I doubt if anyone knows the mechanism, but geotropism is well-understood and I assume that more or less the same applies. My point in writing is to draw attention to the same trait in Ansellia (Africa) some Cymbidium (chiefly mainland Asia) and Grammatophyllum (Pacific Asian).
All of these are closely related, of course, yet maintain this relationship across thousands of miles of ocean. Africa and South America were last in contact in the Jurassic, and India freighted from Africa to Asia starting 120 mybp and arriving 60 mybp, carrying this freight and e.g. Acampe and Eulophia. This is a lot earlier than the dates in which flowering plants are usually credited with existence, yet the alternatives - stratospheric seeds, or migratory birds with seeds, or logs washed up on the shore - are rather hard to take. Why does the African orchid flora seem migrate out, but nothing seem to migrate into Africa if the reason is random movement or bird routes? Continental drift seems a sensible alternative; and one which also explains the "intermediate" nature of the Madagascar flora, as between Asia and Africa. ______________________________ Oliver Sparrow +44 (0)1628 823187 www.chforum.org _______________________________________________ the OrchidGuide Digest (OGD) [email protected] http://orchidguide.com/mailman/listinfo/orchids_orchidguide.com
