Plant Ancestry – Where are the Lines of Descent? – Part 1

 

Peculiar Characteristics of Plants Defy Evolution

By Margaret Helder, PhD (Botany)

Some moss is famous

Have you ever seen a pleasure boat proceeding smoothly through a grain field? It so happens that the beautiful province of Friesland in the Netherlands is riddled with canals. With the water almost level with the top of the soil, it looks as if boats on nearby canals are improbably proceeding across farmers’ fields, when actually they are navigating along narrow canals.

These beautiful canals are the historic result of wholesale mining of peat from raised bogs in this region of the Netherlands including the provinces of Groningen and Drenthe. Apparently large-scale peat cutting spread over this region in the 16^th and 17^th centuries in order to supply fuel for fast growing urban areas farther to the south. It was the same in other nearby European countries such as Denmark, Germany, England and Ireland.

The wholesale extraction of peat has continued in many regions even up to present times. Now we know that peat is largely composed of Sphagnum moss which famously causes its environment to become much more acidic. This acid drastically inhibits microbial action and decay. Thus Sphagnum and other plant matter build up over time (hence the raised peat bogs). Apparently also other organic remains can be preserved in these bogs. Still, it was a surprise in May of 1950 when two peat cutters in Denmark unearthed the corpse of a man with a leather noose around his neck. The man was buried under 2 m (6.5 ft) of peat. Further investigation suggested that the man had lived during the 4^th century B.C. Apparently the body of a woman had already been discovered nearby in 1938. Since then similar discoveries have been made in England, Ireland, Germany and the Netherlands.

Unusual lifestyles

Most people are aware of moss plants, at least the remains of Sphagnum, which we call peat moss. Like Sphagnum, most mosses are at most about 2 inches (5 cm) tall. They grow in moist, shady areas for the most part. What many people do not know is that the green moss plants which we see, are one of two generations which must follow each other. The cells of the green plants contain only one set of chromosomes in their nuclei whereas most plants, with which we are familiar, have two sets. So the green moss plants are haploid (means one set of chromosomes) and they bear tiny multicellular sexual organs at their tips. Some plants have flask shaped structures which shelter a single egg. Others have tiny globose structures which release lots of swimming sperm. With the help of splashing rainwater, the sperm make it to the egg. The fertilized egg (now diploid with two sets of chromosomes) grows in place, emerging upward from the female archegonium (sexual organ).

A new thin stem grows vertically attached to the original moss plant. This new diploid growth (generation) is topped by a capsule covered by a cute little cap. The cap falls away and the capsule disperses spores which are once again haploid. So we have a green haploid sexual generation (gametophyte) followed by an attached non-green diploid spore-producing generation (sporophyte) followed again by a haploid obvious green moss generation.

There are other plants which exhibit the same alternation of generations. The liverworts are small, flexible flat green growths with curled edges, which have obvious pores all over their upper surface. These pores open downward into chambers which contain photosynthetic cells. On the surface, there are conspicuous cups which catch rain water and use it to eject miniature new plants.  These green plants are all haploid. From the leafy base, they produce some stalks with flat expanded tops. These contain the sex organs. Once a swimming sperm has been splashed to a female stalk, a sac-like sporophyte (diploid generation) produces spores which are haploid and which germinate into the leaf-like haploid generation.

Yet a third kind of plant, with the same kind of alternation of generations, are the hornworts. The haploid generation looks much like liverworts, but the diploid generation grows taller and is more elaborate, containing green cells, a cylinder of sterile cells as well as spore producing cells and special pore cells on the surface called stomata, which open and close, allowing for exchange of gases between the plant and the air outside.

 

New study involves land plants

None of these plant types has conducting tissue (xylem and phloem) or roots, or leaves and many other features that vascular plants display. Because the mosses, liverworts and hornworts are small with fewer features, they have seemed like the obvious candidates to propose as ancestors of all the rest of the land plants. But when we look at the details, this idea seems not to work. A cover story on plant evolutionary relationships in the October 31, 2019 issue of Nature, displays moss gametophytes and sporophytes on the cover. The article admits concerning the three groups of small plants (collectively called bryophytes) that: “Resolving relationships among bryophytes (mosses, liverworts and hornworts) and their relationships to the remaining land plants has long been problematic …”¹ Nevertheless in three short paragraphs they promote a line of descent, and move on to other plant groups. But a more detailed look at the details of these plants and of the vascular plants (also called tracheophytes) reveals an interesting story.

