Dispersal by Open Sesame!

In certain instances, “open sesame” might be something you exclaim to magically open the door to a cave full of treasure, but for the sesame plant, open sesame is a way of life. In sesame’s case, seeds are the treasure, which are kept inside a four-chambered capsule. In order for the next generation of plants to have a chance at life, the seeds must be set free. Sesame’s story is similar to the stories of numerous other plant species whose seeds are born in dehiscent fruits. But in this instance, the process of opening those fruits is fairly unique.

Sesamum indicum is a domesticated plant with a 5000 plus year history of cultivation. It shares a genus with about 20 other species – most of which occur in sub-Saharan Africa – and belongs to the family Pedaliaceae – the sesame family. Sesame was first domesticated in India and is now grown in many other parts of the world. It is an annual plant that is drought and heat-tolerant and can be grown in poor soils and locations where many other crops might struggle. However, the best yields are achieved on farms with fertile soils and adequate moisture.

image credit: wikimedia commons

Depending on the variety and growing conditions, sesame can reach up to 5 feet tall and can be unbranched or highly branched. Its broad lance-shaped leaves are generally arranged directly across from each other on the stem. The flowers are tubular, similar in appearance to foxglove, and are typically self-pollinated and short-lived. They come in shades of white, pink, blue, and purple and continue to open throughout the growing season as the plant grows taller, even as fruits formed earlier mature. The fruits are deeply-grooved capsules with at least four separate chambers called locules. Rows of tiny, flat, teardrop shaped seeds are produced in each chamber. The seeds are prized for their high oil content and are also used in numerous other ways, both processed and fresh. One of my favorite uses for sesame seeds is tahini, which is one of the main ingredients in hummus.

The fruits of sesame are dehiscent, which means they naturally split open upon reaching maturity. Compare this to indehiscent fruits like acorns, which must either rot or be chewed open by an animal in order to free the seeds. Dehiscence is also called shattering, and in many domesticated crop plants, shattering is something that humans have selected against. If fruits dehisce before they can be harvested, seeds fall to the ground and are lost. Selecting varieties that hold on to their seed long enough to be harvested was imperative for crops like beans, peas, and grains. In domesticated sesame, the shattering trait persists and yield losses are often high.

Most of the world’s sesame crop is harvested by hand. The plants are cut, tied into bundles, and left to dry. Once dry, they are held upside down and beaten in order to collect the seeds from their dehisced capsules. When harvested this way, naturally shattering capsules may be preferred. But in places like the United States and Australia, where mechanical harvesting is desired, it has been necessary to develop new, indehiscent varieties that can be harvested using a combine without losing all the seed in the process. Developing varieties with shatter-resistant seed pods, has been challenging. In early trials, seed pods were too tough and passed through threshers without opening. Additional threshing damaged the seeds and caused the harvest to go rancid. Mechanically harvested varieties of sesame exist today, and improvements in these non-shattering varieties continue to be made.

In order to develop these new varieties, breeders have had to gain an understanding of the mechanisms behind dehiscence and the genes involved in this process. This research has helped us appreciate the unique way that the capsules of the sesame plant dehisce. As in the seed bearing parts of many other plant species, the capsules of sesame exhibit hygroscopic movements. That is, their movements are driven by changes in humidity. The simplest form of hygroscopic movement is bending, which can be seen in the opening and closing of pine cone scales. A more complex movement can be seen in the seed pods of many species in the pea family, which both bend and twist as they split open. In both of these examples, water is evaporating from the plant part in question. As it dries it bends and/or twists, thereby releasing its contents.

dehisced capsules of sesame (Sesamum indicum); photo credit: wikimedia commons (Dinesh Valke)

The cylindrical nature and cellular composition of sesame fruits leads to an even more complex form of hygroscopic movement. Initially, the capsule splits at the top, creating an opening to each of the four locules. The walls of each locule bend outward, then split and twist as the seed falls from the capsule. In a study published in Frontiers in Plant Science (2016), researchers found that differences in the capsule’s inner endocarp layer and outer mesocarp layer are what help lead to this interesting movement. The endocarp layer is composed of both transvere (i.e. circumferential) and longitudinal fiber cells, while the mesocarp is made up of soft parenchyma cells. The thicknesses of these two layers gradually changes along the length of the capsule. As the mesocarp dries, the capsule initially splits open and starts bending outwards, but as it does, resistance from the fiber cells in the endocarp layer causes further bending and twisting (see Figure 1 in the report for an illustration). As the researchers write, “the non-uniform relative thickness of the layers promotes a graded bi-axial bending, leading to the complex capsule opening movement.”

