The Cosmopolitan Quickweeds (Galinsoga) of the World

Quickweeds. Shaggy soldier, Galinsoga quadriradiata (L), Gallant Soldier, Galinsoga parviflora (R), Photo 528808, (c) Kyle Jones, some rights reserved (CC BY-NC).	Shaggy Soldier, Galinsoga quadriradiata. Two disc flowers and fruit with acuminate pappus scales (L); one ray flower and fruit with pappus scales nearly as long as corolla tube (R).Photo 9072390, Daniel Atha, public domain.

Introduction
Galinsoga (Quickweed) is a genus of 15–30 species indigenous to the Americas and centered in Mexico (Canne, 1977; Canne-Hilliker, 2006). Two species are cosmopolitan, occurring in disturbed places in most countries of the world: Galinsoga quadriradiata and Galinsoga parviflora. Currently, only these two species are known from the continental US (USDA NCRS, 2020). Judith Canne-Hilliker who studied these plants for decades published works on the taxonomy of the genus and her work is the basis for modern floras that treat the species. A few additional studies have been done as well. For example, Braden and Cialone found that achenes of Galinsoga quadriradiata are significantly shorter and wider than those of Galinsoga parviflora (Braden & Cialone, 1971). Based on the literature and what could be observed in the field it was clear that the two species were distinct, but I was frustrated by the challenges in separating them, especially from fresh material in the field and from photographs on iNaturalist. The study described here was my attempt to address this problem. See the observation here for images of seedling development.

Materials and Methods
I examined all the herbarium specimens of Galinsoga quadriradiata and Galinsoga parviflora from North America at the New York Botanical Garden (NY) and the Brooklyn Botanic Garden (BKL) (over 300 specimens). Using a compound microscope and strong light, I sorted all specimens by the characters used by Judith Canne-Hilliker and flora writers to separate the two species. I ignored identifications and used only the traits of: rays with well-developed pappus about equaling the tube vs. rays with vestigial pappus scales or scales absent; pappus scales of the disk flowers awn-tipped vs. blunt.

Here is a good view of the pappus scales of the disk flowers and the pappus of the ray flowers in Galinsoga quadriradiata. In Galinsoga parviflora the ray pappus is absent.

I ended up with three piles, one much larger than the other two. The smallest pile consisted of specimens for which it was not possible to determine nature of disc and ray pappus scales. I then examined the two larger piles for hair and leaf teeth character states.

Results
All of the specimens (100%) that sorted by lack of ray pappus and blunt disc scales (the middle sized pile) were found to have very short (<0.5 mm), unicellular hairs (on stems and leaves) and narrow-ovate leaves with mostly entire or merely crenate margins. The larger pile with ray pappus scales present and acute or awn-tipped disc pappus scales were all found to have long (> 0.5 mm) multicellular hairs (on stems and leaves) and wider leaves with definite acute teeth. It was then possible to sort the smaller, undetermined pile on the basis of hair and leaf characters alone so that all specimens were identifiable as one or the other species.

Outside of Mexico, Galinsoga quadriradiata is the most common of the two species. On the continent of Europe, the species occurs at just over twice the rate of Galinsoga parviflora (2,296 to 988, as of 17 Nov 2020). But in the United States, the species occurs at over ten times the rate of Galinsoga parviflora (2,214 to 214). In Asia it is 537 to 120. On the continent of Australia there are thirty-six observations of Galinsoga parviflora and no Galinsoga quadriradiata--so far.

Key to the Cosmopolitan Quickweeds of the World

1a. Stems and leaves pubescent with multicellular hairs 0.5–1(-2) mm long; leaf margins dentate, the teeth obtuse to acute; limb of ray flowers 2–5 mm long, the pappus scales about as long as the tube; pappus scales of disk flowers sharp-acuminate, lacerate..... Galinsoga quadriradiata

1b. Stems and leaves glabrous to sparse pubescent with unicellular hairs 0–0.5 mm long; leaf margins crenate, the teeth rounded; limb of ray flowers 0–2 mm long, the pappus scales minute or absent; pappus scales of disk flowers truncate to obtuse, fimbriate..... Galinsoga parviflora

Discussion
The results obtained here and those of previous work elucidate several traits that are discrete, objective and unambiguous. Further, these traits are consistently correlated with each other to form a distinct assemblage of character states that unambiguously and consistently define the species. Each couplet (1a and 1b) in the dichotomous key above is a series of characters (separated by semicolons). The two species share these characters (such as pubescence or leaf margins) but not the values or attributes of that character. The values of each character are called character states. The character states exhibited by a species (such as hairs longer than 0.5 mm or shorter than 0.5 mm) are discrete and unambiguous for the two species. They are true for one species or the other, but not both. Each couplet is thus a summary of the assembled traits (characters and values) that define that species and can be used as a brief description (often called a diagnosis in older literature).

The process undertaken in the present study can instruct others seeking to distinguish taxa based on simple terms understood by a general audience and what can be observed with the naked eye or a hand lens. In photographs and often even in the field, it is not possible to dissect the specimen to examine minute and often highly technical features. But monographs, floras and even field guides may rely solely on minute and difficult characters to distinguish species (as in Galinsoga). For nearly all photographs of Galinsoga in iNaturalist, these characters are impossible to see and thus only tentative identifications were possible prior to this study. Working with preserved specimens I could examine under a microscope, I used the technical characters in a process of reciprocal illumination to find other, more easily observable traits by which the species could be identified. Based on the simple traits described here and summarized in the dichotomous key above, it is now possible for anyone to confidently identify Galinsoga in the field and from photographs such as those on iNaturalist.

This procedure may fail to reveal easily observable traits to distinguish cryptic species in other genera, but I have used it with success in Smartweeds (Persicaria) and in genera of other families.

Outside their native range, the two weedy species, Galinsoga quadriradiata and Galinsoga parviflora are often found growing side by side in mixed populations. Care should be taken when taking multiple photos in a single population.

It may only be a matter of time before additional Galinsoga species are found in the continental United States, especially in areas adjacent to Mexico, center of diversity of the genus.

References
Canne-Hilliker, J. 2006. Galinsoga Ruiz and Pavón, Pp. 180–182. in Flora of North America Editorial Committee (eds.). Flora of North America North of Mexico, Vol. 21. Magnoliophyta: Asteridae, part 8: Asteraceae, part 3. Oxford Univ. Press, New York; Canne, J.M. 1977. A revision of the genus Galinsoga (Compositae: Heliantheae). Rhodora 79: 319–389; USDA NCRS. 2020. Galinsoga. United States Department of Agriculture, Natural Resources Conservation Service accessed 21 October 2020; Braden, D.A. and J.C. Cialone. 1971. Characterization of two Galinsoga R. & P. species from New Jersey by achene length/width ratio and the presence of marginal cotyledonary hairs. Bulletin of the Torrey Botanical Club 98: 50–52.

Acknowledgements

This post is dedicated to Judith Canne-Hilliker, student and master of Galinsoga, who passed away in Guelph, Canada on October 27, 2013.

Except where explicitly stated, the words and research in this article are entirely my own. The facts belong to everyone. Please acknowledge that you heard it here first.

Posted on Οκτώβριος 21, 2020 0522 ΜΜ by danielatha