Evidently at this period classification was in the air, and at length it began to precipitate and to crystallise in the work of Andrea Cesalpino (Caesalpinus: 1519-1603), Professor in the University of Pisa, whose De Plantis Libri XVI, published in 1583, is one of the most important landmarks in the history of systematic Botany. Here for the first time a system is propounded which is based definitely upon morphological observation. Cesalpino turns to the "fructification," that is the flower and the fruit, for his distinguishing characters. "Enitamur igitur," he says (Lib. i. cap. xiv.), "ex propriis quae fructificationis gratia data sunt, plantarum genera investigare"; and he goes on to point out that the observable differences here depend on number, position and form of the parts:—"ad organorum constitutionem tria maxime faciant, scilicet, partium numerus, situs et figura." These principles he illustrates as follows:—the flower being the outermost covering of the fruit, a single flower may cover a single seed, as in the Almond: or a single seed-receptacle as in the Rose: or two seeds, as in the Umbelliferae: or two seed-receptacles, as in the Cress: or three seeds, as in the genus Tithymalus (Euphorbia); or three receptacles, as in the Bulbaceous plants (petaloid Monocotyledons): or four seeds, as in Marrubium: or four receptacles, as in Euonymus: or many seeds, as in the Cichoriaceae: or many receptacles, as in the Coniferae. The feature of the relative position of the parts which he especially emphasizes is whether the flower is inserted upon the top of the fruit (i.e. is epigynous): or is inserted lower around the fruit (hypogynous or perigynous). Moreover, the form of the seed, of the seed-receptacle, and of the flower, is to be taken into account.

The practical application of these principles led to a classification of plants which, though of course imperfect, was at least a good beginning. Following Theophrastus, Cesalpino divided plants into two main groups, (1) Trees and Shrubs, (2) Undershrubs and Herbs: each of these groups was then subdivided according to the nature of the fruit and of the flower. It will be observed that Cesalpino, as was customary at that time, designated as "seeds" all indehiscent one-seeded fruits, such as nuts and the varieties of achene. The following abstract will suffice to give an adequate idea of the results obtained. The author's own words are given as nearly as possible.

In spite of its inherent imperfections and of errors of observation, the method succeeded in bringing together a considerable number of the plants dealt with, into groups which are still regarded as natural. For instance, among the trees and shrubs, the leguminous genera, and the coniferous genera, respectively, are so brought together: and among herbs, the leguminous, umbelliferous, cruciferous and composite genera. Moreover, though many of Cesalpino's sections consist of what seems to be a heterogeneous assemblage of plants, yet they include groups of closely allied genera, representing several of the natural orders of more modern times, which his method was incapable of distinguishing. With all its shortcomings, the method produced a classification of plants which has proved to have been natural in no slight degree.

The very numerous botanical works which were published in the century after the appearance of Cesalpino's De Plantis afford evidence that his system of classification did not meet with an enthusiastic reception. Though his plant-names were generally quoted, his arrangement was entirely ignored: in fact the very idea of classification seems to have gradually faded out of the minds of botanists, whose attention was more and more engrossed with the description of the new species that the rapid extension of geographical discovery was bringing to light. This condition of the science is well illustrated by the most authoritative systematic work that the 17th century produced, the great Pinax Theatri Botanici (1623) of Caspar Bauhin (1560-1624), a work which contains about six thousand plant-names, and was the product of forty years' labour. It might be expected that in such a work, special attention would have been paid to classification, that at least the best available system would have been used: as a matter of fact, the arrangement adopted is far inferior to that of Cesalpino and may be described as simply haphazard for the most part. The general lines of it are indicated by the following enumeration of the contents of the twelve Books of which the work consists; the modern equivalents of his plant-names being given.

Summary of the Arrangement adopted in Bauhin's Pinax.

Liber I. Gramineae, Juncaceae, Cyperaceae, Typhaceae, Ephedra, Equisetum, Hippuris, Asphodelus, some Iridaceae, and Zingiberaceae.

Liber II. De Bulbosis; bulbous Monocotyledons, including Orchids with Orobanche, Monotropa, and Lathraea.

Liber III. Olera et Oleracea; most Cruciferae, Polygonaceae, and Chenopodiaceae, with some of the Compositae.

Liber IV. Other Compositae; Delphinium, Fumaria; the Umbelliferae (so named); Valeriana.