Answers are grounded in this paper and cite their sources.
I have assumed that I am expected to base my discussion primarily on some features of phytogeography which I have personally examined, rather than to attempt extensive generalizations concerning floras with which I am absolutely unfamiliar.
No takes yet. Share an insight, caveat, or question.
Merritt Lyndon Fernald (1931) studied this question.
Synapse has enriched 10 related papers. Consider them for comparative context: