They are often combined into the one phylum, the bryophytes. The essential common feature of mosses and liverworts is that the sporophyte generation is a kind of symbiont with the gametophyte. The latter grows out of the ground in moist places; the former grows out of the gametophyte. The life cycle of a moss is shown in Fig. 12-4. As is characteristic of gametophytes and sporophytes, the one producing the gametes is haploid, the other is diploid. Meiosis occurs in the diploid sporophyte, and haploid spores are formed and then released to germinate and thus start a new gametophyte generation.

In mosses, water is absorbed through rhizoids. These are filamentous chains of cells that extend underground, like roots. The stem is a vertical growth of cells from the rhizoids. Never over a few inches long, it brings water and dissolved minerals up to the small leaflets that grow directly out of the stem. Active photosynthesis occurs in the leaflets. Because the cells are small, water, nutrients, and products of photosynthesis can be distributed by diffusion throughout the plant body. This is thought to be the reason why cells and tissue specialized for a transport function are notably absent. Vascular plants without seeds. Not only do all the remaining land plants show vascular tissues, their sporophyte and gametophyte generations differ from those seen in the mosses and liverworts. From among the various seedless vascular plants, let us look at the ferns as typifying the essential features of this group.

A fern, as most commonly encountered, is a sporophyte, or diploid plant. Its most visible part is the frond (leaf), which extends gracefully upward from a root stock which in most cases has roots extending into the soil. On the underside of the frond, which can be undivided or subdivided into a delicate array of subparts, there are quite often sporangia containing spores. After meiosis, the spores formed here are released; upon germination, a spore produces a tiny, haploid gametophyte. This tiny plantlet develops both sperm-forming and egg-forming parts. A sperm fertilizes an egg and initiates a new diploid sporophyte.

Note, especially, the relative roles of the sporophyte and the gametophyte. Both are necessary for a complete life cycle, but here they draw independently from their environment to exist autonomously. There is no dependence of sporophyte on gametophyte, such as the symbiosis seen in liverworts and mosses. Furthermore, the sporophyte is the more conspicuous of the two plant stages or generations. As regards the details of sporophyte structure, we find the already-mentioned root structures and frond. Vascular tissues are quite well developed in the stem of the frond. Here we can see the xylem, which functions principally to transport water and dissolved minerals, and the phloem, which distributes dissolved foods (mostly carbohydrates)

Land Plants

The transition from an aquatic environment to a terrestrial one involves many adaptive changes. To set the stage for a more careful examination of that transition, we can start by looking at those features common to land plants.

1. There must be protection against evaporation. This, probably the most obvious adaptation, has been achieved by the development of epidermal tissue and, for some structures, a special waxy layer or cuticle external to the epidermis or outermost plant tissue. Delicate tissues, such as leaves, most need the epidermis and its cuticle. Other structures, such as roots and stems and their counterparts in the less complex land plants, are effectively protected by the epidermis. The bark of trees is an obvious example.

3. Gaseous exchange with the environment must be possible. The leafy tissues must not be so completely covered that the release of oxygen and the uptake of carbon dioxide is prevented. Such a two-way flow of gases is essential to life. In the leaves of higher plants this flow is facilitated by special structures called stomata. Under conditions of low humidity, cells on either side of the stomatal opening expand and effectively close the opening. Under other conditions, and depending on the needs of the nearby tissues, the stomata are opened to varying degrees.

4. Water must be absorbed. Roots or root-like structures called rhizoids perform this function. The actual absorption by roots is carried out by root hairs. These microscopic cells extend at right angles from the root surface and provide the cell surface needed to absorb water and salt and other dissolved nutrients that may be present in the water.

5. Materials must be transported throughout the plant body. Photosynthetic products from leafy parts must be available to stems and roots and materials absorbed by roots must be available to stems and leaves. In the more complex land plants, special cells, which constitute the vascular tissue, conduct nutrients throughout the plant. Many land plants lack vascular tissue, but nonetheless transport is achieved by more generalized tissues. But since aquatic plants are surrounded by water, the plant tissues can exchange materials directly with the water, and there is no need for vascular tissue in the algae.

6. Land plants need support to keep them upright. Water is a buoyant medium and in its absence algae collapse into rather pathetic heaps. The special development of tough cell walls is used by plants for support. Additionally, some stem cells are specialized; there, the fiber cells carry the weight of plants. Consider, in particular, a giant redwood; it holds up thousands of tons, for centuries.

