DANIEL & TRIPP: LOUTERIDIUM : TAXONOMY, BIOLOGY, AND CONSERVATION 59 
wetter forests of northeastern (i.e., L. tamaulipense ) and southeastern (i.e., L. parayi) Mexico, both 
on the Gulf of Mexico versant. Louteridium chartaceum and L. costaricense are both endemic to 
Central America, with the former restricted to Belize and the latter known only from Costa Rica 
and Panama. Both of the Central American endemics occur primarily (or exclusively?) on the 
Caribbean versant. Only three species have relatively broad ranges (i.e., of at least 32,000 km 2 ), 
based on extent of occurrences (EOOs; IUCN 2017): L. costaricense, L. mexicanum, and L. don- 
nell-smithii (Figs. 9 to 11). In elevation, Louteridium occurs from 1 to 2100 meters, with only one 
species, L. donnell-smithii, spanning that entire range. In general, species of section Tetrandrium 
occur at lower elevations (i.e., 10 to 750 [to 1400] m) than those of sections Louteridium and 
Parcostamium (i.e., 1 to 2100 m), but there is significant overlap. 
The distributions of two pairs of species are currently known to overlap. Both L. donnell- 
smithii {Breedlove 50875 ) and L. mexicanum {Breedlove & Keller 49526) occur in northeastern 
Chiapas, and grow within less than one kilometer of each other. The ranges of L. mexicanum and 
L. parayi also overlap, with the EOO of the latter occurring entirely within the EOO of the former. 
These two taxa have been collected from the same site in the Uxpanapa region of southeastern 
Veracruz (i.e., Dorantes 2801 of L. mexicanum and Dorantes 2850 of L. parayi), and both species 
were observed at another site in the Uxpanapa region where L. parayi was collected {Daniel & 
Wendt 5804). These same species also occur within less than one km of each other in west-central 
Chiapas, northwest of Berriozabal (i.e., Miranda 7834 of L. parayi and Breedlove 24803 of L. mex¬ 
icanum). Although no hybrids between species have been detected among collections examined, 
considering the interspecific fertility shown for several genera of Acanthaceae (e.g., see partial 
summary in Daniel 2007), close phylogenetic relationship between L. donnell-smithii and L. mex¬ 
icanum, considerable overlap in the flowering seasons for both of these sympatric pairs of species, 
and likelihood of common pollinators (see below), hybridization between these pairs could be pos¬ 
sible. 
Although species of Louteridium have been reported from several plant communities, it is 
noteworthy that the sections of the genus recognized here generally correspond to wet vs. dry 
ecosystems. The six species of sections Louteridium and Parcostamium all appear to be restricted 
to moist to wet plant communities. These have been described by collectors as: lowland and 
montane moist to wet forests (or rain forests), mesophytic montane forests, cloud forests, and trop¬ 
ical semi-evergreen and evergreen forests. In contrast, the five species of section Tetrandrium occur 
in tropical deciduous forests, tropical subdeciduous forests, and evergreen seasonal forests. Among 
these species, only L. chartaceum, which is restricted to steep and well drained limestone hills on 
the Caribbean coastal plain, occurs in the evergreen seasonal forests (cf. tropical evergreen 
seasonal broadleaf lowland forests fide Meerman and Sabido 2001). This forest type receives more 
rainfall than the dry forests in which the other species of section Tetrandrium occur. Louteridium 
chartaceum is also the only species of this section that occurs on the Caribbean versant and it is 
phylogenetically early diverging with respect to other members of its clade (Fig. 1). Thus, it is 
possible that L. chartaceum is somewhat transitional in its habitat between the wet-forest species 
of sections Louteridium/Parcostamium and the four dry forest species of section Tetrandrium that 
occur in western and southern Mexico. Within their respective communities, most data on speci¬ 
men labels and our observations reveal that species of Louteridium generally occur on exposed 
rocks, rocky slopes, and rocky ground. The exposed rocks or bedrocks are mostly (exclusively?) 
limestone, commonly with some form of karst topography. Plants frequently occur within riparian 
zones, but are also found on upland sites not associated with waterways. 
