OLEACEAE:
Forsythia.
GENTIANACEAE:
Menyanthes.
Limnanthemum.
Villarsia.
POLEMONIACEAE:
Gilia.
CORDIEAE:
Cordia.
BORAGINEAE:
Pulmonaria.
VERBENACEAE:
Aegiphila.
POLYGONEAE:
Polygonum.
THYMELEAE:
Thymelea.
MONOCOTYLEDONS.
PONTEDERIACEAE:
Pontederia.
In some of these families the heterostyled condition must have been acquired at
a very remote period. Thus the three closely allied genera, Menyanthes,
Limnanthemum, and Villarsia, inhabit respectively Europe, India, and South
America. Heterostyled species of Hedyotis are found in the temperate regions of
North and the tropical regions of South America. Trimorphic species of Oxalis
live on both sides of the Cordillera in South America and at the Cape of Good
Hope. In these and some other cases it is not probable that each species
acquired its heterostyled structure independently of its close allies. If they
did not do so, the three closely connected genera of the Menyantheae and the
several trimorphic species of Oxalis must have inherited their structure from a
common progenitor. But an immense lapse of time will have been necessary in all
such cases for the modified descendants of a common progenitor to have spread
from a single centre to such widely remote and separated areas. The family of
the Rubiaceae contains not far short of as many heterostyled genera as all the
other thirteen families together; and hereafter no doubt other Rubiaceous genera
will be found to be heterostyled, although a large majority are homostyled.
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