Violets can be smelled for only a few moments at a time because they get their scent from ionine which shuts the smell receptors off after simulating them. After a few breaths the scent pops up again. – WTF Fun Facts
Source: https://www.ncbi.nlm.nih.gov/pubmed/12643663
WTF Fun Fact – Violet Ionine
The picture is of a pansy , not a violet