May 2024.
This winter we spent several months in Florida. Our beach-fronting residence had no enclosed garden for our Jack Russel terrier to roam freely, so I had to provide “comfort” walks several times a day. On these jaunts I intentionally sought out “waste places” to accommodate the natural needs of my canine companion. Besides this humane and practical purpose, the walks offered the opportunity to observe, identify and photograph wild plants, forbs and herbs, i.e. “weeds”..a long-practiced hobby of mine.
Spring (for weeds) occurs early in February in northern Florida. Many flowering plants are in bloom. One of the earliest and most common species is a tiny plant with white flowers arising from a basal rosette of heart shaped leaves, known as HAIRY BITTERCRESS (Cardamine hirsuta). This form of bittercress is a native of Europe, Asia and Africa and is today found widely distributed in the USA, and many other places around the world.
Bittercress was common bordering north Florida walkways, in untended lawns, waste places, disturbed areas and weed patches. If seemed to prefer places also preferred by my dog. So by late February to early March, I had seen much of it when the weed had grown to about ten to 12 inches (30 cm) high. At this time, it was topped with tiny clusters of white flowers, each with four petals in a “cross pattern”. This “cross pattern” revealed it to be a member of the “Cruciferae” or Mustard Family (Brassicaceae). The plant arises from a basal rosette of low growing leaves which are round (or “liver”shaped). These appear first, often in the fall of the year. By early spring (or February in the south) the plant produces 6 to 10 inch long “flower” stems. These are toppex with a cluster of white cruciform flowers. The flower shoots sprout compound leaves composed of four or six leaflets, each ending in a single terminal leaflet.
Each of the tiny flowers, when pollinated, mature into a long, narrow, purple-colored seed-pod (like a tiny, straight and elongate pea pod). The pods grow vertically to stand upright at the top of the plant like a spike. The numerous seed-pod spikes (called siliques) each about 1-2mm thick and about one inch (2.5cm) long, are clustered at the top of the plant and give the species its “hairy” (or in Latin, hirsuta) look. The spiky siliques are the source its name: “Hairy” Bittercress. Later in the season, when the plant has died and turned brown, the hairy look intensifies and persists to provide even greater justification for its name.
The Mustard, crucifera or Brassicaceae family to which this plant belongs includes many common food crops, such as mustard greens, broccoli, cabbage, turnips, bok choy, rape (canola) and even radishes. Like the other plants in this group, the leaves, and flowers, of Hairy Bittercress are edible. Though I make this statement without personal knowledge, having made no taste tests (given the source of my observations in waste places) . The name “Bitter cress” is claimed a misnomer since its taste is reported as not bitter but more peppery-like. It is harvested and used as a peppery salad green or cooked with other pot herbs. (I assume for this purpose it is picked from areas where canines, pesticide applications, and other contaminant sources are excluded)
But in truth none of these interesting attributes motivated me to write this piece. What did spark my interest was the way its seeds were dispersed…by ballistics. This surprising observation became apparent as my dog, Finn nosed around in the waste places he preferred, which were dominated by specimens of Hairy Bittercress. When he did the tiny Bittercress seed pods exploded around him launching hundreds of the tiny pale or cream colored mustard seeds two or more feet above the surface and a good three to four feet away. Many of the tiny seeds settled on his coarse furry coat.
Though floating down stream or on an ocean currents (cocoa nuts) carried by wind (Dandelion), rolling down hill by gravity (Black walnuts) , whirligiging Maple “polynoses”(samaras) and many other dispersal systems are well known, actually firing seeds like a Roman ballista into the air is quite uncommon. I wondered about the mechanism. We often think of plants as immobile. The ability to discharge seeds by a mechanical means like that of a Roman ballista has given this process its name ..” ballistic” seed dispersal. It is an unusual seed dispersal mechanism though not unknown phenomenon in the plant world. The pods are hair-triggered. Any slight touch or breeze and certainly the snuffling nose of a dog immediately elicit a violent explosive eruption too fast to see what was actually occurring.
