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Plant Crystals

December 5, 2022

Cold weather has arrived at Tikalon's home in Northern New Jersey, and that's a signal for our snowbirds to head south for the winter. For those not familiar with the term, a snowbird is not a migratory bird, but someone who flees the cold North for the sunny South in the winter. However, unlike birds who neither sow, nor do they reap, you need to be retired with a sizable nest egg to be a snowbird.

Many bird species migrate south for the winter, as the incessant and loud honking of Canada Geese and the sight of their V-shaped flight formation clearly demonstrate in our area. Despite a migration of more than a thousand miles, these birds are known to return to the same nesting ground every year. That brings us to the question of how these birds navigate. One way to head south is to keep the rising Sun at your left, and the setting Sun at your right. There's also the possibility of a learned route of landmarks, such as lakes and rivers, passed on through generations (A Canada goose has a lifespan of about 10-20 years).

Canada Geese in V-formation

Canada Geese in V-formation.

This photo was taken at Northey Island on Blackwater Estuary, Essex, England.

(Wikimedia Commons image by Glyn Baker of Northey Flypast, licensed under the Creative Commons Attribution-ShareAlike 2.0 Generic License. Click for larger image.)

Canada geese also fly at night, when the Sun isn't visible and landmarks are seen only by moonlight. It's thought that geese have a physical compass in their head that allows navigation by detecting the Earth's magnetic field. Birds have materials containing iron in their upper beak that might enable this magnetoreception. As might be expected, numerous experiments have been done on homing pigeons, which also have iron clusters in their upper beak. It's thought that the trigeminal nerve allows sensing of the magnetic forces on these iron particles.

Iron is the fourth most abundant element in Earth's crust and the first abundant element after the aluminosilicate elements, oxygen, silicon, and aluminum. As a consequence, iron has been incorporated into many organisms, and some bacteria have been found to contain crystals of magnetite (Fe3O4), a magnetic iron oxide. As a consequence, these bacteria are magnetotactic; i.e., their motion is affected by a magnetic field.

Magnetotaxis in bacteria was discovered in 1963 by Salvatore Bellini. This discovery was published only in Italian and it was overlooked for many years. The aphorism, "publish or perish" should have instead been stated as, "publish in English or perish." Magnetotaxis in bacteria was subsequently rediscovered in 1975 by Richard Blakemore.[2] The bacteria do not sense the magnetic field as in magnetoreception; rather, their internal magnets force the bacteria into alignment with the field, and the response is seen also in dead bacteria. Bacterial movement was observed by Blakemore in magnetic fields as weak as 0.5 gauss, about the strength of Earth's magnetic field.[2] It's conjectured that the evolutionary advantage in having such magnets aids these bacteria in reaching areas of optimal oxygen concentration.

Magnetotactic bacteria magnetosomes

Magnetosomes of magnetite in magnetotactic Alphaproteobacteria.

Left is a transmission electron microscope image of a vibrioid magnetotactic bacteria from Lake Mead, Nevada, showing its contained chain of elongated magnetosomes.

Right is an image of two double chains of elongated magnetosomes from a freshwater coccus.

(Portion of a Wikimedia Commons image by Mihály Pósfai, Christopher T. Lefèvre, Denis Trubitsyn, Dennis A. Bazylinski, and Richard B. Frankel.)

Plants don't move very much and they would have little need of magnetic navigation crystals. However, they often contain another type of crystal, raphides, needle-shaped crystals of calcium oxalate monohydrate or calcium carbonate found in specialized cells called idioblasts that can also contain other substances, such as oil. These crystals are found in more than 200 families of plants, and they appear to act as a calcium reservoir, a defense against grazing insects and animals, and a means of mechanical support. In some cases, calcium oxalate amounts to more than 5% of the plant's dry weight.

Raphides were first described by pioneer microbiologist, Antonie van Leeuwenhoek (1632-1723), in the late 17th century.[3] Seventy-five percent of flowering plants contain crystals.[3] Aside from raphides, plants can contain small silica crystals.[3]

Some common plant items containing raphide are as follow:
• Beta (Beets, Swiss Chard)
• Spinacia (Spinach)
• Rheum rhabarbarum (Rhubarb)
• Ananas comosus (Pineapple)
• Actinidia (Kiwifruit)
Epipremnum aureum, the devil's ivy

Left, the raphide-containing Epipremnum aureum, appropriately named the devil's ivy. Right, a 600 magnification image of raphides from such a plant. (Left image, a Wikimedia Commons image by Joydeep. Right, a Wikimedia Commons image by Agong1. Click for larger image.)

