Great photostudy, Helena.
The thought is that the elaisosome (the white part on the side of the seed proper) is mimicking an ant larva/pupa. We can see there is a similarity in size and colour. But I think it goes deeper than that: the elaiosome actually smells right. All that means that, not only do ants sniff out the violet seeds (rather than just stumbling across them), they take them back to their nest - underground and in nice finely tilthed earth!
Because Viola odorata and related violets have evolved this strategy with ants as a means of dispersal, their seed pods lie on or near the ground and when they open, the seeds just tumble gently out. That's in contrast to other violets, such as V. riviniana, which accomplish dispersal by ballistic ejection of the seeds from the pods, which are held upright above the leaves. It is no coincidence that the seeds of the latter type of violet do not have elaiosomes.
Just to pick up on violet seed pods being 'held upright above the leaves'...
Most of the time, the top of the flower stem is turned over, so the flowers face somewhat downwards. That persists after the seed pod is formed. But, in species that use ballistics for seed dispersal, shortly before the pod starts ejecting seeds, the top of the stem turns up, thus aiming the pod at the sky to achieve furthest dispersal. You can see what I mean in the last photo in my article on Viola elatior.
http://www.mikehardman.com/plants/viola/elatior.htmBecause violets can spend quite a while in the 'fat pods' stage without the seeds being ripe, it can be tricky when it comes to collecting seeds. The trick is to wait until the pod turns up: that's the signal that the seeds are very nearly ready. Also, the pod's green colour may become yellowish or brownish. Then you can collect the seed pod; best done by nipping it off at the base of the stem - leaving the sap in the stem to allow a bit of final ripening. Put the pod+stems in a closed paper (not plastic) bag and wait. You'll hear the seeds pinging off the inside of the paper.