Tuesday, 28 January 2014

Wednesday Wildflower: tarweed

Tarweed is flowering at the moment.  Parentucellia viscosa is an erect herb with small yellow flowers and its leaves are covered with glandular hairs so dense they feel sticky to the touch.  You'll see it in damp patches beside roads and tracks, along the edges of ditches, and wet hollows in grassland.
Tarweed, Parentucellia viscosa, Karori, Wellington
It's a hemiparasite, which means it derives some of its nutrients parasitically from other plants, but it's also green and able to generate its own energy through photosynthesis.  Full parasites (holoparasites) usually lose the ability to photosynthesise and to make green pigments, so they are often brown or pale.

Tarweed flower.
It's related to Euphrasia, another genus of hemiparasites, of which we have a large number of native species in New Zealand, and to the introduced broomrape, Orobanche, which are holoparasites.

Orobanche minor, broomrape, near Nelson.
The hemiparasites Parentucellia and Euphrasia used to be classified along with Veronica in the family Scrophulariaceae, but it was discovered a decade or so ago ago that Scrophulariaceae as it was then drawn up wasn't a natural group (of related plants).  So that previously large family has been split up. Veronica was transferred to be classified with its relative Plantago (Plantaginaceae), and Parentucellia and Euphrasia joined their relatives the broomrapes in Orobanchaceae.

Ngaio, Myoporum laetum, Wellington
Although Scrophulariaceae has been dismembered into 7–10 different families to make a more natural classification, it still exists as a much smaller family, many of them African.  Our only native member is Myoporum, ngaio.