142 PLANT PHYSIOLOGY AND ECOLOGY 



Experiment 45. The behavior of tendrils. Experiment with the ten- 

 drils of Cucwbita or Micnunpdis to determine the sensory zone as well 

 as the period during which the tendril is sensitive. Find the length 

 of time necessary for contact to produce curvature in the tip, and for 

 the transmission of the stimulus as shown by the formation of the coil. 

 Note the behavior of tendrils that fail to reach a support, and of re- 

 cently formed coils from which the support is removed. Sketch a ten- 

 dril in various stages. Ascertain by sectioning the behavior of the cells 

 on the convex and concave surfaces. 



162. Response to shock. While all protoplasm possesses in 

 some degree the power of response to mechanical shock, this 

 reaction is readily seen only in moving or streaming protojjlasm, 

 and in certain specialized organs or plants, such as the stamens 

 of some cacti, and the leaves and stems of sensitive plants. The 

 best illustration of response to shock is afforded by the common 

 sensitive plant, Mimosa pudica. The normal reaction consists 

 of the folding of the leaflets and the drooping of the whole leaf 

 at the point of union between petiole and stem. A slight shock 

 merely causes the leaflets to close, and it is quite possible to touch 

 the leaves so lightly that it does not constitute a stimulus. At 

 least, no visible reaction takes place. The vigor of the plant also 

 has much to do with the response. The healthy leaves of plants 

 that had at one time been subjected to drouth and cold responded 

 but feebly to heavy blows, merely moving the leaflets slightly, 

 while those of a normal plant reacted fully to a gentle touch. 



The i)erception of shock by the sensitive plant is scarcely if at 

 all localized. Nearly all the epidermal cells of stem and leaf 

 have the power of perception, except certain cells of the base of 

 the petiole, i.e., the pulvinus. When the leaf or stem is struck 

 vigorously, the stimulus probably acts directly upon the pulvinus, 

 the leaflets folding, and the petiole drooping almost at once. The 

 perceptive power of any leaflet may be readily shown by striking 

 it gently. The impulse travels down the axis, closing the leaflets 

 as it goes, until it reaches the pulvinus, when the whole leaf droops. 

 The manner in which the stimulus is transmitted is not certainly 

 known. It is supposed to take place through rows of turgid 

 tubular cells, which lie near the bundles, by means of disturbances 

 in the cell-sap. 



The movements of leaflets and leaves are due to changes in 

 the pulvinus, a mass of swollen tissue at the base of the petioles 



