How Callus

Since graduation, I’ve been on a bit of a plant kick–not just because I needed to learn plant physiology and ecology for my GRE subject test and for my exploration of the flora and fauna near my new home near a suburban conservation area–but also because of my newfound appreciation for horticulture as art and discipline distinct from the botanical and ecological sciences, and for the therapeutic effect that gardening has had for my recent depression. As a side to my experimentation with tropical hibiscus propagation this summer, I was recently able to use my hibiscus calli in class in addition to the plant roots required for the day’s lab protocol. We performed assays to demonstrate areas of high enzymatic activity, and therefore identify zones of rapid cell growth and division, in these plant tissues. One of these was an assay for peroxidase, which neutralizes reactive oxygen species generated by metabolism that can destroy cell machinery and DNA.

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Callus cells scraped off a hibiscus stem look like clear, sausage-shaped bubbles. 9/20/2016.

Callus tissues are essentially stem cells in the somatic tissues of land plants. These stem cells are derived from more specialized cells that undergo rapid growth and division in the presence of growth hormones in the correct proportion. These callus form disorganized masses before differentiating into new roots and shoots, depending again on the appropriate mixture of hormones. In nature, callus tissue is encountered in plant wounds, where they grow and divide rapidly to heal over the injury. Calli are most frequently studied in vitro, however, since they are commonly used in plant tissue culture to produce numerous clones of a desired plant from single mature cells. In such situations, calli are the plant analogs of bacterial colonies in agar dishes.

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Exposed to peroxidase substrate, the cells’ endogenous peroxidases break down the substrate, producing a purple product. 10/11/2016.

I personally find it fascinating that such tissues can be generated on a hibiscus cutting in a jar of water, without sterile technique or specialized tools or media. Someday, I’d like to see if such in vivo callus cells are capable of forming plant embryos in culture, without having to induce calli in vitro. In fact, hibiscus cuttings are quite unique in how they develop such extensive masses of callus tissue all along the submerged part of the stem, not just in wounds. This disorganized tissue differentiates into functional roots in time, even without rooting hormone. This propensity to produce callus prior to rooting makes tropical hibiscus as a group rather easy to propagate, a trait that has made them so amenable to cultivation by enamored enthusiasts around the world. Soft green stem cuttings, taken any time of year, are relatively easy to root in water or soil, as long as adequate warmth and high humidity are provided.

 

My Wild Adventures With (Rooting) Datura Stem Cuttings

Taking a walk in Allegheny Commons a month ago, I noticed a weedy looking shrub peeking out amid the cannas and elephant ears flopping lazily under the wilting sun. Weedy, yes, but somehow it managed to fit in with its tropical ornamental neighbors. Its succulent purple stems and erect but bushy habit were attractive enough to catch my attention even next to the taller and flashier cannas. Velvety, dark green, heart-shaped leaves sheltered spiny seedpods nodding on thick stems from the forking shoot tips, like botanical maces melting away à la The Persistence of Memory. But these unusual features formed merely the backdrop to the huge, striking white blossoms, their scroll-shaped buds flushing a slight lavender as they unfurled into the late afternoon sky.

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The appearance, orientation, and ripening of the seedpods are important characters to pay attention to when identifying Datura. 10/13/2016.

Anyone with an eye for gardening would recognize these exotic features anywhere on the planet as belonging to the plants of the genus Datura. Their common names are much more descriptive: jimsonweeds, thornapples, moonflowers, angel’s trumpets. Although their taxonomic history is rather complicated, I later tentatively identified this particular species as D. inoxia subsp. quinquecuspida, based on this old-ish online key from Erowid, adapted from one by Richard Sanders at the American Brugmansia and  Datura Society (designated as the authority on Datura taxonomy by the ICNCP in 2004).

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Datura inoxia in Allegheny Commons, 10/13/2016. The opened flowers resemble flaring trumpet horns, hence the Chinese name, la ba hua (trumpet flower). With the ribbing and the thin bending petiole, I think they look more like an old-fashioned grammophone horn.

Datura‘s seductive pure white flowers and weedy abundance in out-of-the-way areas stand in eerily stark contrast to its storied reputation as an unpleasantly hallucinogenic drug and a deadly poison. I’ve seen Datura bushes in front of scruffy old homes in college neighborhoods (!) and cultivated in a row in front of suburban apartments filled with retirees (!!). Tom Scocca at the old Gawker website has a great essay in which he stumbles upon a Datura sprouting innocuously from a patch of bare dirt in the middle of New York City, blooming and reseeding and as wholly ignorant of its own toxic potential as all the daily passersby occupied with their own duties to grow and reproduce. Indeed, every afternoon when I would pick several D. inoxia flower buds to bloom at home, I would get strange looks from the dog walkers and outbound commuters who could not comprehend why I was so intently stomping around in an overgrown garden in an urban center.* But for me, the plant’s treacherous snake-in-the-grass character only augments the rare combination of grace and hardiness that convinced me that I had to have one of my own. Truly, for those who grow Datura on purpose (whatever purpose that might be), this is a plant that not only sits and looks pretty on its own, but has a dangerous reputation whose story adds to its mysterious, literally intoxicating charisma.

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I took two D. inoxia cuttings on September 23 in the late afternoon, the worst season and time of day to take cuttings for most plants. Even worse, these were tip cuttings on a mature and blooming plant. For whatever reason (hmmm) there are not many definitive online guides on how to propagate Datura compared to its less infamous showy relative Brugmansia; many sources recommended propagation by roots or from seed, and only described stem cutting propagation for Brugmansia. However, these were stems I had used to feed my Manduca sexta hornworm caterpillar; after it died I only kept them around because I was too lazy to get rid of them. The whole time they sat neglected on my dresser, they remained succulent and green. The only change seemed to be the development of numerous small white bumps along the submerged sections of the stems. Meanwhile, the cutting wound appeared to turn brown and deteriorate.

Two weeks later, on October 6, I had to tidy up my room and noticed the first short root coming down out of the seemingly rotting wound on one cutting. By Saturday, October 8, that root had doubled in size and many more white tips were poking out of the wound; the other cutting had begun to root as well.

In retrospect, my success is not that surprising considering how related solanaceous plants are also commonly propagated by stem cuttings, like tomato plants and the aforementioned allied Brugmansia. In fact, I wonder if propensity to root by certain cutting techniques might be a way to differentiate similar populations or species. Certainly propensity to root is selected for under cultivation and some lines or varieties have higher success rates than others, so it probably has a genetic basis. It might even be a character state that develops and reverses in plant lineages; certain species might become better at rooting, like aquatic plants and succulents, or worse at rooting,like woody plants, as part of an adaptive syndrome to life in certain environments.

But TL;DR, here is how I would now recommend propagating Datura inoxia from stem cuttings:

  1. Get yourself some shoots from a flowering plant in late summer.
  2. Put them in water; leave alone for some weeks. No rooting hormone or any other cheats needed.
  3. When you’re done, now plant in suitable fast drying, low organic content soil and hope it overwinters in your house.

Not really complicated stuff, even if you’ve been hallucinating nightmares, your eyes are maximally dilated and you’re severely dehydrated, nauseous, dizzy, and just starting to separate dream from reality.

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With the cold setting in rapidly, the aboveground portions of the Datura will be killed by frost in the coming weeks. But they’ll be back…

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*One morning at the park, I did have an interesting conversation with a nice old lady who was picking the unripened alkaloid-laden fruits, which she claimed to extract in oil as a topical application for her back and leg pains. Hmm, OK, I said.