7. The gametes and especially, the early stages of new generations, must be protected. The terrestrial environment can be a relatively hostile place in which to germinate and survive. Spores and in particular seeds, which carry embryonicplants, are adapted to survive dry conditions and to respond to wet conditions by germination and subsequent growth.

Present Status of Protistan Origins

We have then two competing views of the origin of the protistan eukaryotes. Both views agree that the protists arose from the prokaryotes; both can account for the observed molecular homologies; and both can explain the morphological gap that separates the prokaryotes and eukaryotes. Also, both suggest that there was a multiple or polyphyletic origin of the Protista. (They both disagree with Pascher.) In other words, both can explain the same set of phylogenetic data. To determine which view is the correct one, other predictions from the two hypotheses will have to be made and tested. This has not yet been done and therefore we are left with both views as options. A summary of protistan phylogeny. Note that other views on protistan evolution differ from this one. Five problem areas can be mentioned: (1) Origin of the protists. This could be by endosymbiosis or by transformation. (2) Diversification of the protophyta. This could be the result of a polyphyletic origin of the algae from the Monera. (1) Origins of the protozoa. There is convincing evidence for a polyphyletic appearance of animal protists (protozoa) from certain colorless algae. (4) Pseupodial evolution. Many different evolutionary experiments among ameboid forms. (5) Kinetidal evolution. Polyphyletic zooflagellates were presumably replaced as free-swimming forms by ciliated cells.

In closing, one more complication should be noted and that pertains to the validity of macromolecular homologies. Usually the argument against homology is stated in terms of random events producing comparable arrays of amino acids or nucleotides, and the argument for homology is stated as due to simi-larity from a common ancestry. That is not correct. It is really convergence versus homology, not random chance versus homology. And we really do not yet know how precise macromolecular convergence can be. The fact that hemoglobin appears in such diverse animals as flatworms, annelids, certain molluscs and insects, some echinoderms, and vertebrates has been widely attributed to convergence. Unfortunately, only the sequence of the vertebrate hemoglobins has been studied; therefore we cannot determine the nature of molecular convergence between different phyla precisely. But it is conceivable that there are only a limited number of ways to build an oxygen-transport molecule. Perhaps only a rather specific sequence of amino acids will be functional, and hence, similar sequences will appear due to similar selection pressures, a process quite different from chance. The same might also apply to similar RNA molecules--their role in ribosomal function might well be so precisely defined as to select for highly similar nucleotide sequences. Evolution produces remarkably similar, complex organs of sight in the cephalopod molluscs (octopus and squids) and vertebrates, which are convergent, not homologous. Perhaps less complicated structures, such as certain functionally identical molecules, will also be precisely convergent. It will be of great theoretical interest to see if the Remanian criteria for homology can be used to detect convergence of molecules.

Molecular Homologies

A recent report from the laboratory of Margaret Dayhoff and her colleagues at the National Institutes of Health ( U.S.) shows that the respiratory protein cytochrome c and the small, iron-containing proteins, the ferredoxins, as well as the 5Sribosomal RNA molecules, all show homologous similarities among monerans and protistans. Of particular interest is the fact that in the eukaryotes the cytochromes are located in the mitochondria and the ferredoxins are obtained from plastids (they are involved in photosynthesis). Similarly other workers have compared the RNA from the plastids of Euglena with moneran RNA and have come up with good evidence of homology. All these molecular studies are based on point-topoint similarities of amino acid sequences in proteins and of nucleotide sequences in RNA.

This kind of evidence strongly supports what is now called the serial endosymbiont theory of protistan origins. This theory was first advanced in the nineteenth century and given its modern form by Lynn Margulis, of Boston University, in 1970. This theory argues that protists are symbiotic associations of monerans. Or, stated more exactly, prokaryotes enter into cooperative associations to form eukaryotes. Margulis and others differ somewhat on the exact details of how this occurred, and those differences are being investigated. Margulis proposes that plastids, mitochondria, and the microtubular structures of eukaryotes are derived from different prokaryotes. For example, a host cell engulfed a blue-green alga that evolved into a plastid. An engulfed aerobic prokaryote could have provided a mitochondrion. And association with a motile prokaryote like a spirochaete could have given rise to flagella, kinetosomes, and other microtubular elements, in short, the primoridal kinetide. In fact, Margulis has proposed that a colorless ameboid prokaryote was the host cell and phagocytosed other prokaryotes of the sort just mentioned. This point of view suggests that protophyta and protozoa, both, are products of serial endosymbiosis.

