seed dormancy

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Book Review: The Gyroscope of Life

Gyroscopes are entertaining toys and incredibly useful tools. They retain their balance and resist changes to their orientation as long as their flywheel is spinning. As the flywheel slows or stops, the gyroscope wobbles out of control and ultimately quits. Considering their design and function, it’s easy to find parallels between gyroscopes and living systems. Consistent energy inputs keep living things alive. Changes can bring imbalance; major disruptions can lead to death. There is a reason we often describe the natural world as a sort of balancing act. It is the work of an ecologist to make sense of this balancing act. The better we understand it, the more equipped we are to protect it and operate responsibly within it.

It is through this lens that David Parrish writes about the biological world in The Gyroscope of Life, a book that Parrish refers to as “a love song to the field of biology.” Parrish has spent much of his life observing and studying the natural world and, as professor emeritus of Crop and Soil Environmental Sciences at Virginia Tech, undoubtedly shared much of what he presents in his book with countless students over the years. The Gyroscope of Life reads like part memoir and part last lecture, and is the work of someone who has an obvious passion for science and nature.

Parrish spends the first few chapters of his book writing mostly about his life and how he came to be a biologist. He acknowledges his privelege – “born male, white, and American in an era where each of those attributes provided me major advantages” –  having essentially been placed on third base from the start, “well down the third base line.” An aspiring zoologist turned botanist, he spent his early years in graduate school studying seeds and seed dormancy. It’s a topic that obviously interests him, as several pages of the book are spent considering what’s going on inside of a seed. “Seeds provide the widest-spread examples of suspended life,”  Parrish says. Are they alive or dead or neither?

Two additional, major life events play a prominent role in the arc of Parrish’s book. One being his break from organized religion and the other his battle with advanced prostate cancer. He grew up in an orthodox Christian home with a very literal understanding of the Bible. His education put him at odds with what he was taught growing up about (among other things) the age of the earth and its creation. Eventually he came to understand that science and religion “exist in separate non-overlapping spheres – the physical and the metaphysical.” He doesn’t necessarily see science and religion as being inherently at odds with each other, but his understanding of science makes it difficult to “find resonance in religion” due to the “cacophony of dissonance” it offers.

In addressing his prostate cancer, Parrish underwent an operation that gave him a newfound perspective on gender. Freed from “testosterone poisoning,” he was able to more fully consider sex and gender from a biological perspective, which he says he had been doing for decades prior to the operation. He spends a good portion of the book “demystifying sex and gender.” One compelling example he offers involves avocado flowers, which actually change gender over time, a phenomenon known as synchronous dichogamy.

avocado flowers (Persea americana) via wikimedia commons

Over the course of its pages, The Gyroscope of Life covers a significant number of topics in the fields of biology and ecology. It’s a relatively short book, but as it careens through such wide-ranging material, it does so in an approachable and suprisingly succint manner. Parrish’s sense of humor, which doesn’t waver despite how bleak the discussion sometimes gets, helps carry the story along and keeps things interesting. Parrish covers evolution (“[Biologists] argue that, if evolution didn’t happen, it should.”), taxonomy (“the name for naming things”) and sytematics, ecological niches (“[humans] are essentially living niche-free and ecosystemless”), domestication, and so much more. The last chapter is spent discussing agroecosystems (“the organisms and abiotic environment that interact in a human-managed agricultural setting”), a topic he spent much of his career studying.

The underlying message of this book, as I see it, is a simultaneous celebration for life on earth and a concern for the direction things are going considering how humans have managed things. Parrish has some admonition for humans in light of how we’ve treated our home planet, but he isn’t too heavy-handed about it. Overall, reading the book felt like sitting in on a lecture given by a friendly and dynamic professor who has obviously given a lot of thought to what he has to say.

Check out the following video to see David Parrish describe the book in his own words.

