Thanks Marco,
the first thing that is obvious is that the three illustrations in Hartlieb, the alchemical herbal and Cadamosto are completely different. For the alchemical herbal, no great similarity should be expected, and the one in Cadamosto looks closest to the real plant.
Comparisons with nature, even when this is being done by many (see You are not allowed to view links.
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This will work for the later herbals, which were more tending to botany than to medicine.
For the Voynich MS we simply don't know.
Typically, the placement of the leaves cannot be relied upon, and this aspect is indeed different for all three illustrations of Hundszunge / Cynoglossa above.
The colour of the flower is another problem. Can we rely on this for the Voynich MS?
A very large number of plants in the Voynich MS have blue flowers. Now it could be, that the MS intentionally deals with blue flowers. It could also be that the colour is arbitrary. Finally, whoever painted the MS did not have purple available, so may have used blue also for purple flowers....
All in all, I don't see this as more than an attractive possibility, one to add to the list, and only when the list gets long enough, a pattern might emerge.