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AN
OVERVIEW OF PLANTS |
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Notes for Plant Taxonomy. Botany 3400 at Utah State University. Prepared by Mary E. Barkworth |
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Key
words: Alternation of
generations; Angiosperms; Bryophytes;
Circumscription of Plantae; Conifers;
Cycads; Evolution
of land plants; Gnetophytes; Horsetail
lineage; Land plant lineages;
Mycorrhyzae; Plantae;
Progymnosperms; Requirements
for land plants; Rhyniophytes;
Seed ferns; Tracheopytes;
Trimerophytes; True
ferns. References: Graham, L.E. 1993. Origin of land plants. John Wiley & Sons., Ltd. Stewart,W.N. 1983. Paleobotany and the evolution of plants. Cambridge University Press. W.N. Taylor, T.N. and Taylor, E.L. 1993. The biology and evolution of fossil plants. Prentice Hall. |
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What
belongs in the kingdom Plantae? The green algae are sometimes placed in separate kingdom from the land plants, but there is little doubt that all land plants have evolved from green algae, the class Charaphyceae in particular, so it is difficult to support their placement in a different kingdom. Evolution
of Land Plants (a subset of Plantae) Initially, the earth’s atmosphere had very low concentrations of oxygen, perhaps less than 0.01% of present levels. The first evidence of life is from 3,500 MYP (Millions Years before Present) and it shows considerable diversity, suggesting that it had been around for some time. Oxygen generating organisms appeared over 1,000 MYP (note the long time that it took). The organisms involved were bacteria, both blue-green and others. Oxygen is important for metabolism, but it also important as a source of ozone, O3. The ozone in the atmosphere absorbs much of the sun’s ultraviolet light. The problem with ultraviolet light is that it disrupts DNA. Most such disruptions lead to disadvantageous changes in both plants and animals (for instance, skin cancer), so land plants did not evolve until after the aquatic bacteria and algae had generated enough oxygen to create thick enough ozone layer to lower the ultraviolet levels to a suitable level. This took rather a long time, the first records of land plants being from the Ordovician, a mere 400-500 MYP. Multicellular fungi and algae, including green algae, had been around for several hundred million years by that time. As you are probably aware, the ozone layer is now getting thinner, thanks largely to human activity. It is unlikely that one alga all of a sudden became a terrestrial plant. In all likelihood, some plants acquired the ability to withstand short periods of desiccation. These may have given rise to those that were able to do so for almost all of their life cycle. Some characteristics are critically important to plants living on land. Several fossil plants seem to have had some of these characteristics and are, therefore, thought to have spent some or all of their life on land. What abilities are needed for life on land? 1. Ability to withstand desiccation. Extant land plants have a cuticle and guard cells. 2. Ability to withstand the effects of more intense radiation, particularly DNA-damaging radiation. Extant land plants have several compounds in their vacuoles that absorb UV. Since the vacuole of a plant occupies most of a mature cell, this helps protect the DNA in other organelles. 3. Ability to protect their spores from desiccation. Early land plants have spores that are encased in a sporopollenin wall. Sporopollenin is a very resistant polymer, resistant to UV and almost everything including desiccation, squashing, etc. To remove sporopollenin from spores, one boils them in a mixture of acetic and hydrochloric acid. 4. Ability to move solutions from the ground to portions of the plant that are not in contact with the ground, and from the photosynthetic portions of the plant to non-photosynthetic portions. Some land plants do this better than others. 5. Ability to support themselves. Aquatic plants float; terrestrial plants cannot do so. Most terrestrial plants have lignin in some of their conducting cells. There is some debate as to whether this was selected for by the advantages of growing tall or the need to protect against embolism in the conducting cells. Since both are important, it seems most realistic to accept that both contributed to the success of plants with the ability to manufacture lignin, the tracheophytes or vascular plants. 6. Ability to acquire the carbon dioxide required for photosynthesis from the atmosphere. This ability is associated with stomatal cells, specialized cells that surround openings (stomates) in the outer cell layer of land plants. Some individuals have suggested that the evolution of land plants was promoted, or even dependent on, the formation of mycorrhizal associations. There is fossil evidence that at least some early land plants had mycorrhizae, so this is feasible. How important it was is not clear. We know that many of today’s plant either require or do very much better if they establish a mycorrhizal association. Many weeds are exceptional in this regard, which enables them to move more rapidly than non-weedy species into areas where the soil fungi have been destroyed. LAND PLANT LINEAGESThere are two major groups of land plants, bryophytes and tracheophytes. I am not going to worry too much in these notes about the rank at which these are recognized, which is why I shall use the English-language form of their names. Notice that I used the term ‘group’ rather than lineages. Both groups are thought to be descended from green algae, but how many different Charaphycean lineages are involved is not known. It may well be that some of the groups lineages within each of the two groups originated from a different Charaphycean lineages, but that need not concern you in this course. There are two major differences between bryophytes and tracheophytes (aka vascular plants):
ALTERNATION OF GENERATIONSIn discussing different plants groups, it is useful to distinguish between two phases of their life cycle because one of the major differences between the groups is the proportion of their life cycle that is spent in each phase.
