Resemblance between central rosette's "towers" and small-plants' "containers".

[Searcher]

The idea that the jars might represent the elements is interesting. Of course, a major difficulty is that six elements is a strange number. Four is normal, five appears sometimes, but six seems difficult to explain. 

You are not allowed to view links. Register or Login to view. hosts a large image of the Yale copy You are not allowed to view links. Register or Login to view. (1570 ca).

In this case, the figures hold flasks, as you said. There are seven towers labeled 1st-7th imbibing (imbibissio): apparently, the towers represent phases in the alchemical process.

Some of alchemists assigned four stages of the Great Work, which well agried with the conception of the four elements.

• You are not allowed to view links. Register or Login to view., a blackening or melanosis
  
  
• You are not allowed to view links. Register or Login to view., a whitening or leucosis
  
  
• You are not allowed to view links. Register or Login to view., a yellowing or xanthosis
  
  
• You are not allowed to view links. Register or Login to view., a reddening, purpling, or iosis
  

Later phases were expanded to 7 (7 planets) and, even, 12 points (Zodiac), etc. You are not allowed to view links. Register or Login to view.
But, anyway, the number 6 in alchemy is always the number of the Great Work (six days of the Creation), so, some found the explanation of this number in alchemical/magical context, probably, to attach value to every point of this number, therefore 4 elements became 6 elements (components): water, fire, air, earth, dark and light.