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 Rh of his dated Nov. 26, 1565 (Epistolae Medicae, 1577, p. 113) that he too was seeking for the basis of a natural system of classification and that he thought he had found it in the flower and the fruit:—"Ex his enim notis (a fructu, semine and flore) potius quam foliis, stirpium naturae et cognationes apparent."

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