appeared in 1865 and ten years later he revised and enlarged it as a book under the title "The Movements and Habits of Climbing Plants," using, as always, not only his own detailed and extensive observations, but also the published writings of other botanists, among them the paper on tendrils by Hugo de Vries, who was subsequently destined to throw such a flood of light on the phenomena of variation. Darwin grouped climbing plants into twiners, leaf-climbers, tendril-bearers, hook-climbers and root-climbers. He maintained that the climbing habit has been developed to enable vines to reach the light and free air; tropical forests show conclusively that this is the case. He showed that circumnutation, the bending of growing tips successively to all points of the compass, is a general phenomenon among flowering plants, and he thought it of high importance to them. The sensitiveness of tendrils to external influences interested him deeply and he made many original experiments upon them. Following the subject much further, he published in 1880 the work entitled "The Power of Movement in Plants," a treatise abounding in records of original observations on seedlings and parts of mature plants, including further studies of circumnutation, of the sensitiveness of plants to light and to other forces, and of the phenomena of geotropism and apogeotropism, which he regarded as modified phenomena of circumnutation.
The value of the impulse given by Darwin to botanical investigation in all its branches is beyond estimation; his power of exact observation and record has seldom been equaled and certainly never excelled; his deductions were highly philosophical and most of them have stood the test of thirty years' inquiry and criticism; he was searching for truth and his absolute honesty in research is plainly evidenced by his repeated criticism of his own conclusions.
The immense number of plant species which had been described and named, and the lack of any complete index to them led Darwin to provide in his will for complete enumeration of the names of published species of flowering plants. This great work was prepared at the library of the Royal Gardens, Kew, England, and published in 1895 in four large quarto volumes, to which several supplements have since been added. This "Index Kewensis" is a great boon to all investigators, and is quite indispensable to those who have to take plant names into consideration.