DANIEL & TRIPP: LOUTERIDIUM : TAXONOMY, BIOLOGY, AND CONSERVATION 49 
(e.g., dry forests, savannas) likely arose repeatedly from wet forest ancestors (Tripp 2007; Tripp 
and Tsai 2017). We cannot, however, rule out the likelihood that extinctions within Louteridium, 
whether representative of early diverging or later diverging lineages, may be confounding patterns 
of character evolution. 
Within Louteridium, it is noteworthy that relationships resolved among species are compatible 
with morphological data and that RAD loci resolved all species as reciprocally monophyletic (Fig. 
1). Our study thus adds to growing body of literature that documents the utility of RADsequencing 
within and across genera of flowering plants (e.g., Hipp et al. 2014; Tripp et al. 2017; Wang et al. 
2017; Bateman et al. 2018). Within Acanthaceae, the Petalidium RAD dataset (Tripp et al. 2017) 
is the only one to have been published to date; however, the present study together with unpub¬ 
lished RAD datasets on two other genera ( Justicia , Kiel et al., in prep.; Ruellia, Tripp et al., in 
prep.) brings the total number of lineages of Acanthaceae successfully sequenced using RAD loci 
to four. 
Morphology 
Because several species of Louteridium are not well known, discussions of some of the mor¬ 
phological variation in and unique or otherwise unusual characteristics of the genus are discussed 
below. Some morphological attributes for several species in the descriptions below are lacking due 
to their being absent or obscure on specimens, especially those that lack mature flowers or suffi¬ 
cient flowers for dissection. 
Habit.— Most species of Louteridium are reported to be trees (sometimes with multiple 
trunks, and sometimes described as “soft-wooded”) to 12 meters tall and with trunks to 25 cen¬ 
timeters in diameter, but several species can also occur as perennial herbs and shrubs. The three 
species of section Parcostamium are reported only as perennial herbs, shrubs, or small trees (to 
3 meters tall). Both L. costaricense and L. tamaulipense are only known as perennial herbs or 
shrubs. Louteridium parayi is apparently only epipetric or epiphytic; and Richardson (1972:63) 
observed that L. tamaulipense was “sprawling and sending out adventitious roots on the rocks.” 
Based on limited substrate data, L. costaricense has been reported to be terrestrial. Species of sec¬ 
tion Tetrandrium are shrubs or usually trees. At least three of them are known to be mostly or 
entirely either epipetric or epiphytic. All three species of section Louteridium consist of both shrubs 
and trees and are terrestrial (or sometimes also epipetric in L. donnell-smithii ). 
Bark of the trees has been observed or reported as smooth to warty and gray to light yellow¬ 
ish brown or golden brown in color. Prop or adventitious roots have been observed in several 
species (e.g., L. brevicalyx, L. dendropilosum; Fig. 2A, B, F) and they have been noted by collec¬ 
tors on herbarium labels for others, including both trees and shrubs. Trees with multiple trunks and 
prop or adventitious roots are also known among several taxa of core Trichantherinae (e.g., 
Bravaisia, Daniel 1988; Trichanthera, Daniel 2015). 
Pubescence.— As in most other genera of Acanthaceae, a diversity of trichome types is 
encountered among species of Louteridium. Although some species are mostly (e.g., L. parayi ) or 
entirely (i.e., L. chartaceum ) lacking in elongate trichomes, either punctate glands (minute and 
sometimes flattened glands lacking a stalk), eglandular trichomes, or glandular trichomes (stalked 
glands) are apparently present on one or more surfaces in all species. Often mixtures of two or 
more of these trichome types co-occur. Dendritic (i.e., eglandular and branched one or more times) 
trichomes are known only in L. dendropilosum (Fig. 3 A, B). Presence or absence of different types 
of trichomes on various parts of the plant, their relative lengths, and their shapes/stances are all 
useful in distinguishing among the eleven species treated here. 
Inflorescence.— The basic unit of the inflorescence in most Acanthaceae is a dichasium 