The sporophyte phase starts with sex, the process during which when two gametes (a sperm and an egg) combine to form a zygote. This zygote then divides by mitosis to form either multiple single-celled sporophytes or a single multicellular sporophyte. Every cell in a sporophyte contains two complete sets of chromosomes. At some point, this sporophyte will make specialized cells that undergo meiosis, producing cells that have only one set of chromosomes. These are the first cells of the gametophyte phase. The haploid cells that were the result of meiosis will undergo mitotic divisions, producing the gametophyte phase of the plant. Like the sporophytes, the gametophyte phase may be unicellular or multicellular. In land plants they are multicellular or at least multinuclear. Each cell in a gametophyte contains only one set of chromosomes. Eventually the gametophyte phase will produce gametes (egg and sperm cells) in some part of its body. In plants, gametes are produced by mitotic division because they are produced by cells that already have only one set of chromosomes. [In humans, there is no phase corresponding to the gametophyte phase of plants gametes are the direct result of meiosis in a cell that contains two sets of chromosomes]. The gametes fuse, forming a zygote, the first cell of the sporophyte phase. Appreciating this basic pattern of alternation between a diploid, sporophytic phase and a haploid, gametophytic phase will make it easier to understand some of the major changes that have occurred in the evolution of land plants. The sporophytic phase can be regarded as ‘genetically safer’ than the gametophytic phase. If a mutation occurs in a cell of the haploid, gametophytic phase, it is likely to be fatal. There will not be a second copy of that portion of the DNA in the cell. If a mutation occurs in one strand of DNA in a cell of the diploid, sporophytic phase, it is possible that the strand in the second copy of the DNA present in the cell will be able to meet the cells needs. In other words, diploid cells can be thought of as having a backup set of genes. This is, as you might guess, an oversimplification, but it is a reasonable hypothesis. There is another aspect to this possession of backup genes that is important. It is possible that some mutations that occur in a cell, although not immediately advantageous, may become so when circumstances change. The extra set of chromosomes can, in other words, can store potentially useful mutations. The last two paragraphs may suggest to you that it would be advantageous for a plant to spend most of its life in the sporophytic phase. Such a suggestion is reinforced by the observation that the dominant plants today, the angiosperms, spend the smallest proportion of their life cycle in the gametophytic phase and the fact that, so far as we can tell, more recently evolved groups spend a lower proportion of their time as gametophytes than groups that evolved earlier. I shall return to this theme again as we discuss the various groups. BryophytesPlants in the
bryophyte lineage have a cuticle, specialized absorbing tissue, spores
that are protected by sporopollenin, flavonoids (a class of UV-absorbing
compounds), and stomates, but they failed to develop effective supporting
tissue and an effective within-plant solute transportation system
(otherwise known as a vascular system). Mosses are probably the bryophytes
with which you are most familiar, but there are at least two other major
lineages in the group – liverworts and hornworts.
Some bryologists are also arguing that Takakia and Sphagnum
represent two other lineages. We shall not be discussing bryophytes in
this course, but they are too intriguing to be completely overlooked.
Tracheophytes
The subsequent evolution of tracheids was a key feature in evolution of land plants. Tracheids eventually became more specialized, some becoming more efficient for transport of solutions from the soil to leaves, others for transport of sugars from leaves to other parts of the plant, and some for support. We shall leave such details for now. Tracheophytes have been far more successful than bryophytes, but they have not eliminated bryophytes. They are considered to represent a different lineage from bryophytes. Where would you expect bryophytes to do particularly well? The earliest vascular plant known is Cooksonia, which dates from 430 MBP, a mere 20-40 million years after the first algal land plant. The fossil record suggests that many lineages of tracheophytes have gone extinct. I shall be pointing out the major changes that gave the groups that we see today an edge over their competitors. I shall also point out major changes in climate that have occurred because these have contributed to the extinction of some lineages and survival of others. You might wish to consider what abilities the founders of a new highly successful lineage of plants should have in order to become the dominant plant group of the future. As you do so, remember that an important factor is “What can the competition do?” [Business plans also have to address this question]. RHYNIOPHYTES Some of the rhyniophytes combine features that suggest bryophytes with features that suggest tracheophytes. They probably represent several different lineages but, in the absence of a time machine, it is difficult to develop a clear picture of the relationship of the many different fossils to extant organisms. Moreover, most fossils are incomplete; this exacerbates the difficulty of determining their relationships. LYCOPOD
LINEAGE – from Rhyiophytes TRIMEROPHYTES HORSETAIL
LINEAGE TRUE FERNS SEED FERNS PROGYMNOSPERMS CYCAD
LINEAGE CONIFER
LINEAGES ANGIOSPERM
LINEAGE |
