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Arctium tomentosum [Woolly Burdock]: Another Example


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Rørslett, B. 2014. Another example of Arctium tomentosum Mill. (Asteraceae). Woolly Burdock. Flowers photographed in visible and ultraviolet light. http://www.ultravioletphotography.com/content/index.php?/topic/946-arctium-tomentosum-woolly-burdock-another-example/

 

Arctium tomentosum has been presented in UV earlier (Rørslett 2013). This is another example to show the UV rendition of the cobweb hairs covering the spiny hooked phyllaria ('burrs') of each flower head.

 

In my region, around Oslo, Norway, A. tomentosum-like features are found with a majority of the sampled specimens. Estimates of their frequency varies from 60 to 80 % of the sampled individuals.

 

Stands of burdocks of any kind might be perceived as ugly, prickly weeds by humans, yet one cannot help notice how all kinds of insects are strongly attracted to these flowers.

 

The genus is native to Europe and Asia and there are four recognised species of Arctium. These species can be difficult to identify in the field because hybrids apparently form easily.

 

The key features for A. tomentosum are as follows,

  • Flower heads are clustered in the end of stem and branches, often forming nearly flat-topped corymbs. Do note this character is less constant and unique in nature than the impression given by the floras.
  • Each flower head is approx. 2.5 cm broad, including the burrs.
  • The spines extend 6-7 mm from the main capitulum. The lower ones are not bent downwards as in A. minus. The phyllaria are dull greyish green, or occasionally, reddish in colour.
  • The flower head itself is roundish in outline. This separates A. tomentosum from A. minus, which has a an urn-shaped capitulum and distinctly reflexed lower phyllaria.
  • The disk flowers have a dark red to violet perianth. The anther tube is dark blue whilst the exserted styles are pink. White flowers are occasionally observed interspersed with normal-looking flowers so may just be an albino form of no systematic value.
  • The innermost row(s) of phyllaria are non-hooked and tips are dark, often brownish red, in colour. A. lappa can have occasional non-hooked innermost phyllaria, but these are always bright green like the others.
  • The burrs are covered with long cobweb-like strands of hair. All other Arctium species either lack this kind of hair (A. lappa) or they are very sparsely developed (A. nemorosum, A. minus).

Floras will furnish additional detail of the flowers and foliage.

 

ARCT_TOM_I1407182035_UV.jpg

 

 

References:

Rørslett, B. 2013. Arctium tomentosum Mill. (Asteraceae). Flowers photographed in visible and ultraviolet light. http://www.ultraviol...woolly-burdock/

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