488 



BOTANY 



response, both on the part of naked protoplasmic organisms, and of 

 multicellular organs. 



Pressure, Contact. The effects of contact-stimuli are especially 

 marked in the case of climbing plants, this being particularly evi- 

 dent in tendrils. In the latter, the tip of the young tendril is extraor- 

 dinarily sensitive, the sensitive area being sometimes confined to 

 the concave surface (Cucurbita), in others (Cissus, Cobaea) all parts 

 of the young tendril are sensitive. Pressure alone does not seem to 

 be sufficient to induce coiling of the tendril, but there must be a 

 certain amount of roughness in the surface to which the tendril is 

 applied. Pfeffer found that a layer of moist gelatine excited no 



FIG. 459. Tendrils of Sicana. (After BAILEY.) 



response in very sensitive tendrils, and Peirce found the same to 

 be true in Cuscuta. 



The effect of contact in tendrils is a checking of growth upon the 

 side in contact with the support, and a greatly increased growth upon 

 the opposite side, causing the close coiling of the tendril about the 

 support (Fig. 459). In tendrils like those of Ampelopsis, the press- 

 ure-contact causes the development of suckers at the tips of the 

 tendrils. 



Many flowers show sensitiveness to contact in the stamens or 

 pistil. Thus Berberis and Sparmannia have the stamens sensitive 

 to touch, springing violently inward when they are touched near 



