The frog

[-JKP-]

You are not allowed to view links. Register or Login to view. shows a plant with a toad-like root.

The plant is called Unctuosa, Campotos, Bubula. 
The leaves are described as "wide at the top and thin at the bottom" (as often seems to be the case in this ms, the illustrated interpreted this in a non-naturalistic way).
The flower is said to be "large, similar to convolvolus (volubilis), but yellow". Some of the illustrated flowers compare rather well with those of the Voynich plant (but for the color).
The root is explicitly described as toad-like ("ad modum buffonis formata").

Now that's interesting, because "unctuosa" is sometimes associated with the plant called Orpine (Sedum telephium), which has quite broad flowers and a lot of thick root tendrils that tend to mass up at the point of the stalk. Also, one of the names for Sedum telephium is "frog's stomach".

Unfortunately, it doesn't resemble the VMS plant (the leaves and roots of orpine resemble VMS 50v, but not the flower). Orpine has pear-shaped leaves with bumps, and umbellate flowerheads, but it might be the one mentioned in Trinity O.2.48.