Mississippi Valley. Seventeen years later, after an investigation of the whole genus in the principal herbaria of this country and of Europe, he published a systematic arrangement of all the Cuscutæ, giving seventy-seven species, besides a number of varieties.
Dr. Engelmann's authority upon the Cactaceæ was of the very highest. He established the arrangement of these plants upon floral and carpological characters. This work was carried on through a series of papers beginning with his sketch of the botany of Dr. A. Wislizenus's expedition from Missouri to northern Mexico, and continued in his account of the giant cactus of the Grila, in his synopsis of the Cactaceæ of the United States, and in his two memoirs upon the southern and western species contributed to the Pacific Railroad Reports and to Emory's "Report on the Mexican Boundary Survey." He 
Fig. 11. Nicholas Riehl;
from a photograph kindly loaned by his son, Mr. E. A. Riehl. had made preparations for a revision of at least the North American Cactaceæ, but upon his death much knowledge of this difficult group was lost.
His papers on the American oaks and the Coniferæ are of the highest interest, and are some of the best specimens of his botanical work; and the same is also true of his study of the vines. Nearly all that we know of this genus scientifically is directly due to Dr. Engelmann's investigations.
His work is characterized by a minuteness and carefulfulness of observation, coupled with a nicety of discrimination which made him a master in systematic work, his treatment of the yuccas and agaves, the genera Juncus, suphorhia, Sagittaria, Isoetes, the Loranthaceæ, Sparganum and Gentiana giving him an eminence among fellows botanists to which few attain. His name was upon the rolls of many societies devoted to the investigation of nature, and he was the recognized authority upon those departments of his favorite science which had most interested him. His name has been given to a monotypical genus of plants, Engelmannia, by Torrey and Gray. Numerous species also bear his name.
Shortly after Dr. Engelmann settled in St. Louis, Nicholas Riehl.
