It’s hard to get too excited about eating celery, but if you can manage to see a dip-drenched celery stick as a dynamically loaded cantilevered beam, then its stringy bits suddenly start to look like incredible feats of bioengineering. The mildest mannered member of the crudité platter turns out to be a misunderstood superhero.
If you are about to celebrate Thanksgiving, chances are good that you have a lot of celery in your immediate future. It shows up in dressing and cranberry relish and especially in leftovers, like turkey salad sandwiches. When I was growing up, my sister and I were tasked with picking the carcass for turkey hash, which, in our family, was basically turkey soup stretched with lots of celery and potatoes and never enough salt. Although frugal and nutritious, this one-pot crusade against food waste did not inspire a lifelong love of cooked celery. But you don’t have to like celery the food to admire its alter ego, celery the plant.
Leaves, not stems
Celery the food may not excite you, but celery the plant – the bundle of dynamically loaded cantilevered beams – is a biomechanical superhero worth exploring in the kitchen. Celery (Apium graveolens) is one of the clearest examples of how a plant’s life in the wild over tens of millions of years yielded anatomical adaptations that determine how we use it now. Because of its evolutionary responses to biomechanical challenges, it is now perfectly built to hold peanut butter or scoop dip, and when sliced, its crescent moon shapes are pretty in soup and chopped salads. On the other hand, its tough strings catch between teeth and are not easy to digest.
Celery stalks are the petioles (“stalks”) of compound leaves. They are not stems, in spite of widespread misrepresentation in elementary school lesson plans. They may look like stems to some people because they are thick and fleshy and have prominent veins running lengthwise through them. But there are several morphological clues to their leafy identity, including these:
- They are crescent shaped, not circular, in cross-section. In other words they are bilaterally symmetrical, whereas most (not all) stems are radially symmetrical.
- Their tips end in a fully developed flat leaflet, whereas stems are usually topped by a growing point (an apical meristem) that gives rise to tiny new leaves or flowers.
- The leafy parts are leaflets (the subparts of a compound leaf) which do not develop like separate leaves along a stem.
- They are arranged in a Fibonacci spiral around a central axis, like many leaves. Branching stems can be arranged in a spiral as well, but only because they are closely associated with leaves. If celery stalks were branches, there would a small leaf below each one.
Understanding that a celery stalk is the petiole portion of a leaf is useful, and not just to score botanical pedantry points. Celery petioles are long and tall structures that support the flat photosynthetic part of the leaf. They experience some downward compressive forces, but they are mostly subject to bending under their own weight, so in the language of mechanical engineering, they act like cantilevered beams. The weight they bear changes as the leaves grow and are buffeted by the wind. Engineers call this dynamic (vs static) loading. In addition, they may be subject to torsion as the wind twists the leaves side to side. Twisting is much less dangerous than bending, so the leaf must be able to give in to twisting in order to reduce the bending forces that might snap it. All of these challenges help explain why celery petioles are scoop shaped and reinforced with stretchy strings and how best to prepare celery in the kitchen.
The strength of a structure depends on both its material composition and its shape. The half-pipe or trough shape of a celery petiole makes it much better at holding peanut butter and much more resistant to bending compared to a flatter petiole. This is particularly true when the bending force is away from the center of the bunch, which is the condition the leaf would face as it grew. It’s much easier to bend a stalk towards the bunch because the c-shape of the trough is deformed and flattens out. It loses its special shape in that direction.
The basal end of a celery petiole also resists bending. It widens out and hugs the bottom of the plant to add stability where the forces are greatest. Fennel, a close relative, has an even more dramatically enlarged leaf base, and the overlapping leaves form a “bulb.”
Although its built to resist bending, a celery petiole can be twisted very easily. Twisting in the wind or during growth within the bundle of leaves takes some of the forces off of the leaf that might otherwise bend it.
The stringy stuff: vascular bundles and collenchyma
Most people peel celery before serving it raw because its long tough strings either catch in your teeth or pass through unchewed and undigested. (A quick Google search reveals the alarm caused by undigested celery strings, which apparently look like hookworms to some people.) But there are actually two different kinds of string, and using a peeler probably catches only the strings lying just under the surface of each narrow longitudinal rib. To avoid postprandial panic, you have to get all of the strings.
