Gall on nettle - geograph.org.uk - 947367
Aside from causing obvious swelling, the gall can be seen to be dotted with numerous so-called cluster-cups, which have orange discs and a paler yellow margin; these cups are called "aecia" ("aecium", singular).
The rust fungi, of which this fungus is one, have an extremely complex life-cycle. Some of them can produce up to five different kinds of spore; the different spore-producing structures are designated, in the scientific literature, 0 (zero), I, II, III, and IV. Species that produce all five kinds of spore are said to be macrocyclic; those that do not are microcyclic. Puccinia urticata is a macrocyclic rust; two of its five stages are hosted on nettle, while the other three are hosted on a species of sedge.
As "British Plant Galls" (Redfern, Shirley & Bloxham; 2002) states, "it seems likely that, amongst all living things, the rust fungi have the most complex life cycles and nuclear arrangements".
The cluster-cups shown here produce asexually-formed "aeciospores" (stage I of the life-cycle); inspection through a hand-lens will often show that some of these discs have a yellow powdery mass adhering to them, made up of spores that have emerged from the discs.
This specimen was on a plant growing beside the path shown in 916482.
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