brown stuff secreted by plants??

[Chris Skory]

This question will probably reveal my ignorance in plant physiology, 
but, I was curious what general types of compounds do plants secrete 
from their roots.   I ask this because I saw some plants (eg lettuce, 
carrots, etc) growing in sterile medium the other day.  The medium 
became increasingly more turbid and brown as the culture grew.  Is 
this protein, alkaloids or some other secondary metabolite.  Any 
help would be appreciated.

Chris Skory
skorycd at ncaur1.ncaur.gov