Botanical Brain Teaser #4: A Yellow Sea
The following is an article that I wrote as an intern at the herbarium of the Royal Botanical Gardens. It appeared in RBG’s magazine Paradise Found and in every issue, the current botany intern composes a puzzle for readers to solve. They choose a specimen from among the cabinets and describe the species leaving enough of a mystery that readers must guess and write-in their answers. I loved writing these, so I’m sharing them here along with images of the herbarium specimens that accompanied them. I cropped this top image to obscure the label so you can make a guess and post it in the comment section at the bottom. Then, click the “Answer” link to see the full herbarium specimen image including the label to check if you were right. Enjoy!
Can you name the plant represented in this RBG herbarium specimen? The specimen was collected on August 2nd, 1941 by Luc LeRoy, who contributed a few dozen specimens collected in Hamilton and surrounding areas to the RBG herbarium. This particular plant was collected beside Lawrence Road near Kenilworth Avenue. The scientific name of this specimen has been changed by subsequent botanists twice since it has been with us, indicating that the group of plants to which it belongs can be challenging to identify. This plant has a smooth, hairless stem, which is the key to distinguishing it from its close relatives.
In the wild, this plant (as well as another related species) sometimes has an abnormal growth on the stem called a gall. These are caused by insects that lay their eggs into the fleshy middle part of the stem. When the egg hatches, the larva begins to feed on the tissues of the plant, causing a reaction that is something like a cross between a scar and a tumour. These galls can be spherical “ball-galls”, elongated “spindle-stem-galls” or densely leafy “bunch-galls” depending on which insect is inside. The larva spends the winter protected within the structures of the plant, then emerges as an adult in the spring, mates and re-starts the life-cycle. The flies responsible for “ball-galls” (in the genus Eurosta) either prefer to lay eggs in this smooth-stemmed, early-August blooming mystery species or in its hairy-stemmed, late-August blooming relative. If a fly lays eggs in the smooth-stemmed plant, those eggs hatch about two weeks earlier, in synchronicity with the growth period of their plant host. Since host preference is an inherited trait in this case, flies with different host preferences do not mate with each other very often. Because of this reproductive isolation, they have formed two distinct fly “races” based on their preferred plant host.
This plant has flowers, called “florets”, which are extremely small: just a few millimetres long and less than a millimetre wide. A few dozen tiny florets are arranged into a “head” or “capitulum,” many of which, in turn, are arranged in a yellow pyramid-shaped structure called a “capitulescence.” The plants can spread by underground stems called “rhizomes,” so that what looks like a patch of many plants might actually be a single, large plant connected under the soil surface. They live in sunny areas such as old fields and open woods, often in moister areas. At RBG, they can be found in a few places including Hendrie Valley and along the Pinetum Trail about half-way between York Road and the Arboretum. Contrary to popular belief, this plant and its relatives do no cause allergies or “hay-fever”, which is actually caused mainly by wind dispersed pollen such as that of Ambrosia species (Ragweed).
