170 LIFE : OUTLINES OF GENERAL BIOLOGY 



petiole only partially expanded, they are highly sensitive; a single 

 light touch with a twig on the inferior or concave surface near the 

 tip caused them to bend quickly, as did occasionally a loop of thread 

 weighing i of a grain." He gave the highest place to the tendrils 

 of the passion-flower: "A bit of platinum wire .-/o of a grain in 

 weight, gently placed on the concave jx)int, caused a tendril to 

 become hooked, as did a loop of soft, thin cotton thread Vi of 

 a grain." The tendril's resix)nse to touch followed in 25-30 seconds 

 in the passion-flower. 



Like the tips of growing shoots in general, the tendrils move 

 slowly round (nutating) in circles or ellipses. In the common pea the 

 tendril takes about an hour and a half to complete its elUptical 

 orbit. In the much thicker and more sluggish tendril of the vine the 

 ellipse is so narrow that it looks as if the movement was from side 

 to side. 



In the course of its nutation the tendril may touch some adjacent 

 support, round which it proceeds to curve. This curvature movement 

 is due to differential growth. The touched side, which is in most 

 tendril-bearers the lower, immediately begins to grow more slowly, 

 the opposite side grows more rapidly; the result must be curvature. 

 Hut as the free tip of the tendril goes on growing and nutating, a 

 new px:)int comes into contact with the support, and another curva- 

 ture follows. This goes on till the whole of the free part has been 

 attached. 



Then the behaviour of the tendril changes. The coiled part becomes 

 more woody and grips the support firmh'. Then, perhaps after a day 

 or two, the straight portion, between the attachment and the base 

 of the tendril, twists into a tight spiral, reversed once or twice, and 

 aftenvards becomes woody. The advantage of the corkscrew is 

 twofold; it draws the shoot nearer the support, and it forms a cable 

 that does not readily snap in the wind as an uncoiled part would be 

 apt to do. This is admirably illustrated by Darwin's observations on 

 the bryony. "I have more than once gone on purpose during a gale 

 to watch a Bryony growing in an exi^sed hedge, with its tendrils 

 attached to the surrounding bushes; and as the thick and thin 

 branches were tossed to and fro by the wind, the tendrils, had they 

 not Ix^en excessively elastic, would instantly have been torn off and 

 the plant thrown prostrate. But as it was, the Bryony safely rode 

 out the gale, like a .ship with two anchors down, and with a long 

 range of cable ahead to serve as a spring as she surges to the storm." 



An interesting point is that the spiral twisting of the lower part 

 of the tendril does not occur when there has been a merely temporary 

 curvature, such as is seen when the tip of the tendril has been 

 affected by casual pressure. This is what might be called a negative 

 adaptation. 



As a climax of jxxsitive adaptation we may notice the adhesive 



