Most of the differences between A and B plants that Koen and Cary have noted seem indeed real. But they do not imply a change of Scribe. They are not really differences in "style" but in "contents".
The two different leaf shapes on the Viola Tricolor page (f9v) are an actual feature of the plant: the leaves simple lance-shaped at the bottom, changing to multi-fingered higher up. I pointed that out back in the mailing list days, after I saw that detail on a "scientific" herbal. When I was at the Mondragone conference, by luck there were potted pansies in the Villa's courtyard, so I was able to show a photo of the leaves in my talk.
For of that detail, more than for the flowers (which are upside down -- could that be a Chinese subspecies?

) I consider You are not allowed to view links.
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Login to view. the only plant that has been identified with near certainty -- and the only one which can be assumed to have been drawn from nature. But unfortunately it grows wild all over the world...
The "color annotations" seem to be mostly a feature of the first few bifolios of the Herbal, not of Herbal-A as a whole. An explanation that seems plausible is that the Scribe at first copied those annotations from other books, together with the plants, without understanding what they meant; and stopped doing so when the Author noticed and told him to ignore any letters or text when copying a drawing.
The "line separating root from stem" is, in many cases, better described as the root ending abruptly with a broad flat top, from which there grows a much thinner stem or stems. I think that detail is due to many plant parts in the Herbal, especially roots, having been copied from the Pharma section. The latter may have lost two folios f97-f98 after the pages were numbered, and maybe more before that; so it is possible that even more Herbal plants were copied from it.
Now, I think that many Pharma drawings depict roots with the top cut off to show the internal appearance, or even cut into sections as the root would be found in a farmer's market or apothecary shop. And the drawings that show both roots and leaves were not depicting the whole plant, but only those two parts -- disconnected, but drawn one over the other for "artistic" effect. Thus the flat-topped roots in the Herbal would be more examples of the Scribe misunderstanding the figures that he was copying.
More generally, I believe that the differences between Herbal-A plants and Herbal-B plants can be explained by the two sub-sections having been scribed in that order, but with a long pause between them; and/or by the Scribe having used a different set of existing herbal(s) for "inspiration" on each sub-section. Whereas the
style of the drawings, in the stricter sense of the term, seems to be the same through the whole book (apart from variations that can be ascribed to variable amounts of BEEEP).
We have seen even more dramatic differences between the nymphs on different pages of the Zodiac, or the jars on different pages of Pharma; but in these cases we can clearly see the
gradual evolution over time.
All the best, --stolfi