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To Dr. George Engelmann I am, also, much indebted for a list of the plants found in the southern part of the state, mostly opposite St. Louis. His accurate knowledge of the plants of this country is well known, and will give confidence to any statements that rest upon his authority. In Wood's Class Book of Botany a number of species are credited to Illinois.
But, in addition to all these sources of information, I have had an opportunity to examine the extensive collections of plants made by Mr. Robert Kennicott, Mr. Emile Claussen, and others, and exhibited at the state fair held at Chicago, in 1855. This collection, besides adding quite a number to the species before known as belonging to the state, was of material aid in showing their distribution.
It will be readily understood, therefore, that the following is a pretty full list of the plants growing naturally within the state of Illinois. Further researches will undoubtedly reveal some new treasures; but compared with the whole, their number will be small. A glance at the geographical position of Illinois, with a knowledge of her principal topographical features, will prepare us to look for a rich flora. Bordering on the great lakes; bounded on her whole western side by the great Mississippi river; with the valley of the Wabash on the east, and of the Ohio on the south, she is almost entirely surrounded by water. The southern extreme of the state lies in latitude 37 deg., or nearly as far south as the lower part of Kentucky and Virginia, and has an elevation of only two hundred and seventy-five feet at low water, at Cairo, above the ocean level. Here are found quite a number of plants that do not extend into the central and northern portions of the state. These are distinguished in the following catalogue by the letter S. The north line of the state is in latitude 43 deg. 30 min., and partakes in a considerable degree of the character of the south part of the adjoining state of Wisconsin. The plants in the following list found here, and which do not extend into the central and southern parts of the state, are distinguished by the letter N.
The central portions of the state lie on a nearly uniform plain, whose average elevation is considerably below the level of Lake Michigan, (578 feet above the ocean) and constitute the great prairie districts. These are rapidly being subdued and converted into "improved lands," from which almost every trace of the original