378 EFFECT OF MOLAR AGENTS UPON GROWTH [Ch. XIT 



studied by experimental methods the movements of these 

 organs. The conditions which induce the twining of tendrils- 

 are peculiar. It appears that tendrils are not irritated by 

 mechanical shaking, such as is produced by a powerful current 

 of air issuing from a narrow tube ; nor by a drop or strong cur- 

 rent of water, provided it contains no hard particles ; nor even 

 by a stream of mercury, or the surface of pure mercury or oiL 

 All solid bodies, however, with the exception of moist gelatine,, 

 when rubbed against the stem stimulate it so as to produce a, 

 bending at the irritated point. 



The behavior of moist gelatine is instructive. Pfbpper. 

 first observed that it was exceptional, and Pbirce has found 

 the same exception to hold for Cuscuta. Pfeffee's experi- 

 ments showed that, no matter how great the pressure or 

 strokes made by drops of 5 to 14% gelatine, or by glass rods 

 covered by a la3'-er of moist gelatine, no response occurred ; 

 and Peiecb ('94, p. 66) found that when the Cuscuta stem, 

 came in contact with a rod covered with wet gelatine it made 

 only the long, steep turns characteristic of the free-growing- 

 stem. Later, when it reached the part of the rod not covered 

 with gelatine, it formed at once a close, tight coil. That it is- 

 not the chemical composition of gelatine which prevents the 

 twining is shown by the fact that dry gelatine, even to 25%,. 

 irritates ; also, if some sand is mingled with the moist gelatine^ 

 tropism occurs. The fact that wet gelatine does not irritate is 

 explained by Ppepfbe upon the ground that the stimulus is- 

 not given by mere contact, for a blow does not produce it. A 

 certain degree of adhesion between the tendrils and the irritat- 

 ing substance is necessary. The moist surface of the gelatine 

 prevents such adhesion taking place. The effective irritation 

 is a sort of tickling. 



The degree of this tickling may, however, be extremely 

 slight. Thus Pfeffer ('85, p. 506) placed a rider of cotton 

 thread, weighing 0.00025 milligramme, on a tendril, and 

 found that no effect was produced so long as it was quiet ; 

 whereas, when the rider was put in motion by a current of air, 

 a considerable bending occurred. Under similar conditions, a 

 thread weighing 0.00012 milligramme produced no reaction. 

 MacDotjgal ('96, p. 376) finds that a spider's web, 43 cm. 



