194 
AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
The most common and abundant species is Pinus taeda. Other common 
species are: Quercus stellata, Q. michauxii, Pinus palustris, Taxodium disti- 
chum, Hicoria spp., Ulmus spp., Magnolia spp., Liquidamhar styraciflua, 
Nyssa spp., Ilex spp. From the southwest, Parkinsonia aculeata, Prosopsis 
glandulosa, and Vachellia Farnesiana are invading. Characteristic of the 
salt marshes are species of Spartina and Sporobolus. 
Nine wild malvaceous species were found in this area. The only species 
found on the prairie was Callirrhoe involucrata. In the fresh water swamps 
Hibiscus lasiocarpus was common and abundant, H. militaris and Kosteletz- 
kya althaeifolia were infrequent. In the brackish marsh at the mouth of the 
Trinity River in Galveston Bay, Kosteletzkya althaeifolia grew very vigor- 
ously and abundantly. Hibiscus militaris and H. lasiocarpus rarely. Mal- 
vaviscus drummondii occurred commonly and often abundantly in fairly 
open woods along streams, lakes, bays, and in shrubby thickets on the 
edges of woods. Sida spinosa and Malvastrum americanum were found 
rarely in very open woods. No malvaceous plants were found in the pine 
or swamp forests. Four malvaceous weeds were found: Sida rhombifolia 
abundant and common in waste places, 5. spinosa frequent in fields, Mal- 
vastrum americanum frequent in waste places, and Modiola caroliniana 
rare in low, moist, waste places. 
B. Xerophytic Section 
Topography: Low and flat, broken only by streams and very low hills 
and ridges. In the northern part of Cameron County an extensive area of 
sand dunes extends inland from the Gulf. Soil: Chiefly compact clay. 
Elevation: Most of the area is below lOO feet; Corpus Christi is 20 feet, 
Brownsville 38 feet, Mercedes 68 feet. Rainfall: The annual mean at the 
97th meridian is about 35 inches, at the Rio Grande about 20 inches. The 
annual mean at Corpus Christi is 27 inches, at Brownsville 27 inches (maxi- 
mum 60 inches, minimum 9 inches), at Mercedes 19.8 inches. Temperature: 
The mean annual temperature from Laredo to the Gulf is 73° F., at Browns- 
ville 73° F., at Corpus Christi 70.0° F. Clear Days: The percentage of 
clear days in 1918 in Corpus Christi was 36, in Mercedes 12. i. Humidity: 
The amount of water evaporated from a free- water surface, according to 
Hill (9), is less than 60 inches annually. 
Most of the Xerophytic Section is covered with chaparral vegetation. 
Woody species of the Leguminosae; mesquite {Prosopsis glandulosa), 
huisache {Vachellia Farnesiana), mimosas, acacias, Parkinsonia aculeata, 
prickly pear {Opuntia spp.), Leucophyllum texanum, and others characterize 
this vegetation. Along streams and lakes, Ehretia elliptica, Celtis pallida, 
Siderocarpus flexicaulis, and large mesquites and huisaches are common. 
Dense clumps of the Mexican palm, Sabal mexicana, occur along the Rio 
Grande in the vicinity of Brownsville. A greater abundance of tropical 
vegetation is prevented from developing by the periodic freezes caused by 
"northers" in winter and by the aridity of the region. 
Several of the malvaceous species found in this area were very rare, and 