For reasons given above regarding the homologies between protozoa and colorless protophyta, it seems unnecessary to suggest that the protozoa originated by endosymbiosis. But the suggestion that the protophyta originated by endosymbiosis is attractive and is supported by the molecular evidence given above.

The Protistan Plesiomorph

Pascher, influenced by Haeckel's phylogenetic speculations, constructed a hypothetical ancestor of the Protista. It was a unicellular, photosynthetic cell with two flagella--a phytoflagellate. It had one plastid and a cell wall. From such an ancestor, Pascher believed there could be derived all the algae and from them the higher plants, fungi, and protozoa. In other words, this was the start of eukaryotic evolution. This hypothetical ancestral phytoflagellate of Pascher illustrates the weakness of this kind of phylogenetic speculation. No experiments can be done with it, since it does not exist. Rigorous comparisons for homology are impossible, since it does not exist. The only thing in its favor is that it alerts us to the phylogenetic problems in this area by saying that a biflagellate, unicellular eukaryotic producer is the kind of cell we are looking for. But the worst effect of this kind of thinking is that it is prejudicial to any other theory. The implication is that there was a unitary origin of the Protista. But was there? What is the evidence? It suggests that a certain moneran species evolved into this type of protistan. Did it? We will see shortly that some people favor a startlingly different point of view. In summary, Pascher's suggestion and others like it must be held at arm's length to avoid the distortions they can introduce. Perhaps better, we should ignore them altogether and simply work from plesiomorphs.

Working from plesiomorphs, we immediately face the situation already described, namely, there are many plesiomorphs and there is a huge gap between the plesiomorphs and the prokaryotes. The next step is to look for homologies, which, as we have also noted, will probably have to be molecular. Are there any? The growing answer is yes, there seem to be some, but the story is not yet entirely convincing.

Molecular Algae

The green algae have sometimes been classified among the Metaphyta for the obvious reason that many of their member species are organized as highly integrated colonies or even as multicellular plants. We have treated them as protistans because of their obvious derivation from unicellular algae. By contrast, the brown algae (Phaeophycophyta) show no unicellular forms except for gametes, and in the red algae (Rhodophycophyta) there are only a few unicellular species.

Multicellular plants typify the red and brown algae. They are differentiated into the same general structures we encountered in the green algae, i.e., tissues specialized as holdfasts to anchor the plant to the substratum, and above that a buoyant thallus or vegetative structure. The thallus varies enormously in size and shape. The largest ones, found in the California kelps, are long blades of brown algae 50 m or more in length. Furthermore, the thallus can be organized variously into unbranched or branched structures and can be flattened blades or cylindrical stems. Often in these algae reproductive structures form another kind of tissue specialization with male and female gametes being produced in different parts of the plant. Finally, alternation of generation occurs. In the red algae there can be three different generations of plants before a life cycle is completed. This involves various combinations of haploid and diploid stages.

No motile cells have ever been found in the red algae, not even among their gametes. The sexual structures typically produce well-differentiated eggs and sperm--so well differentiated, in fact, that fertilization is referred to as oogamy. The male gamete is released into the watery environment of these plants and apparently, through random motion, comes into contact with and fertilizes the egg. The brown algae, however, have flagellated gametes and zoospores. Fertilization of gametes produces a zygote that germinates and develops into a sporophyte--the diploid phase of these plants. Meiosis occurs in the special cells that form zoospores, and these flagellated cells swim free and attach to the bottom where they then develop into haploid gametophytes.

There are various parallels between the organization of thethallus in the red and brown algae and the tetrasporalian line of evolution in the green algae. The result is an impressive array of multicellular aquatic plants. Quite naturally, the question arises as to which of them might be ancestral to the land plants. Or perhaps, the land plants are polyphyletic and have ancestors in all three algal phyla. Answers become clearer when we make certain other comparisons between these algae and the terrestrial plants. All these plants contain chlorophyll a among their photosynthetic pigments, but differ in terms of the other pigments, except for the Chlorophycophyta and the terrestrial plants. These latter two groups have the same photosynthetic pigments, food reserves, and cell wall components. This strongly suggests that the green algae were ancestral to these terrestrial plants. The red algae are the most different in terms of the characters cited. Note their photosynthetic pigments: these include phycoerythrin and phycocyanin, which occur only here and among the blue-green algae. This too has important evolutionary implications.

