Women as allegory for the life force of plants - or just a plant biology lesson?

[Koen G]

There are elements of some of the drawings that imply the author had a good understanding of the underlying structure of both plants and bodies, but the execution is rather shoddy. 

You are very observant
It's true, some things are really well drawn, while others look weird even for a non-standard medieval work. 
For example, many nymphs' arms look like weird rubber appendages. But then the horizontal proportions of their bodies (head to body, knee to navel etc) are really quite consistent. And as you say, some faces are quite well drawn. Circles are drawn with care (just look at the large foldout) while lines are almost never straight - there are hardly any squares, triangles etc.

I think many of the large plant drawings are actually well done, but they are ruined by the paint job. This is why some researchers even prefer to ignore color altogether because the painting is so irregular, and some of the thicker paint may have been added later. 

BUT then again, on some pages the paint job is decent! Just look at You are not allowed to view links. Register or Login to view. , the leaves on You are not allowed to view links. Register or Login to view. and so on. There are many examples. Now compare this to the sloppy painting on You are not allowed to view links. Register or Login to view. for example. Just to say, it's complicated...