All this considered, a rock rolling away from the entrance of a cave after giving the command, “Open sesame!” almost seems simpler than the “open sesame” experienced by the fruit of the sesame plant.

See Also: Seed Shattering Lost – The Story of Foxtail Millet

Permaculture Lessons, part one

This is a guest post by Laura-Marie River Victor Peace Nopales (see contact info below).


Hello, I’m Laura-Marie. I love plants, permaculture, and learning what grows wherever I find myself. This guest post is about the permaculture lessons I’ve learned gardening with my spouse Ming. Ming is a kind, brilliant person who enjoys interdependence, being a street medic, and helping our garden grow. He’s a long-time permaculturist with two permaculture design certificates.

I’ve studied permaculture for ten years. I enjoy it for many reasons: responsibility and interdependence of organisms, long view, appreciation of small. Thinking a lot about water storage, microclimates, and what makes sense for a particular place. That “you don’t have too many snails–you have too few ducks” mentality. Anything you have too much of to use can be pollution, even something usually considered good.

origins

Ming and I are both from California. We moved to this Las Vegas desert from Sacramento, which is at the north part of the Central Valley and inland from the Bay Area. I love Sacramento for its diversity of humans, plentiful parks, and proximity to many other wonderful places. Ming likes that it’s compact, but not too dense. Things are close together and easy to get to, but not overly scrunched up and piled on each other. 

Sacramento gets hot in the summers but cools off at night with a breeze from The Delta. There are rivers, a wet feel, many trees. We liked helping with Food Not Bombs and being part of the peace community there.

When Ming and I gardened in Sacramento, our relationship grew and changed as our plants grew. At Fremont Community Garden, I turned compost for the first time. I ate the most delicious Asian pears I’ve ever tasted and learned what espalier pruning is, for easy reach of fruits. 

I learned how to be a good garden neighbor. The man who grew long beans in the plot next to ours went on vacation and asked us to water his plot. Our reward was harvesting from his garden. I never knew green beans could grow like that, and so delicious.

In that climate and soil, the oregano we had in our herb spiral went wild, like mint does some places. It turned into a delicious weed, and we would harvest whole trays of oregano to deliver to a local restaurant in exchange for cookies and drinks. It was informative to watch the oregano choke out the tarragon, as the herb spiral spiraled out of control.

Our lavender bush got bigger and bigger–I liked my fragrant attempts to divide it, as I learned how to use a shovel. I enjoyed our basil forest, pinching its flowers, and seeing basil wood for the first time.

tree collards

Tree collards are a quintessential permaculture plant. People who want food forests do well to grow this charming perennial brassica. Ming and I grew lush, gorgeous tree collards in Sacramento. They are so productive and delicious to eat. I loved to make curried greens with beans, and I added ripped up tree collard leaves to a stir fry or any veg dish for more deliciousness. Yes, I adored those pretty leaves, whether they were green or purple.  

Our biggest, first tree collard we called Sideshow Bob. Its leaves floofed up like the hair of Sideshow Bob on the cartoon tv show The Simpsons. Sideshow Bob got infested with Harlequin bugs, and I learned how to save a tree collard from Harlequin bug infestation. Squishing around 300 Harlequin bugs between my gloved fingers and putting their bug bodies into a bucket of soapy water was a thought-provoking scene of carnage.  

What am I willing to do, to defend my favorite plant and meal ingredient? I considered what must die to keep my own body alive, what’s worth it. I miss those cute orange gardening gloves that I would never look at the same way again.

Sideshow Bob tipped over, and Ming found ways to support its “trunk” as it grew diagonally. It was fun to watch Sideshow Bob’s adjustment to sideways life, and we liked to give cuttings away.