The deeper strings are the less interesting. They are vascular bundles – the strands of sugar-conducting (phloem) and water-conducting (xylem) tissue that occur in virtually all plant tissues. The xylem is strong and elastic and will spring back after it has been stretched, but it breaks fairly easily. It is the xylem that stars in those shameful elementary school lesson plans that mislead our impressionable youth about the nature of celery stalks. If you put a clean-cut leafy celery stalk into colored water, the dye will move up through the xylem and highlight the bundles.
Xylem strands are not easy to chew, but the more shallow strands, made of collenchyma, are four or five times harder to break. The strength of this tissue was documented by one of my botanical heroes, Katherine Esau, in 1936, who by all accounts knew what it meant to be tough. After fleeing the Ukraine with her family, whose politics did not sit well with local officials, Esau continued her education in Germany and then in California where she made her mark as a highly accomplished woman scientist in early 20th century. She lived to be 99 years old.
Collenchyma is very unusual tissue. Unlike many tough tissues (xylem, fibers, stone cells) that build stiff walls and then die, collenchyma is alive (even while it’s stuck in your teeth). Its cell walls are made of relatively soft cellulose and pectins that can absorb a lot of water and act like a stiff gel (Leroux, 2012). The way these materials interact in the cell wall makes collenchyma very plastic – that is, it will stretch relatively easily without breaking – but it does not bounce back, so it is not elastic (Niklas, 1992).
Collenchyma can be found in a lot of petioles or other structures that elongate rapidly while having to maintain stiffness against bending forces. Collenchyma allows for this rapid growth by stretching, while it keeps its strength. By contrast, the cells that make up xylem vessels will stretch only because the stiff parts of their walls are built like springs. Once they are stretched too far, however, the vessels collapse. Younger vessels, built once growth has slowed, take over for them.
The importance of collenchyma is obvious when you remove only those strands from a celery stalk and then try to bend it. When I carefully removed only the collenchyma strands (and overlying epidermis), I could bend the celery stalk to the point of breaking.
My mother taught me how to remove both kinds of string: snap the petiole near one end, leaving the strings intact, and then pull the short piece up along the stalk, unearthing the strings along the way. If there are some strings left, they will probably be sticking out of the broken end, and you can easily pull them up with a knife. Trim the ragged end, and you have beautiful tender celery.
The celery marvel
There is a lot more to be said about the biomechanical properties of celery. It’s not only shape and strings that keep celery petioles from buckling under their own weight. It’s the placement of those strings within the shape and the way the tissues interact that are so impressive. In the words of another highly influential botanist, Karl Niklas, “Indeed, when we come to look at a representative cross section of a petiole through the anatomically critical eye of a biomechanicist, we see that the composite tissue construction and spatial allocation of materials found in petioles reflect one of the most elegant expressions of evolutionary adaptation encountered in all of biology” (Niklas, 1992, pg 167).
A note about flavor
Although we often use celery as a filler or a neutral vehicle for something rich and fatty, celery itself is actually very aromatic and has distinctive flavor. Its name Apium graveolens means “strong smelling bee favorite.” As Jeanne has pointed out in a couple of posts, celery and its many edible relatives contain some complex-tasting terpenes, especially limonene and pinene. In addition, various celery pthalides seem to be responsible for enhancing the complexity and umami flavor of broth, even when used at levels we cannot perceive (Kurobayashi et al. 2007). Finally, there are the furanocoumarins, which taste harsh and can irritate or numb your lips. Furanocoumarins can also cause photodermatitis, an allergic reaction triggered by light exposure. Eating normal amounts of celery is unlikely to cause much trouble, although farm workers have suffered reactions after hours of harvesting celery in the sun.
If indeed there is a lot of celery in your Thanksgiving day recipes, I hope that you can be relaxed enough in the kitchen to marvel over it. If not, please at least pass along this lesson to an elementary school student: celery is not a stem! Happy Thanksgiving from Jeanne and Katherine.
References and further reading
Esau, K. (1936). Ontogeny and structure of collenchyma and of vascular tissues in celery petioles. California Agriculture, 10(11), 429-476.
Kurobayashi, Y., Katsumi, Y., Fujita, A., Morimitsu, Y., & Kubota, K. (2007). Flavor enhancement of chicken broth from boiled celery constituents. Journal of agricultural and food chemistry, 56(2), 512-516. http://pubs.acs.org/doi/pdf/10.1021/jf072242p
Niklas, K. J. (1992). Plant biomechanics: an engineering approach to plant form and function. University of Chicago press.