36 RELATIONS OF PLANTS TO MECHANICAL FORCES 



to injury in this manner, and it is supposed that the Wound acts 

 simply in setting off or starting the machinery of irritability de- 

 signed to give movements as reactions to other stimuli, a thing 

 not unknown in other kinds of irritability. 



52. Repeated Movement in Response to Injuries. Secure well- 

 grown specimens of Biophytum sensiiivum and snip away the ter- 

 minal pair of leaflets with the scissors. The pairs remaining close 

 in succession toward the base of the petiole or rachis. After a 

 few minutes, when the leaflets have begun to recover from the 

 contracted state of the pulvini and regain their former position, a 

 second partial closure ensues, which also is in exact imitation of 

 the normal reaction of the plant. 



53. Traumatropic Curvatures of Tendrils. Select a number of 

 active tendrils of Passiflora or any cucurbitaceous species, and 

 snip off a section a centimeter in length from the tip with a single 

 sweeping stroke of a sharp razor given on the non-sensitive sur- 

 face. If a razor is not available use a pair of scissors though errors 

 are introduced by the stimulus given the mechanism of ordinary 

 curvature. Observe the wounded tendril, and note the peculiar 

 curvature which is most marked in a region about 5 to 10 mm. 

 from the end of the wounded organ. 



54. Tissues Formed in Response to Injuries. The destruction 

 of any of the living portion of a plant is followed by various re- 

 generative processes, the most general of which consist in the for- 

 mation of a layer ,of cork over the injured surface in herbaceous soft- 

 bodied species and of callus in woody plants. Such production 

 of cork is always due to embryonic tissues, usually the cambium, 

 and the cork thus formed is a fixed tissue and not capable of 

 further differentiation. Wounds in woody plants are healed or 

 recovered by means of a special mass of embryonic tissues which 

 may arise from any tissue except wood, hard bast and epidermal 

 cells. Callus sometimes undergoes suberization of the walls and 

 protects the wounded portion, or it may give rise to a phellogen 

 forming an outer corky layer which serves this purpose. In the 

 dicotyledons and gymnosperms the outer corky layer is differ- 



