282 COMPENDIUM OF GENERAL BOTANY. 



(support) causes the formations of the curvatures or windiugs. 

 The support aud the tendril or climbing plant are drawn toward 

 each other ; that is, the tendril has a tendency to form coils 

 whose radii of curvature are less than that of the support, pro- 

 vided the support is not too slender nor the tendril too thick. 

 Besides these windings due to contact there are produced turn- 

 ing-points, or places where the direction of the coil changes. 

 There may be one or several of these changes, and they always 

 occur in the free portion of the tendril, tliat is, in the portion 

 between the support and the stem of the plant. The origin of 

 such coils is due to a mechanical cause, and may be very readily 

 illustrated as follows : A narrow stretched strip of india-rub- 

 ber is firmly cemented along another strip of rubber not 

 stretched. Upon releasing the tension of the former rubber, it 

 contracts and forms the inside of a spiral, the outer side of 

 which is formed by the strip that was not stretched. Tendrils 

 without a support usually coil slowly in the form of a spiral, 

 but imthout the formation of turning-points. 



According to de Vries, the first effect of the contact-stimulus 

 is to increase the turgor of the side not stimulated. (For par- 

 ticulars see the text-books of Sachs aud Pfeffer.) 



Careful studies of irritability have been made by the Darwins 

 (father aud sou), later also by Wiesner, Detlefsen, Haberlandt, 

 and others. According to Haberlandt, the irritable stimulus in 

 Mimosa is propagated along a system of special stimulus-con- 

 ducting cells which have highly permeable plasmic membranes 

 (pore-membranes) along the transverse septte. These sjoecial 

 cells lie either within or along the outside of the leptome-bundle. 

 The permeable membranes allow the ready passage of water- 

 currents, which are supposed to be the cause of the irritable 

 movements. 



The propagation of stimuli in tendrils is but little under- 

 stood ; also the propagation of stimuli causing geotropic aud 

 hydrotropic curvatures. Opinions differ even in regard to the 

 observed facts of the geotropic curvatures of roots. I will 

 briefly state the results of Krabbe's investigations, which have 

 verified the observations made by Cisielskis and Darwin. 



The sensible or irritable portion of the root-tip is never more 

 than 2 ram. long. Tl;e portion of the root which is really 

 capable of curving is not wholly located in the 2 mm. of the 



