At the western edge of the southern forest, multi-stemmed mesquite trees accompany scrub oaks and several species of "cedars" of the genus Juniperus. Mesquite, with wood characteristic of tropical hardwoods, has ecological significance. Known as the "forgiving wood," it is praised by cabinetmakers for its stability and gluing ease: where other hardwoods change shape with changing humidity and therefore may be disagreeable to work with, this beautifully grained wood of Texas and Oklahoma serves well. Cattle, on the great drives to Kansas City and other stockyard markets a century ago, carried seeds of the leguminous trees on the long march northward. Before the drives, mesquite was principally confined to the Rio Grande country hugging the Mexican border or as scattered trees where fire reduced the number of stems. At least its range was much less extensive than at present. Once on the drive, the bovine consumed the highly nutritious bean pods, dropping them in their pats along the trail some days later. Soon the beans in the manure piles germinated, grew to seed-producing trees, and provided browse for other cattle a few years later. Softened by acids in animal stomachs, the seeds were naturally stratified for ready germination. Now, mesquite trees occur in dense nuisance stands nearby pine and hardwood forests.
A tropical hardwood species, the wood contains quartz minerals. A hand lens enables one to see crystals of silica dioxide ingrained in the wood. How particles of the fourth-hardest mineral get into the wood remains a mystery. Although silica is not an essential element for plant growth, and although it is highly insoluble (else the sands of the seashores would not persist), the mineral moves from the soil through roots to the xylem of the tree, there to precipitate as crystals among the fibers of cellulose and lignin in the wood.24 The crystals dull the cabinetmakers' tools.
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