The period at which the movement of closing and opening of flowers takes place is very varied. Ordinary leaves, as has been stated, close toward evening and open in the day. The periods of opening and closing in the case of flowers vary considerably, being affected, no doubt, by the visits of insects, which carry the pollen from plant to plant belonging to the same species. By this means flowers are fertilized, and the seeds resulting from plants that are so fertilized are much more numerous than those resulting from self-fertilized plants. Some plants, such as the pimpernel, close their petals when the sky is overcast. This is doubtless to protect the pollen from the injurious effects of rain. This kind of closing, however, is not to be confounded with the regular and periodic closing and opening of flowers. The diversity in the regular and periodic opening and closing of flowers in regard to time is so great that Linnæus was able to arrange flowers in a list in accordance with their times of opening and closing. This list he named a _Horologium floræ_, or floral clock, the time of opening or closing representing each succeeding hour. Some closing flowers open under the influence of strong artificial light, such, for example, as Crocus and Gentiana verna; on others, however, such as Convolvulus, artificial light has no effect. The closing of flowers is usually a slow process, as may easily be observed, but there are exceptions to this. “In Desmodium gyrans” (the Indian telegraph-plant) “the trilobate compound leaf has a large terminal leaflet and a smaller one on each side. When the plant is exposed to bright sunlight in a hothouse, the end leaflet stands horizontally, and it folds downward in the evening, but the lateral leaflets move constantly during the heat of the day, advancing, edgewise, first toward the end leaflet, and then returning and moving toward the base of the common petiole alternately on each side, in a manner very well compared to the movements of the arm of the old semaphore telegraphs.” Such are some of the more striking movements of plants. Even in cases where the precise advantage, as far as regards the economy of plant life, is not fully ascertained, it can not be doubted that such movements are advantageous. In strict accordance with the accepted theory of evolution, no peculiarity would be continued from generation to generation of either plants or animals, if it possessed no essential characteristic which helped the plant or animal to hold its own in “the struggle for existence.” [Illustration: Cacti, Rare Flowers, and Fuci Cacti--1 and 3, Mamillaria; 2, Echinocactus; 4, Cereus. Fuci--5, Sargassum; 6, Agarum; 7, Thalassophyllum. The Wool Tree (Bombax) and the Rafflesia Arnoldi] MOVEMENT IN PLANTS --CHARLES DARWIN