"The 'Flora of North America,' by Drs. John Torrey and Asa Gray, was commenced in 1838, and appeared in numbers, at convenient intervals, until 1840, when, having reached, in the accepted arrangement of orders, to the end of Compositæ, its publication ceased. So valuable was this 'Flora' to the working botanists, that its discontinuance was a source of great disappointment; and those who were not aware of the reasons which made its intermissions almost imperative were not a little impatient. To those best advised, the discontinuance of the work was known to be really in the interest of American botany. The acquisition of Texas and of new territory at the close of the war with Mexico essentially changed our botanical as it did our geographical area. Up to that time, our knowledge of the far Northwest, the far West, and the Pacific coast, was mainly due to the labors of European explorers, to which were added the results of the journeys of Nuttall, Wyeth, Long, and a few others, but nothing like a general exploration had been made of those vast fields which have since yielded such rich botanical harvests. The two journeys of Fremont, the forced march to the Pacific on the line of the Gila, by Emory, with the volunteer expeditions of Lindheimer, Wislicenus, Fendler, Wright, and some others, resulted in such a wealth of new material, opening, in some cases, an entirely new flora, that it at once became evident to its authors that the 'Flora of North America' could not yet be written with any approach to completeness. These explorations indicated that the number of genera in the families, and the number of species in the genera, already published in the 'Flora,' were in many cases doubled, and it became evident that the authors must either continue a work which would be unsatisfactory when finished, or suspend it for a time, and devote themselves to the elaboration of the rapidly-accumulating new material, which would otherwise pass into other hands. They wisely adopted the latter course, a decision which proved all the more judicious when the survey of the Mexican boundary and the surveys of numerous routes for a Pacific railroad added most essentially to the already rich collections, and opened to botanical exploration such a breadth of territory that but few important localities were left unvisited. Besides these Government expeditions, and nearly contemporaneous with the later of them, came the private explorations of Parry, Hall, and Harbour, and others, while the rapid settlement of mining and other localities brought out a number of local collectors, who made important additions to the rapidly accumulating treasures. The State Geological Survey of California, and the survey under Mr. Clarence King under the fortieth parallel—thanks to Sereno Watson's energy and perseverance—also yielded important botanical results. The time has now arrived when the 'Flora of North America' may be written with the hope of presenting a fairly complete record. Of course new species and new genera are yet to be discovered, but no such bonanza of botanical riches as the past thirty years have developed can be looked for in the future, and our present knowledge may properly be embodied in a work which will serve as a standard, around which the clustering of future accessions will be an easy matter.
"One of the illustrious botanists, whose name appeared as joint author of the earlier 'Flora,' and which will ever be identified with North American botany, has passed away; but the results of Dr. Torrey's many years of labor since the first 'Flora' was discontinued will appear in the new work, the pages of which will show how industriously he labored during that long interval.
"We have said that the present is a most fitting time for making a 'Flora of North America';" it is so, not only in the fullness of materials, but especially so that its author is in the fullness of his industrious and useful life. Prepared, as no other can be, by years of study of our plants from every part of the country, and also by the experiences of extended field-observations on two journeys to the Pacific coast, our first botanist presents this, which we may regard as his crowning work. It is fortunate that, just at this time, those eminent botanists Bentham and Hooker have presented in their 'Genera Plantarum' a complete revision of the genera, made, so far as American genera are concerned, in full sympathy and correspondence with Dr. Gray. While, in the 'Flora,' Dr. Gray may not adopt all the views of these gentlemen, it is not the less gratifying to American botanists to know that the genera, so recently elaborated by three such botanists as Gray, Bentham, and Hooker, are likely to be accepted as established for a long while to come.