Page:Insectivorous Plants, Darwin, 1899.djvu/42



The Inflection of the Exterior Tentacles as directly caused by Objects left in Contact with their Glands. [The researches of Pfeffor ('Unters. aus d. Hot. Institut zu Tübingen.' vol. I., 1885, p. 483) on the sensitiveness of various organs to contact show that the conclusions as to the sensitiveness of Drosera cannot be maintained in their present form (see p. 24).

Pfeffer shows, both in the case of the tendrils of climbing plants, and also in that of the tentacles of Drosera, that uniform pressure has no stimulating action; the effect which is ascribed simply to contact is in reality due to unequal compression of closely neighbouring points. Tendrils which move after having been rubbed with a light stick fall to be stimulated when they are rubbed with a glass rod coated with gelatine. The gelatine has the same uniformity of action as drops of water falling on the tendril, which are known to produce no effect. If the gelatine is sprinkled with fine particles of sand, or If the water holds particles of clay In suspension, stimulation results. Analogous experiments were made on Drosera (p. 511). It was found impossible to produce movement of the tentacles by rubbing the glands with a surface of mercury, whereas by rubbing or repeated touches with solid bodies movement is called forth. Other experiments of Pfeffer's show conclusively that continuous uniform pressure has no stimulating effect. He placed small globules of glass on the glands, and convinced himself, by examination with a lens, that contact was effected. Some of the tentacles moved, but the majority showed no movement, a long as the plants were so placed that no vibration from the table or floor could reach them. When they were exposed to vibration, and when, therefore, the glass globules must have rubbed against or jarred the gland, the tentacles moved. The results detailed above in the text must presumably be set down to the same cause, namely, the vibration of the table and floor. The sensitiveness of Drosera, therefore, by no means ceases to be astonishing. Instead of believing in movements caused by the steady pressure of very small weights, we set down the results as being due to the jarring of the gland by these same minute bodies.— F. D.]

I made a vast number of trials by placing, by means of a fine needle moistened with distilled water, and with the aid of a lens, particles of various substances on the viscid secretion surrounding the glands of the outer tentacles. I experimented on both the oval and long-headed glands. When a particle is thus placed on a single gland, the movement of the tentacle is particularly well seen in contrast with the stationary condition of the surrounding tentacles. (See previous Fig. 6.) In four cases small particles of raw meat caused the tentacles to be greatly inflected in between 5 and 6 m. Another tentacle similarly treated, and observed with special care, distinctly, though slightly, changed its position in 10 s. (seconds); and this is the quickest movement seen by