1977] 
Porter — Mesostenines 
31 
ilar Argentine Chaco at the austral extreme of the Neotropics. 
Almost all my fieldwork on Valley mesostenines was done in the 
500 acre Bentsen Park near Mission and the 20 acre Valley Botan- 
ical Garden at McAllen. Otherwise, except for the Santa Ana Na- 
tional Wildlife Refuge near Alamo, most natural vegetation has 
been extirpated from the Valley and replaced by citrus groves, 
truck farms, sugar-cane fields and other agricultural systems. 
The Valley Botanical Garden is about 16 km. from the Rio 
Grande and thus lacks gallery forest and other really humid asso- 
ciations but offers a sample of scrub communities and moderately 
humid woods. Here the most abundant or conspicuous larger 
plants are: Acacia greggii (rare), A.farnesiana (common), Baccharis 
sp. (common), Bumelia celastrina (rare), Celtis lindheimeri (com- 
mon), C. pallida (common), Cercidium floridum (rare), Condalia 
obovata (common), C. obtusifolia (common), Ehretia anacua (com- 
mon), Forestiera texana (rare), Karwinskia humboldtiana (com- 
mon), Leucaena pulverulenta (scarce), Morus rubra (rare), Opuntia 
sp. (common), Parkinsonia aculeata (common), Phaulothamnus 
spinescens (rare), Pithecellobium flexicaule (rare), Porlieria angus- 
tifolia (rare), Prosopis juliflora (common), Sabal texana (rare), 
Salix nigra (common), Serjania sp. (common), and Xanthoxylum 
fagara (scarce). Within its 20 acres, the Botanical Garden contains 
several more or less distinct associations. Dry areas are dominated 
by Prosopis juliflora and Opuntia sp. with Condalia obtusifolia 
and Parkinsonia aculeata often common also. Moderately dry 
habitats have at least some of the foregoing species along with 
Condalia obovata, Celtis pallida, Baccharis sp., and Bumelia cel- 
astrina. Moderately humid sites usually are dominated by Celtis 
lindheimeri in the tree stratum, C. pallida in the shrub layer, and 
by Serjania vines at ground level. They also may contain the small 
tree Ehretia anacua, the large shrub Xanthoxylum fagara, and the 
small shrub Karwinskia humboldtiana. The large trees Leucaena 
pulverulenta and Salix nigra also occur in the Garden but only 
near ponds and irrigation canals. Finally, some abandoned farm- 
lands near the Garden support open Acacia farnesiana woods with 
a monotonous undergrowth of tall grasses. 
The Bentsen Park is much larger and floristically more varied 
than the Botanical Garden. Only Sabal texana occurs in the Gar- 
den but not at Bentsen, while Amyris texana (common), Fraxinus 
berlandieriana (common), Mimosa berlandieriana (moderately com- 
