In selecting the form of the corolla as the basis of his classification, Tournefort has, perhaps, contributed more to the progress of botany than any other savant of any age. The task of instruction was rendered a pleasure by thus taking, as a subject of scientific inquiry, the most attractive part of the plant. He soon made adepts of those who had hitherto only contemplated flowers as the source of an agreeable sensation. The system of Tournefort for the classification of plants met with great favor among his contemporaries, on account of its simplicity. Nevertheless, in its application, this system presented many difficulties. The form of the corolla is not always so exactly appreciable that the class to which that plant belongs can be settled from that character alone. But the gravest defect of the system is, that by it the vegetable world is divided into two classes, namely, Herbaceous Plants and Trees--a division which has no existence in nature. The division destroys the natural analogies, for the size of a plant has no bearing upon its organization and structure. In conclusion, the continually increasing number of new species, which were unknown in Tournefort’s time, tests, in the strongest manner, the defects of his system of distribution. The greater number of vegetable species discovered since Tournefort’s time could not be placed in either of his classes. This defect soon became very apparent, and the system fell by degrees out of favor with botanists even among his own countrymen, with whom it had found most admirers. In England the study of plants had taken a more philosophical direction. About the middle of the Seventeenth Century the microscope was first applied to the study of the organs of plants; and in 1661 spiral vessels were detected by Henshaw in the walnut tree, and shortly afterward the cellular tissues were examined by Hooke. These discoveries were followed by the publication of two works on the minute anatomy of plants by Malpighi and Grew. They examined the various forms of cellular tissues and intercellular passages in their minutest details, and with an exactness which causes their works still to be recognized as the groundwork of all physiological botany. The real nature of the sexual organs in plants was demonstrated by Grew; the important difference between the seeds with one and those with two cotyledons was first pointed out by him. Clear and distinct ideas of the causes of vegetable phenomena were gradually developed, and a solid foundation laid on which the best theories of vegetation have been formed by subsequent botanists.