Bill from San Jose, California (aka bbum@Flickr) submitted today's photograph of bull thistle (taken last July) via the BPotD Flickr Group Pool (original image). Bill has recently written about some of his experiences with nature photography on his weblog, bbum's weblog-o-mat. Thanks again, Bill!
Cirsium vulgare is native to Europe and Asia, but is now found throughout most of North America (including all of the continental US and Alaska). Part of its success as a weedy invader is due to it being a myrmecochore – a plant whose seed is dispersed by ants (in a 1973 paper in Biotropica by Fittkau & Klinge, the authors estimated the biomass of ants to be 4x that of all terrestrial vertebrates in the Brazilian rainforest – I don't know what the numbers might be elsewhere, but you can read more about ants in Wilson EO, Hölldobler B. 2005. The rise of the ants: A phylogenetic and ecological explanation. Proc. Natl. Acad. Sci. USA 102(21): 7411-4).
Dr. Peter Harris, emeritus scientist with Agriculture and Agri-Food Canada, has written an extensive article describing Cirsium vulgare, its economic impact, methods of biological control and its ecological properties. The advantages of myrmecochory are fully outlined in the article, but I'll summarize as well. Seeds transported to ant colonies are discarded (the ants eat the protein- and fat-rich elaisome on the fruit) into abandoned galleries in the nest. In the nest, the seed is: 1) protected from above-ground predation (rodents and birds), 2) provided with a nutrient-rich environment in which to germinate (if the seed is non-dormant) and 3) if it is dormant, can remain so until the seed is exposed to light. The last advantage might require some explanation. For a biennial species like Cirsium vulgare, two bad years of no seed production (due to biological controls, fire, chemical control, etc.) could theoretically wipe out a local population. However, by having buried seeds reside in ant nests that may not germinate for many years, the long-term sustainability of the entire local population of plants is secured.
Art resource link: Ruth Jones – Art of Tapestry, with images of pieces in the Galleria and Range of Work. Tapestry, like most other artwork, is certainly better witnessed in person. Still, it is possible to observe elements of composition, colour and form online (with an eye to continued attentiveness of such in photography). Site suggested by BPotD reader Margaret from Vancouver (thank you!).