Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/209

looked at after an interval of 1^h. 30^m. the pollinium was found ejected. Another flower was placed in water at 90° F. (32.2° C.), and after 25^m. the pollinium was found ejected: two other flowers left for 20^m. in water at 87° F. (30.5° C.) did not explode, though they were afterwards proved to be sensitive to a slight touch. Lastly, four flowers were placed in water at 83° F. (28.3° C.); two of these did not eject their pollinia in 45^m., and were then found to be sensitive; whereas the other two, when looked at after 1^h. 15^m., had spontaneously ejected their pollinia. These cases show that immersion in water raised to a temperature only a little higher than that to which the plant had been exposed, causes the membrane by which the discs are attached to rupture. A thin stream of almost boiling water was allowed to fall through a fine pipe on the antennæ of some flowers on the above plant; these were softened and killed but the pollinia were not ejected. Nor did sulphuric acid, dropped on the tips of the antennæ, cause any action; though their upper parts which had not been injured by the acid were afterwards found to be sensitive to a touch. In these two latter cases, I presume that the shock was so sudden and violent that the tissue was instantly killed. Considering the above several facts, we may infer that it must be some molecular change which is conveyed along the antennæ, causing the membrane round the discs to rupture. In C. tridentatum the antennæ were one inch and a tenth in length, and a gentle touch from a bristle on the extreme tip was conveyed, as far as I could perceive, instantaneously throughout this length. I measured several cells in the tissue composing the antennæ of this species, and on a rough average it appeared that the stimulus must travel through no less than from seventy to eighty cells.