IRRITABILITY. 491 



of tendril-power is prevented. Darwin also observed that 

 the tendrils of the two plants mentioned above are not irritated 

 by the impact of drops of water. This is an important bio- 

 logical arrangement, for it prevents the disadvantage which 

 would ensue were the tendrils capable of being stimulated 

 to curvature by rain. 



Tendrils are not, however, the only organs which are 

 possessed of sensitiveness to contact. Other instances of 

 this are afforded by the petioles of most leaf-climbers. It is 

 not necessary to go into detail concerning them, for these 

 organs closely resemble tendrils in their irritability and in 

 their mode of response to a stimulus. Usually, the petioles 

 are irritable only whilst young, but in different degrees in 

 different plants, and they are sensitive on all sides. 



In some cases shoots are sensitive to contact. It was 

 first observed by Dutrochet that the twining stem of Cuscuta, 

 the Dodder, is irritable like a tendril. Von Mohl indeed 

 suggested that all twining stems are irritable, but both Dar- 

 win and de Vries were unable to detect any trace of such 

 irritability. This view of von Mohl's has, however, been 

 recently revived by Kohl, who finds that the twining inter- 

 nodes of climbing plants are sensitive to long continued con- 

 tact, and that the side in contact with the support grows less 

 rapidly than the opposite side. In Calystegia the irritability 

 is so great that contact with a thin thread of silk, or with a 

 piece of thin platinum wire, or a somewhat prolonged friction, 

 suffices to induce a considerable difference in growth between 

 the side thus treated and the side opposite to it. It must be 

 mentioned here that Darwin found the young internodes of 

 Lophospermum scandens^ which is not a stem-climber, as also 

 the peduncles of Maurandia semperflorens, to be sensitive to 

 touch ; Kerner states that the peduncles of many flowers 

 (Poppies, Anemones, Ranunculuses, Tulips) are also thus 

 irritable ; and Sachs gives instances of the manifestation of 

 sensitiveness to contact by roots. 



From the observation of the behaviour of the radicles of 

 seedlings in their attempts to pass over obstacles which they 

 meet with in the soil, Darwin was led to suspect that the tip 



