Photo credit: Norbert Nagel.
Used under this licence.
The plant to the left is French Lavender. There seems to be some disagreement over its scientific name. I have found references to Lavandula angustifolia and to Lavandula stœchas, both of which are called French Lavender. I am not sure if these are supposed to be the same plant or not. No matter. The name cassidony is or was once applied to one or all of these. Here is what the online OED has to say about the word.
Etymology: Of uncertain etymology: suggestions are that it is the same word as cassidoine n.1, or of the same derivation.
(Skinner’s guess that it might be a corruption of *Stœchas sidonius labours under the fatal objection that no such name is known.)