Tree collards are easy to propagate, so we had several tree collard plants in our garden after some time. We brought one to my mom’s house and planted it at the edge of her garden. She lived in a different part of California, further south near the coast. Her tree collard flourished there. Every time we visited my mom, I felt excited to see how the tree collard had grown.

sharing

For a while we had two gardens. We had our plot at Fremont Community Garden, but we also lived in an apartment complex with shared beds.There were four beds when we got there, and then two more were added.  

We learned about sharing garden space with friends, including emotions, not wanting to encroach on someone else’s space, challenges of communication and expectations. I had a clump of rainbow chard that I loved to eat and watch grow. It got infested with aphids, but I was hoping to win that battle.  After some time, a well-meaning neighbor ripped it out, and I cried.

We learned how much space is the right amount, and which plants we like to eat grow well in Sacramento.  Tomatoes do well there.  I learned a permaculture lesson about the wave of energy: how having a high yield might not correspond to having enough energy to harvest it.  

One summer, so many cherry tomatoes grew that we couldn’t harvest them fast enough. Big changes were happening in the lives of everyone who lived in that apartment building, when the tomato plants were covered in hundreds of fruits. It was sad, not to have the capacity to share surplus with people in need.

There’s a mushroom farm in Sacramento that gave away spent substrate, which intrigued us.  We decided to try using substrate as mulch. “It could take nutrients from the soil, since it’s just sawdust.  Maybe this is a silly idea,” we wondered.  But we opened the bags and spread the sawdust on our garden beds, curious,

Then there was a rainy couple weeks in the winter, and we found ourselves with more oyster mushrooms than we could eat. They fruited out like mad. That felt magical and was a tasty experiment in trying something out just to see what happens.

promise

In future guest posts, I’d like to tell you what I’ve learned doing permaculture in the desert, and how doing permaculture as a disabled person is a great idea. Please let this post serve as an introduction to how my spouse Ming and I see plants and enjoy garden life.  

We enjoy new experiences, and we have a slow, grateful pace of loving the land. We love plants as food and sibling organisms on this beautiful earth.


Laura-Marie River Victor Peace Nopales is a queer permaculturist trikewitch who enjoys zines, ecstatic dance, and radical mental health. Find her at https://www.listeningtothenoiseuntilitmakessense.com.

The Wonderful World of Plantlets, Bulbils, Cormlets, Tubercles, and Gemmae

Probably the most well known strategy that plants have for dispersal is by way of seeds. Seeds are plants in embryo, and new generations of plants are born when seeds, released from their parent plants, find suitable locations to germinate. But one of the most amazing things about plants in general is that they have the ability to reproduce in a variety of different ways, and many plant species are not limited to seeds as their only means of dispersal. A paper by Scott Zona and Cody Coyotee Howard, published in Flora (February 2022), introduces us to the intriguing world of aerial vegetative diaspores – just one of the many ways that plants have to get around.

A diaspore is a plant structure that facilitates dispersal. Seeds are diaspores, as are spores, which are produced by non-seed bearing plants like mosses and ferns. If you’ve ever planted bulbs, you’ve handled another type of diaspore. Bulbs and corms, which many spring flowering plants are grown from, form little offshoots called bulblets and cormels that, when detached from their parent structure, can grow into new individuals. These vegetative diaspores are produced below ground. Aerial vegetative diaspores, on the other hand, are formed on above ground plant parts. This clunky term encompasses a number of different structures that are often simply called bulbils, which Zona and Howard explain is used as “a catch-all term that obscures their morphological identity.”

Compiling a list of plant species that feature aerial vegetative diaspores is a difficult task when plant descriptions from various sources use a broad selection of terminology for the same or similar plant parts. To help complete this task, Zona and Howard defined five distinct types of aerial vegetative diaspores – plantlets, bulbils, cormlets, tubercles, and gemmae – and came up with a list of 252 taxa that are known to feature at least one of these structures.

plantlets on the leaf margin of Kalanchoe daigremontiana (wikimedia commons; Aurélien Mora)

Plantlets are miniature plants attached to another plant. Once mature, they have clearly visible leaves, stems, and roots (or root initials) and are non-dormant, meaning they are ready to grow on their own as soon as they’re given the opportunity. The tiny plants borne along the margins of the leaves of mother of thousands (Kalanchoe daigremontiana) is a great example of a plantlet.

