Fig. 125.
Pinnæ folding up
after stimulus.
Fig. 126.
Later all the pinnæ folded and leaf drooped.
271. Sensitive plants.—These plants are remarkable for the rapid response to stimuli. Mimosa pudica is an excellent plant to study for this purpose.
272. Movement in response to stimuli.—If we pinch with the forceps one of the terminal leaflets, or tap it with a pencil, the two end leaflets fold above the “vein” of the pinna. This is immediately followed by the movement of the next pair, and so on as shown in [fig. 125], until all the leaflets on this pinna are closed, then the stimulus travels down the other pinnæ in a similar manner, and soon the pinnæ approximate each other and the leaf then drops downward as shown in [fig. 126]. The normal position of the leaf is shown in [fig. 124.] If we jar the plant by striking it or by jarring the pot in which it is grown all the leaves quickly collapse into the position shown in [fig. 126]. If we examine the leaf now we see minute cushions at the base of each leaflet, at the junction of the pinnæ with the petiole, and a larger one at the junction of the petiole with the stem. We shall also note that the movement resides in these cushions.
273. Transmission of the stimulus.—The transmission of the stimulus in this mimosa from one part of the plant has been found to be along the cells of the bast.
274. Cause of the movement.—The movement is caused by a sudden loss of turgidity on the part of the cells in one portion of the pulvinus, as the cushion is called. In the case of the large pulvinus at the base of the petiole this loss of turgidity is in the cells of the lower surface. There is a sudden change in the condition of the protoplasm of the cells here so that they lose a large part of their water. This can be seen if with a sharp knife we cut off the petiole just above the pulvinus before movement takes place. A drop of liquid exudes from the cells of the lower side.
275. Paraheliotropism of the leaves of the sensitive plant.—If the mimosa plant is placed in very intense light the leaflets will turn their edges toward the incidence of the rays of light. This is also true of other plants in intense light, and is paraheliotropism. Transpiration is thus lessened, and chlorophyll is protected from too intense light.
Fig. 126a.
Leaf of Venus fly-trap (Dionæa muscipula),
showing winged petiole and toothed lobes.