Soon after his arrival he went on a visit to the west to see whether he had best practise his profession there. On the way he sustained a severe shock to his spine in a steamer accident. It took him several weeks to recover somewhat, but he never fully recovered. He was disappointed in the outlook and returned to the east, where he took up practise in Philadelphia. There he became a friend of Elias Durand, a druggist and botanist, who in the latter capacity was requested to elaborate the botanical collections made by Heermann while with the Williamson Pacific Railroad Expedition. Durand proposed to Hilgard that they should collaborate in this work, and the latter being by nature an expert draughtsman, he not only described, but drew the illustrations of a large number of the "Plantæ Heermanianæ" accompanying the final report of the expedition. The strain of this work seemed to develop the spinal injury into a serious inflammation, from which he was prostrated for months. After recovery which was, however, never complete, he resolved to begin practise in St. Louis, and removed there in 1855.
He continued to practise in St. Louis from that time until 1870, much handicapped by the spinal weakness which obliged him to refuse much lucrative practice. His spare time was chiefly devoted to botanical studies, now more especially to the cryptogams, whose development he studied under the microscope, in the use of which he became very expert. In these studies he found that the then current classification and nomenclature of these-organisms was seriously at fault, many merely developmental forms being classed as separate species, genera and even orders. He also worked zealously in devising a system of arrangement of the phanerogams which would express their mutual cross relations, the best graphic presentation of which on a flat surface he found in the pentagrammatic form. Comparative anatomy and the homotaxy of organs and structural parts also formed a favorite subject of investigation. Most of his work on these subjects was published in the Proceedings of the St. Louis Academy of Sciences, of which he was a charter member; also in the Proceedings of the American Association for the Advancement of Science and in the St. Louis Medical Reporter. He also helped in the organization of the "Humboldt Institute" library which for some time had a very useful cultural influence. In 1865 he married Miss Georgina Koch, daughter of Mr. A, Koch, of Zeuglodon fame. No children came of this union.
As the state of his health precluded his acting as an army surgeon, he remained at St. Louis during the war in hospital and private practise. After the war medical practise seemed to become more and more incompatible with his strength, and he gave it up and joined his brother Eugene at the University of Mississippi, where at that time a lectureship of botany was contemplated. But it failed of realization, and he