26 RELATIONS OF PLANTS TO MECHANICAL FORCES 



upper and lower sides. Examine the epidermal cells of the con- 

 cave surface constituting the perceptive zone (Fig. 1 1). 1 



FIG. ii. Sections showing the character of the cell contours in a longitudinal sec- 

 tion of a tendril. A, epidermis and cortex of convex side before curvature. B, after 

 curvature. C, concave side before curvature. D, after curvature. 



39. Comparison of the Irritability of Tendrils and Mimosa. 

 Allow a stream of water to fall in a shower upon sensitive tendrils 

 as in the experiment with Mimosa (27). Does curvature follow ? 

 Arrange to have the water mixed with a quantity of fine sand. 

 This can be done by placing a quantity of sand in the bottom of 

 the vessel containing the water and stirring as the water flows out 

 through the siphon tube. Note the result of the action of the 

 minute mineral particles. Strike the stem at the base of an active 

 tendril so that it will be shaken violently. Note the reactions to 

 shock. Are they similar to those of contact ? Apply a steady 

 but gentle pressure to the tip of a leaflet of Mimosa taking care 

 not to crush or bruise the tissues. Is Mimosa sensitive to contact ? 



40. Contact Reactions of Drosera. Specimens of Drosera 

 should be cultivated in shallow wooden or earthenware dishes con- 

 taining peat and brought into a room kept at 20 to 25 C. for the 



1 MacDougal. Mechanism of the curvature of tendrils. Annals of Botany, lo ; 

 373- 1896. 