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Dr. Beal’s Seed Viability Experiment

In 1879, Dr. William J. Beal buried 20 jars full of sand and seeds on the grounds of Michigan State University. He was hoping to answer questions about seed dormancy and long-term seed viability. Farmers and gardeners have often wondered: “How many years would one have to spend weeding until there are no more weeds left to pull?” Seeds only remain viable for so long, so if weeds were removed before having a chance to make more seeds, the seed bank could, theoretically, be depleted over time. This ignores, of course, the consistent and persistent introduction of weed seeds from elsewhere, but that’s beside the point. The question is still worth asking, and the study still worth doing.

When Dr. Beal set up the experiment, he expected it would last about 100 years, as one jar would be tested every 5 years. However, things changed, and Dr. Beal’s study is now in its 140th year, making it the longest-running scientific experiment to date. If things go as planned, the study will continue until at least 2100. That’s because 40 years into the study, a jar had to be extracted in the spring instead of the fall, as had been done previously, and at that point it was decided to test the remaining jars at 10 year intervals. In 1990, things changed again when the period was extended to 20 years between jars. The 15th jar was tested in 2000, which means the next test will occur in the spring of next year.

In preparing the study, Dr. Beal filled each of the 20 narrow-necked pint jars with a mixture of moist sand and 50 seeds each of 21 plant species. All but one of the species (Thuja occidentalis) were common weeds. He buried the jars upside down – “so that water would not accumulate about the seeds” – about 20 inches below ground. Near each bottle he also buried seeds of red oak and black walnut, but they all rotted away early in the study.

After the retrieval of each bottle, the sand and seed mixture is dumped into trays and exposed to conditions suitable for germination. The number of germinates are then counted and recorded. Over the years, the majority of the seeds have lost their viability. In 2000, only three species germinated  – Verbascum blattaria, a Verbascum hybrid, and Malva rotundifolia. There were only two individuals of the Verbascum hybrid, and only one Malva rotundifolia. The seeds of Verbascum blattaria, however, produced 23 individuals, suggesting that even after 120 years, the seeds of this species could potentially remain viable long into the future.

moth mullein (Verbascum blattaria)

In the 2000 test, the single seedling of Malva rotundifolia germinated after a cold treatment. Had the cold treatment not been tried, germination may not have occurred, which begs the question, how many seeds in previous studies would have germinated if subjected to additional treatments? Dr. Beal himself had wondered this, expressing that the results he had seen were “indefinite and far from satisfactory.” He admitted that he had “never felt certain that [he] had induced all sound seeds to germinate.”

There are also some questions about the seeds themselves. For example, the authors of the 2000 report speculate that poor germination seen in Malva rotundifolia over most of the study period could be “the result of poor seed set rather than loss of long-term viability.” The presence of a Verbascum hybrid also calls into question the original source of those particular seeds. A report published in 1922 questions whether or not the seeds of Thuja occidentalis were ever actually added to the jars, and also expresses uncertainty about the identify of a couple other species in the study.

Despite these minor issues, Dr. Beal’s study has shed a great deal of light on questions of seed dormancy and long-term seed viability and has inspired numerous related studies. While questions about weeds were the inspiration for the study, the things we have been able to learn about seed banks has implications beyond agriculture. Seed bank dynamics are particularly important in conservation and restoration. If plants that have disappeared due to human activity have maintained a seed bank in the soil, there is potential for the original population to be restored.

In future posts we will dive deeper into seed banks, seed dormancy, and germination. In the meantime, you can read more about Dr. Beal’s seed viability study by visiting the following links:

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Seed Oddities: Vivipary

Seeds house and protect infant plants. When released from their parent plant, they commence a journey that, if successful, will bring them to a suitable location where they can take up residence (upon germination) and carry out a life similar to that of their parents. Their seed coats (and often – in the case of angiosperms – the fruits they were born in) help direct them and protect them in this journey. Physical and chemical factors inhibit them from germinating prematurely – a phenomenon known as dormancy. Agents of dispersal and mechanisms of dormancy allow seeds to travel through time and space — promises of new plants yet to be realized.

