LECTURE XIX. 



IRRITABILITY OF GROWING ORGANS (continued}. 



Contact. 



IN previous lectures we became acquainted with the effect 

 of continued and considerable pressure, upon growth, and we 

 have now to study more particularly the effect of slight 

 pressure continued during a relatively short space of time. 



The most familiar instances of the effect of contact on grow- 

 ing organs are afforded by tendrils, the well-known twining 

 properties of these organs being simply the expression of 

 their sensitiveness to contact. A slight touch, in the case 

 of very sensitive tendrils, such as those of Passiflora gracilis 

 and of Sicyos angulatus^ is sufficient to induce a very per- 

 ceptible curvature, Darwin found the tendrils of Passiflora 

 gracilis to be the most irritable. He says with reference to it : 

 " a single delicate touch on the concave surface of the tip 

 soon caused a tendril to curve, and in two minutes it formed 

 an open helix. A loop of soft thread weighing ^ of a grain 

 placed most gently on the tip thrice caused distinct curvature. 

 A bent bit of thin platinum wire, weighing only -fa of a grain, 

 twice produced the same effect ; but this latter weight when 

 left suspended, did not suffice to cause a permanent curvature." 

 In this case also, the movement after a touch is very rapid : 

 it is generally perceptible, according to Darwin, in half a 

 minute after a touch. It will be readily understood that, in 



