I should clarify at the outset that the ‘Two prickly customers’ does not refer to this article’s contributors!
Having cleared that up I can safely continue – these images of the dead fruiting heads of composites were taken recently by David Haines, as part of early trials in ‘stacking’ digital images.
On the left is spear thistle Cirsium vulgare (constructed from seven images) from Culkein Drumbeg (NC1133), while on the right is burdock Arctium minus (constructed from thirteen images) from the side of Loch Drumbeg (NC1132). Both bear rigid spiny involucral bracts which dissuade animals from grazing the heads before the fruits can be dispersed, but there the similarities between the two end.
The fruits of spear thistle, like those of many composites, have feathery pappus hairs sprouting from the top of the ovary. These hairs spread, when the fruits are ripe and the air is dry, forming rigid parachutes and the means by which they are wind-dispersed, the well-known thistledown.
Those of burdock also bear pappus hairs, but they are short and cannot function as parachutes. So seed dispersal in the case of burdock is by the hooked bracts catching in the fur of passing animals, the whole head, or burr, is detached and the hard black fruits are gradually shaken out as the bearer travels further afield.
Both plants are usually found in disturbed ground and neglected gardens in Assynt, occasionally way up in the hills, but spear thistle is much more widespread. Burdock is a tap-rooted perennial that persists where it gets a foothold; spear thistle is a biennial, which just produces one set of flowers and fruits; contrasting strategies for life.
Of course it should be noted that their defences do not protect either from the predations of birds, in particular goldfinches.
Ian M. Evans