There is rarely a need for a seed to germinate immediately upon reaching maturity. In many cases, such as in temperate climates or in times of drought or low light, germinating too soon could be detrimental. The most vulnerable time in a plant’s life comes when it is a young seedling. Thus, finding the right time and space to get a good start is imperative.

The fruits (and accompanying seeds) of doubleclaw (Proboscidea parviflora) are well equipped for long distance dispersal. (via wikimedia commons)

In rare instances, dispersal via seeds offers little advantage; instead, dispersal of live seedlings or propagules is preferable. For this select group of plants, vivipary is part of the reproductive strategy. In vivipary, seeds lack dormancy. Rather than waiting to be dispersed before germinating, viviparous seeds germinate inside of fruits that are still attached to their parent plants.

Occasionally, seeds are observed germinating inside tomatoes, citrus, squash, and other fruits; however, these fruits are usually overripe and often detached from the plant. In these instances, what is referred to as “vivipary” is not a genetic predisposition or part of the reproductive strategy. It’s just happenstance – a fun anomaly. The type of vivipary discussed in this post is actually quite rare, occurring in only a handful of species and prevalent in a select number of environments.

There are three main types of vivipary: true vivipary, cryptovivipary, and pseudovivipary. In true vivipary, a seed germinates inside the fruit and pushes through the fruit wall before the fruit is released. In cryptovivipary, a seed germinates inside the fruit but remains inside until after the fruit drops or splits open. Pseudovivipary is the production of bulbils or plantlets in the flower head. It does not involve seeds and is, instead, a form of asexual reproduction that will be discussed in a future post.

True vivipary is commonly seen among plant communities located in shallow, marine habitats in tropical or subtropical regions, such as mangroves or seagrasses. The term mangrove is used generally to describe a community of plants found in coastal areas growing in saline or brackish water. It also refers more specifically to the small trees and shrubs found in such environments. While not all mangrove species are viviparous, many of them are.

Seedlings of viviparous mangrove species emerge from the fruit and drop from the plant into the salty water below. From there they have the potential to float long or short distances before taking root. They may land in the soil upright, but often, as the tide recedes, they find themselves lying horizontally on the soil. Luckily, they have the remarkable ability to take root and quickly stand themselves up. Doing this allows young plants to keep their “heads” above water as the tides return. It also helps protect the shoot tips from herbivory.

Viviparous seedlings emerging from the fruits of red mangrove (Rhizophora mangle) via wikimedia commons

Another example of vivipary is found in the epiphytic cactus (and close relative of tan hua), Epiphyllum phyllanthus. Commonly known as climbing cactus, this species was studied by researchers in Brazil who harvested fruits at various stages to observe the development of the viviparous seedlings. They then planted the seedlings on three different substrates to evaluate their survival and establishment.

Epiphyllum phyllanthus is cryptoviviparous, so the germinated seeds don’t leave the fruit until after it splits open. In a sense, the mother plant is caring for her offspring before sending them out into the world. The researchers see this as “a form of parental care with subsequent conspecific [belonging to the same species] nursing.” Since the plant is epiphytic – meaning that it grows on the surface of another plant rather than in the soil – local dispersal is important, since there is no guarantee that seeds or propagules dispersed away from the host plant will find another suitable site. That being said, the researchers believe that “vivipary involves adaptation to local dispersal,” since “the greater the dispersal distance is, the higher the risk and the lower the probability of optimal dispersion.”

Epiphyllum phyllanthus via Useful Tropical Plants

While some viviparous seedlings of mangroves can travel long distances from their parent plant and don’t always root into the ground immediately, they maintain their advantage over seeds because they can root in quickly upon reaching a suitable site and lift themselves up above rising tide waters. As the authors of the Epiphyllum study put it, vivipary is “a reproductive advantage that, in addition to allowing propagules to root and grow almost immediately, favors quick establishment whenever seedlings land on suitable substrates.”

There is still much to learn about this unusual and rare botanical feature. The research that does exist is relatively scant, so it will be interesting to see what more we can discover. For now, check out the following resources:

Also, check out this You Tube video of :