The species of our s, are, on the whole, well enough recognized, but their scientific definition is very insufficient—the characters, given in the books, vague and indefinite. I have had a good opportunity to study the different species and forms from all parts of our country, fresh and living as well preserved in numerous collections; among them those contained in the great of New York and ( and Gray) and those of (), and especially those of Berlin, whence the types of the different Mexican species were sent to me by my late excellent friend, . With the exception of , which with us is always a prostrate plant, all our species occur both in the form of low shrubs or of trees, a few of them of magnificent dimensions. In the arid mountain regions, the trunks of the different species which occur there frequently assume peculiar conical forms, very thick at base and rapidly tapering to a slender point.
— GEORGE ENGELMANN