Some years ago I reffereed a paper on an extract  of what was a 
species of prickly pear from the west coast of Africa, if I remember 
correctly.
The extract was used as an essential "haematoxylin" component of 
Papanicolaou Stain.  The paper was published in cytopathology.

Unfortunately, M y files and collections  from the earlier part of my 
career have had to be culledso I cannot give exact references


Russ Allison, 
Dental School
Cardiff
Wales