268 BOTANICAL GAZETTE [OCTOBER 
The Nuéces river, notwithstanding its course through the 
semi-arid Rio Grande plain, and its distance westward from con- 
tinuous forest areas (below its cafion), is accompanied by a 
narrow belt of mesophytic timber of which the largest growths 
are the species just cited for the other streamways. Other 
species from the southwest, habitually only shrubs, here become 
small trees. 
The timber of the Rio Grande valley presents three types of 
formation. The first is the cottonwood-willow association above 
the Grand cafion, with the Texas green ash (/raxinus Berlan- 
dieriana), and two or three minor species in the side cafions of 
the Great bend. In the heavily timbered Cibolo cajfion, for 
example, Havard reported Populus Fremontii four to five feet in 3 
diameter, and Texas green ash forty to fifty feet high and one 
to two feet in diameter. The second type, which is mainly that 
of the river valleys of the Nuéces and other rivers eastward, 
occupies the middle course of the Rio Grande below Eagle 
pass, but is at best a very insignificant feature. The third type 
occupies the valley below Rio Grande city. This is a mixture 
of eastern river valley species with species of the warmer Mexi- 
can region, as Ehretia elliptica, Bumelia lycioides, and several 
Mimoseae which have here become fair sized trees. The tropical 
affinities are finally marked in the lower course of the river by 
the appearance of an open formation of Sabal Mexicana, a tree 
twenty to thirty feet high. 
Timber formations of the box caftons—Streams which rise in or 
cut across the central erosion area of the Edwards plateau have 
worn deep and narrow channels known as box cafions, which in 
many cases are so sheltered and well-watered as to furnish 
proper conditions for typical mesophytic vegetation. Suc 
cafions, for example, are those of the Guadalupe system and the 
Perdenales. The woody vegetation in these cafions, as well as 
the herbaceous, is that of the interior margin of the Atlantic 
coast plain, and yet it is to be noted how isolated these cafions 
are with respect to the mesophytic regions of the coast plain. 
In the Turtle creek cafion in Kerr county sixteen species of 