The study from the October 31, 2019 Nature compares the transcriptomes of one thousand green plants, everything from green algae to organisms like palm trees and oak trees. There have been many studies which compare the sequences of DNA in various organisms, but not all DNA in all organisms has obvious functions. Some DNA may play roles other than being copied in order to produce needed products for the cell. The obviously functional DNA is copied by the cell into RNA, which is then further edited. It is the sum total of all the messenger RNA which will be used to produce products needed by the cell, that this study compared. The word transcriptome thus indicates that they were studying the information in the cells that directly leads to functional molecules. With all that information and the authors’ confidently indicated lines of descent, what could be wrong with their conclusions? Plenty.

Special features of land plants

Although the mosses, liverworts and hornworts are small and prefer sheltered, moist habitats, they are very different from algae. Whereas in water, whether marine or fresh, algae are surrounded by most of their basic needs. Water contains dissolved minerals and gases and of course water is not in short supply either. Water also provides buoyant support for the organism so that it does not need extra strengthening. Generally, most of the cells in an alga are able to divide indefinitely. It is the angle at which cell division occurs which confers a characteristic shape on an individual algal colony or filament.

Contrast the algal situation with that of land plants. Water must be accessed either from the soil or from sporadic rain or both. Surface water evaporates, so that the plants need a firm epidermis covered by a waxy cuticle to reduce water loss. Most land plants also increase bulk (volume) to reduce surface area drying. One highly important feature in land plants is that they exhibit terminal or lateral meristems or centres of cell division. Behind the meristem, the newly produced cells merely mature into whatever tissue they are supposed to become. In keeping with the meristems and differentiation of specialized tissues, co-ordination of different parts of the organism is essential. Thus, signaling is important in this endeavor and it is an important difference between land plants and algae. For example, Bowman et al. declared: “One hallmark of multicellular life is a plethora of signaling pathways by which cells communicate, influencing cell specification, differentiation, and physiology.” ² Indeed the authors conclude that auxin (signaling protein) is essential for controlling the vast diversity of land plant shapes, even including the liverworts, mosses and hornworts.³

But there are other important differences between algae and land plants. One is the remarkable ability of land plants to respond to gravity. In most plants, the shoot grows up, against gravity while roots (or simple rhizoids in bryophytes) grow down. Thus, textbook authors in 1972 declared: “Land plants have developed highly sensitive methods of detecting gravity and mechanisms for altering growth patters with respect to gravity. This, too, is a necessary evolutionary step if plants are to develop on land.” ⁴

We will investigate differences between bryophytes and tracheophytes next time, and see whether an evolutionary ancestry can be inferred from algae to land plants or between any of these groups.

To Be Continued on Monday, March 30.

References

1. 1. One Thousand Plant Transcriptomes Initiative (James H. Leeben-Mack et al. (total of 38 authors listed on-line) 2019. One thousand plant transcriptomes and the phylogenomics of green plants. Nature 574 # 7780,  679-685. See p. 681.https://doi.org/10.1038/s41586-019-1693-2
2. 2. John L. Bowman et al. (total of 113 authors). 2017. Insights into Land Plant Evolution Garnered from the Marchantia polymorpha Cell 171, 287-304, October 5. See p. 294.
3. 3. Bowman et al. 299.
4. 4. William A. Jensen and Frank B. Salisbury. 1972. Botany: An Ecological Approach. Wadsworth Publishing Company, Inc. pp. 758. See p. 387.

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Margaret Helder completed her education with a Ph.D. in Botany from Western University in London, Ontario (Canada). She was hired as Assistant Professor in Biosciences at Brock University in St. Catharines, Ontario. Coming to Alberta in 1977, Dr Helder was an expert witness for the State of Arkansas, December 1981, during the creation/evolution ‘balanced treatment’ trial. She served as member of the editorial board of Occasional Papers of the Baraminology Study Group in 2001. She also lectured once or twice a year (upon invitation) in scheduled classes at University of Alberta (St. Joseph’s College) from 1998-2012. Her technical publications include articles in the Canadian Journal of Botany, chapter 19 in Recent Advances in Aquatic Mycology (E. B. Gareth Jones. Editor. 1976), and most recently she authored No Christian Silence on Science (2016) which promotes critical evaluation of scientific claims. She is married to John Helder and they have six adult children.