Protistan Origins

There are as many candidates for the protistan plesiomorph as there are protophyte phyla, since we cannot decide which phylum is plausibly ancestral to the others. Although a species of Chlamydomonas would be an excellent plesiomorph for the Chlorophycophyta, and a species of Euglena the plesiomorph for the Euglenophycophyta, we have no good evidence for deriving an euglenoid cell from a chlamydomonad cell or vice versa; and similarly for a chrysomonad or pyrrophycophytan (dinoflagellate) plesiomorph. All are equally good or equally bad as a protistan plesiomorph. This situation can be explained in two ways: (1) we have as many independent origins of the protophyta from the Monera as we have separate plesiomorphs, and (2) there was a unitary origin of the protophyta, but the rapid, tachytelic evolution, in going from the adaptive zone of the metabolically specialized Monera (prokaryotes) to that of the structurally complex Protista (eukaryotes), resulted in rapid adaptive radiation into various protophyte phyla. In both cases we can argue that many intermediate forms, i.e., those that are neither good prokaryotes nor good eukaryotes, were lost and, hence, there is a considerable evolutionary gap. We are trying to peer across this gap; we are trying to reconstruct, conceptually, a phylogenetic bridge. Our phylogenetic methods tell us to look for serial relations to bridge such a gap. But that may well be futile in terms of cellular structure. Researchers have been aware of the difference between prokaryotes and eukaryotes for decades and have looked in vain for intermediates or missing links. They may yet turn up as fossils, but it is questionable that the needed fine-structural detail will be preserved. The best remaining possibility is molecular data. We need to compare conservative or plesiosemic molecules in protophyte plesiomorphs with each other and with comparable molecules in the monerans. Some such comparisons have been made, as we will see. Before turning to them, it is worth mentioning the work of the German botanist Pascher, who was a profound student of the algae in the early part of this century.

The Role of Consumer

The role of consumer or ingestor is also that of predator, which usually means that the predator must be larger than the prey; otherwise, ingestion is impossible. This puts a selective advantage on increase in size, at least for certain predators. That has many functional as well as evolutionary complications. Increase in size means new solutions to locomotion. The kinetide solves that by becoming compound and then complex in response to selection pressures. Increase in size means an adjustment of nuclear content to cells needs. Various solutions are found. Large flagellates (and ameboid forms) become multinucleate. Large ciliates develop a macronucleus, a highly polyploid structure. Large size also demands coordination of various parts of the cell body. The microtubules and microfilaments of the kinetide as well as associated membranes seem to act as coordinators. In addition to coordination of body parts, a predator must coordinate sensory input with body function. Predators must be able to locate prey, capture it, and ingest it. In the ciliates, especially, the kinetides aid in all this. Ciliates do respond chemotactically and thigmotoctically (by touch) to prey in their vicinity. They swim toward their prey. Prey is swept into the cytostome or captured by other special cortical organelles, and then ingested.

This predatory behavior puts a premium on body specialization, which includes sensory, locomotory, and ingestatory apparatuses, all within a single cell. It demands a complexity of the cell body not found in any other living thing. The protozoa are the most complex of all cells, and the ciliates are supreme among the protozoa. We see in them the consequences of the selective pressures arising from a predatory mode of life. In the multicellular animals this means bilaterally symmetrical forms with specialized anterior ends. Theanterior ends typically carry special sensing devices--eyes, ears, noses, taste buds--and ingestatory structures--jaws and mouths. Bilateral symmetry allows these predators to develop the specific orientation--right and left, up and down--necessary for predation. All these evolutionary innovations are anticipated by the ciliates. They will be discussed in further detail when we come to metazoan origins. Now let us make some concluding comments on kinetidal protozoa.

The flagellates and ciliates get larger, develop more complex kinetidal and nuclear apparatuses, and become more complex; this complexity includes permanent diploidy and polyploidy in the ciliates. The key step, in going from the zooflagellates to the ciliates, is the emergence of a permanent mouth, a hallmark of predation. This neosemic character may well explain why most of today's zooflagellates are symbiotic and why the ciliates are mostly free swimming. The now more efficient predators, the ciliates, eliminated their ancestors, the zooflagellates, except in cases in which the latter had invaded specialized niches. These niches appear today as host organisms. But note, when the host organisms are multicellular animals, those hosts evolved after the appearance of the protozoa. Presumably the zooflagellates survived in special, free-living niches and then when opportunities for symbiosis arose, they took them. Extant symbiotic zooflagellates can therefore be viewed as relict populations of once widespread free-living species, now replaced by the ciliates.