stewart wrote:Why "domesticate" purple thorn when the 'cats have already shown a preference for celery.
The extra-Sphinxian colonies can take Sphinx-strain celery. It will likely be easier to grow on Earth-terra-formed planets that can support other terran foods.
It also avoids introducing a non-native species plant (purple thorn) onto other planets.
-- Stewart
As fallsfromtrees noted, Sphinxian Celery might be just regular celery when not grown on Sphinx.
Domesticate Purple Thorn to provide a supply of the "natural" supplement for study and extraction; The Celery version is slightly different and there may be some benefit to the Purple Thorn version not yet known.
Also, despite a preference for Celery, Purple Thorn can be grown without greenhouses. If a greenhouse full of celery should fail, the entire crop would be lost. Increasing the supply and accessibility of Purple Thorn would be a reasonable hedge against celery crop failure.