I'd like to add another reference for dragons. It draws weight on the idea that they simply mean that sap is the ingredient to be taken from the plant, and is not an indication of the name of the plant, or of a particular plant.
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Login to view., a 1487 italian herbal on paper very recently digitized at Museum national d'histoire naturelle in Paris.
If you look at folio 32r you will see two dragons, and it is clear from the text that they refer to
suco (sap), one from the leaves, the other from the roots. The plant itself is
gengiana (italian for gentian). So the name has nothing to do with
drago.
And if you look at folio 50r there are also two dragons pointing/eating the leaves, and the plant there is
carlina (and looks markedly different from the depicted
gengiana).
Since dragons appear twice on different-looking plants that have different-sounding italian names, the dragons cannot refer to a particular plant or a name related to
drago, but only on something in common to both folios. The sap idea being written explictely on folio 32r, it is very probably the correct one.
Now, making the assumption that this is the meaning of the dragon in the VM too, it implies that the VM author knew of that peculiar medicinal convention. It would thus be interesting to trace it carefully, both geographically and chronologically. That would help narrow down the probable background/location of the VM author.
It should also be noted that Ms 326 has several interesting similarities to the VM : a brown snake near the root on its folio 13r is reminiscent of the VM f43v, and heads in roots on folio 33r are akin to VM f33r. Several differences exist too obviously, for instance there are heads on other folios too, and also two full humans on folio 42v and folio 43r.