Today’s photo features one of my favourite plants of all time–Achillea millefolium, or yarrow.
To call Achillea millefolium a species is incorrect. Achillea millefolium is a species complex, or aggregate, that consists of a set of species that are closely related. The members of the Achillea millefolium aggregate have a combination of different ploidy levels and natural hybridization. The relationships between the members of the aggregate are difficult to reconstruct. Guo, Saukel and Ehrendorfer (2008)(pdf) recount some of the attempts to make sense of the Achillea millefolium aggregate, which include placing all members into one giant species, or conversely, breaking the aggregate down into 40+ micro-species. One way of sorting the Achillea millefolium aggregate is by ploidy level (number of sets of chromosomes in a cell). The diploid (2 sets of chromosomes) taxa are limited to distinct areas in Eurasia and are less variable than the others. Achillea millefolium also come as tetraploids (4x), hexaploids (6x), and octoploids (8x). These species groups are far more variable and have an extensive northern hemispheric distribution. They are found in a wide variety of habitats from deserts to sea coasts, lawns and talus slopes.
The diversity of forms and locations that yarrow occupies is one of the reasons that I love this plant so much. In the period of one day, I can see tall pink yarrow cultivars that I have planted in my garden, the dainty fern-like foliage of yarrow growing weedily in lawns, scruffy yarrow growing along the roadside, and short-stemmed yarrow growing wild along an alpine trail. Although I will likely never understand the evolutionary sequence of these members of the Achillea millefolium aggregate, I can still make use of any of them if I am feeling ill or have been hurt. As a child, my mother would make a tea of yarrow flowers, amply sweetened with honey, anytime that I had a cold or flu. A review on phytochemistry and medicinal properties of the genus Achillea lists many medicinal properties for yarrow, including its use in treating wounds, ulcers, and diabetes.
I imagine that most of the Botany Photo of the Day readers in the northern hemisphere are just as intimately familiar with Achillea millefolium as I am. Those of you in the south are likely wishing I had provided a description of yarrow 2 paragraphs ago! Here it is: Achillea millefolium is a rhizomatous perennial that produces flowering stems .2-1 meter tall. The leaves are bipinnate or tripinnate, and distributed spirally along the stem. They are up to 20 cm long at the bottom of the stem, getting smaller as they approach the top. The Achillea millefolium inflorescence is a
panicle corymb of calathidia (it’s like a double inflorescence: the composite inflorescence type (calathidium) further arranged in a panicle corymb). It can be white to pink or yellow. What appears to be an individual flower in today’s image is actually composed of 3-8 ray flowers and 15-40 disk flowers (a typical characteristic of the Asteraceae). The disk flowers are crowded into the centre of each flower cluster, forming what looks like the eye of the flower. The ray flowers form what look like the petals. These flowers are nested within multiple layers of scale-like involucral bracts.