Today’s entry is the second in the series on the Coastal Flora of the Pacific Rim. This species, Cakile edentula, grew along all of the sandy beaches that I visited (a couple years ago) near Tofino, British Columbia. I was impressed by the ability of these plants to grow in the middle of large expanses of sand, in places where few other species could survive.
Cakile edentula, or American sea rocket, is one of the first plant species to colonize coastal dunes. It is extremely tolerant of salinity and salt spray. In order to grow in shifting sands, it first forms a large taproot that anchors it in place, then develops an extensive network of shallower roots. These shallow roots stabilize the dunes, eventually facilitating the succession of other plant species. Despite efforts to secure the sand, American sea rocket plants are frequently buried. Many of the plants that I observed were visible only by their growing tips peeking out of the sand. Fortunately for this species, such burial stimulates its growth.
I was surprised to learn that Cakile edentula belongs to the Brassicaceae, the same family as broccoli, kale, and so many other delicious vegetables. The thick, nearly succulent leaves did not remind me of brassicas at all. The flowers of the plants that I saw on Long beach had all senesced, but today’s other photo of a flowering Cakile edentula, taken by Daniel on Chesterman Beach, gave me the idea of examining the flowers more closely. When I found this macro of the species in flower , I was reminded of the little pink flowers and pods of radish plants (the brassica Raphanus sativus). The flowers of Cakile edentula can be blue to purple, pink to red, or just plain white. The flowers are perfect, radially symmetrical, and have four petals. These give way to silicles, seedpods that are less than twice as long as they are broad, and composed of two carpels separated by a seed-bearing partition. The function of these silicles is evident from their form. Once mature, the round upper segment will snap off and float away, carrying off its single seed to a (possibly) distant shore. The lower segment of the silicle also contains one seed (and sometimes none), but usually remains attached to the mother plant and will likely remain on the same beach. In this way, Cakile edentula ensures that new shores are colonized and that its original populations are maintained. This seems to be a good strategy, as American sea rocket is commonly found up and down the east and west coasts of North America. It has also been introduced in Japan and Australia, where it is invasive.
Interestingly, Cakile edentula has been shown to favour its kin over other members of its species that are not directly related to it. Susan Dudley and Amanda File (2007) found that Cakile edentula seedlings grown in a pot together formed fewer fine roots if they came from the same mother, suggesting that siblings did not compete with each other as much as unrelated plants. Although Dudley and File offer no explanation as to how plants can differentiate between kin and unrelated individuals, they did conclude that the mechanism occurred within the plant’s roots. If you’re in Canada, you can watch Dr. Dudley and her students investigate this phenomenon in “Smarty Plants: Uncovering the Secret World of Plant Behaviour“.