An international research team with members from the Rheinische Friedrich-Wilhelms Universität Bonn (Bonn, Germany), the Nees-Institut für Biodiversität der Pflanzen der Universität Bonn (Bonn, Germany), the University of Wisconsin-Milwaukee (Milwaukee, Wisconsin), the Hessisches Landesmuseum Darmstadt (Darmstadt, Germany), and the Senckenberg Naturhistorische Sammlungen Dresden (Dresden, Germany), has found that impressions of calcium oxalate crystals are found in fossil plant remains, and they can be used as a means of fossil identification.[4-5] Their research is published in an open access article in Scientific Reports.[4]

Such fossil impressions were previously considered to be granular traces of algae or pollen with no convincing explanation.[4] The present study shows that these enigmatic microstructures were caused by calcium oxalate.[5] Says Mahdieh Malekhosseini, principal author of the study at the Institute of Geosciences at the University of Bonn,
"Until now, it was not known how these cavities were formed... For example, it was believed that they came from algae or pollen from other plants that somehow got onto the leaf during fossilization. But after analyzing hundreds of these structures, we can rule that out. Instead, we were able to show that calcium oxalate crystals are responsible for the depressions."[5]

Oxalate crystals are not present in the fossil remains. What are found are cavities in which the oxalate crystals had been. That's because they are soluble in even the weakest acids, including carbon dioxide (CO2)-saturated water.[4-5] Sometimes other minerals containing calcium, silicon, aluminum, sulfur, and iron and organic material accumulate in the depressions left by the dissolved oxalate, where they sit like tiny beads in the fossil leaf.[4-5] Fossils of pines and firs sometimes show these calcium oxalate crystal imprints, but these had not been discovered for angiosperms, the flowering plants and trees.[5] The present study discovered these in fossil angiosperm leaves from the late Oligocene Rott Fossil Lagerstätte, and they were classified by shape, size, and distribution pattern, and compared with crystals in the leaves of extant plants.[4]

Fossil oak leaf of a Quercus neriifolia (left), compared with a leaf from a present-day oak (Quercus variabilis, right)

fossil oak leaf of a Quercus neriifolia (left), compared with a leaf from a present-day oak (Quercus variabilis, right). The brown round deposits of the fossil resemble calcium oxalate crystals of present-day oak leaves (University of Bonn image, also available here. Click for larger image.)

The research team used energy dispersive analysis by X-ray (EDAX) in a scanning electron microscope for element analysis.[4] Some of the questions that they addressed in their study were whether the shape and location of the oxalate crystals of the fossil leaves correspond to those of modern leaves, and what chemical and biochemical processes occurred during fossilization to change their micromorphology.[4] Observation of granular structures caused by crystals can serve as an aid in fossil leaf assignment.[4] As Malekhosseini explains,
"We studied the microstructure of the pits and their distribution on fossil leaves whose species affiliation we knew... In addition, we looked at calcium oxalate crystals in the leaves of present-day plants. We found clear parallels in closely related species. For example, the crystal imprints in a fossil ginkgo leaf strongly resemble the calcium oxalate deposits of a present-day ginkgo in distribution and structure."[5]


  1. Tom Langen, "How do geese know how to fly south for the winter?," The Conversation, November 16, 2020.
  2. Richard Blakemore, "Magnetotactic Bacteria," Science, vol. 190, no. 4212 (October 24, 1975), pp. 377-379, DOI: 10.1126/science.170679.
  3. Ivan Amato, "The Secret Life Of Plant Crystals," Chemical & Engineering News, vol. 84, no. 6 (February 6, 2006).
  4. Mahdieh Malekhosseini, Hans-Jürgen Ensikat, Victoria E. McCoy, Torsten Wappler, Maximilian Weigend, Lutz Kunzmann, and Jes Rust, "Traces of calcium oxalate biomineralization in fossil leaves from late Oligocene maar deposits from Germany," Scientific Reports vol. 12, Article no. 15959, September 24, 2022, https://doi.org/10.1038/s41598-022-20144-4. This is an open access paper with a PDF file here.
  5. New field of research: Crystal traces in fossil leaves, University of Bonn Press Release, October 7, 2022.

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