Try as I might, I could not find an unfired seed pod that I could examine, prior to explosion. It was immediately apparent that the pods exploded on the slightest disturbance. After being triggered, and the seeds launched into the air, all that remained of the seed pod was the empty, blade-like, central-wall of the narrow silique. On close inspection, the interior surface was marked here and there with a few stem portions of the launched seeds which remained attached to this inner central wall of the seed pod. Apparently, seeds formed on both sides of the central pod wall. I counted what appeared to be up to about 20 seed locations on each side. Thus each pod might contain as many as 40 tiny seeds. On a plant with a single flowering stem, I counted ten purple colored siliques. That plant might have dispersed 400 or more seeds when triggered to explode. Most plants must have had many dozens of pods. So the number of seeds dispersed must be very numerous and thus very effective. It may well explain the ubiquitousness distribution of this weed plant.
I did observe as well that the movements of my dog nosing through the weedy patches readily discharged seeds from the siliques and the seeds showered down on the furry 20 pound canine. Many hundreds of seeds must have lodged in his furry coat, dor at least a short time. His movements at setting off the explosive siliques would be similar to small native denizens of this area ,where raccoons, rabbits, squirrels, opossum, armadillos as well as other small rodents might move and cause discharges. Small animals may increase the distances that seeds are dispersed, by first causing the explosive discharge, then become an agent of plant seed dispersal themselves by transporting seeds that fall on their fur of cling to their bodies. This animal transport effect may account for much further and effective seed dispersal than that observed by the immediate explosive aerial discharge of a meter or more.
I was fascinated by this most unusual and effective means of seed dispersal of this tiny inconspicuous plant. But was frustrated that while in Florida I could not determine the mechanism of just how the explosion worked.
My winter season in Florida ended, and in early April I traveled back to New York. En route I found flowering Bittersweet all along my route, attesting to the wide ranfe of this very common weed. In South Carolina, where I continued our necessary dog walks, I observed numerous Hairy Bittersweet specimens on the lawns and waste places and weedy paths of that state. Further north, in Salisbury, Maryland, Hairy Bittercress was also in abundant evidence. Arriving in New York State, Finn and I were greeted with Hairy Bittercress in his favorite weedy places and in our own untended garden. So it seemed clear that Hairy Bittercress is widely dispersed up and down the temperate zone of the east coast of North America. It was in my own garden where I finally had the opportunity to closely examine a silique as it exploded and immediately afterward.
I picked up a plant with several unexploded siliques. As I touched one of them, the pod exploded. It was still to fast to see, but after the seeds launched away. l found several tiny rolled up fragments of the outer walls of the seed pod on my hand. These fragments appeared to be composed of the coiled up outer-wall of the silique. So I tentatively concluded that it was likely the rapid detachment and coiling up of the outer walls of the silique which was the likely ballistic mecahanism. I theorized that on explosive discharge the outer walls rapidly coil up, and as they do, they detach the seeds from the central wall while they remain partly connected to the rapidly coiling outer wall. In this motive force they are launched into the air. In the process the silique wall, fragments into several parts.
An examination of the silique wall cylindrical fragments I recovered suggest that the outer silique walls detach from the base (?) and coil upwards in such a way that the lighter colored interior of the silique wall was exposed and formed the outside of the cylinder fragment, while the outer purple colored wall wound up on the interior by the coiling process. Apparently, the two outer walls detach from the base and coil up violently. As they do, the rapidly coiling pod walls pull away the attached seeds, launch them into the surroundings and in the violent process—the silique walls fragment and fall away as small barrel shaped, coiled up wall-fragments. The central wall of the silique remains behind, stiff and upright, and remains attached to the plant, but with the purplish outer walls and all or mo#t seeds gone, the central, light-tan-colored wall remains intact and upright.
A short time after this happy discovery, I came across a published article from more than a decade ago which actually explained the full mechanical and chemical mechanism of this explosive process of the Cardamine hirsuta. See: Mechanism for explosive seed dispersal in Cardamine hirsuta, August 8, 2011 by K. Vaughn, A. Bowling, K. Rue, in: Amer. Jour Botany, 8-01-2011, Vol 98, Issue 8, p1276-1285. The full story is even more intriguing…I highly recommend reading through it…though it is technical.
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