A bulbil consists of a shortened stem surrounded by scale leaves modified for food and water storage. Sometimes root initials are visible at the base of the bulbil. Bulbils remain dormant until they are dispersed and conditions are suitable for growth. When bulbils start growing but remain attached to the plant, they become a plantlet. A good example of a bulbil can be found on bulbous bluegrass (Poa bulbosa).

Cormlets are comprised of stem tissue and, like plantlets and bulbils, have a single axis of polarity. They have highly reduced scale leaves and are dormant at dispersal. Bulbil bugle lily (Watsonia meriana), despite its misleading common name, is a good example of a plant that produces cormlets.

Tubercles are made up of swollen stem tissue and, like tubers (their underground counterparts), have multiple shoot buds and multiple axes of polarity (meaning there is no right side up like there is in plantlets, bulbils, and cormlets). They lack scale leaves and are dormant at dispersal. Air potato (Dioscorea bulbifera) is an example of a tubercle-producing plant. As you might guess from the common name, potato-like structures are produced aerially on this vining plant that was introduced to North America from Africa and is now invasive in Florida.

A gemma is a tiny cluster of undifferentiated cells. Gemmae are non-dormant and lack polarity. They are the smallest and least common form of aerial vegetative diaspore and can be found on Drosera pygmaea, a species of sundew native to parts of Australia and New Zealand.

Drosera pygmaea (wikimedia commons; Björn S…)

Zona and Howard’s list of plants with aerial vegetative diaspores is the most comprehensive list to date – although it is undoubtedly and understandably missing some – and includes representatives from 42 plant families and 21 plant orders. Plantlets are the most common form of aerial vegetative diaspore at 116 taxa, with bulbils coming in second at 72. Cormlets and tubercles are less common, with 25 and 16 taxa respectively. Their paper includes the full list and offers further information about many of the species listed. It’s worth taking time to explore and is a valuable resource for anyone interested in the topic. In addition, their discussion section highlights a number of questions that warrant further investigation.

Questions surrounding reproductive strategies and the dispersal of aerial vegetative diaspores are particularly interesting. Because these structures are vegetative, they are essentially clones of the parent plant, meaning there is no genetic mixing as occurs when seeds are produced. This can be an advantage when sexual reproduction isn’t possible due to lack of pollinators, environmental restrictions, or chromosomal/polyploidy anomalies. It also assures that new individuals are pre-adapted to the site, and it can help a species colonize an area quickly. This ability to rapidly colonize explains why several of the species on Kona and Howard’s list are known to be invasive in parts of the world outside of their native range.

A species that produces both seeds and aerial vegetative diaspores may have an advantage when it comes to dispersal since both types of diaspores have their strengths. Seeds can remain dormant in the soil and are likely to persist in the environment longer than vegetative diaspores, but vegetative diaspores can be produced without relying on pollinators and can establish new individuals quickly. The modes of dispersal between the two can also vary. Seeds can be dispersed by wind or carried away by animals, while vegetative diaspores often rely solely on gravity to get around. One exception is hitchhiker elephant ear (Remusatia vivipara), whose bulbils are equipped with tiny hooks that cling to animal fur and are transported in a similar manner to burs.

hooked bulbils of hitchhiker elephant ear (Remusatia vivipara) (wikimedia commons; Dinesh Valke)

When the advantages of aerial vegetative diaspores are considered, it is a wonder that we don’t see them more often. Many plants can be easily propagated by taking stem, leaf, and/or root cuttings and placing them in conditions that favor adventitious root and shoot growth. This may suggest that dormant genetic pathways for producing vegetative diaspores exist in most plants. Or maybe not. Genetic studies of species that feature these structures are needed in order to understand why they are found in some species and not others. Kona and Howard leave us with a slew of research questions like this, and it’s a topic I’ll continue to check in on.


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