HOW PLANTS CLIMB. 199 



IV. — Root-Climbers. — Those ascending by means of rootlets 

 attached to their supports. 



Let us take the classes in the above order, and ascertain the 

 manner in which each set of cHmbers pursues its way by studying 

 a few examples. 



I. — Twiners. This is the largest class, and for several 

 reasons appears to be the oldest and simplest type. 



Darwin takes Hiunuhis h/piiltts, the common Hop, as a fair 

 example. Its first two or three internodes are straight and quite 

 stationary. Then comes one that, while young, bends over to one 

 side, and travels slowly round its support in the direction of the 

 hands of a watch ; as the next internode is developed, the two 

 rotate, and usually a third. The ordinary velocity is soon attained, 

 and this was found to be about 2 h. 8 m. for each revolution. As 

 the lowest internode grows old, it gradually ceases to rotate, 

 although the revolutions continue in the terminal two or three of 

 the shoot, so long as the plant continues to grow. Thus, inter- 

 node by internode, the shoot twines itself round its support, each 

 'joint' of the stem gradually becoming stationary, while the last 

 two or three keep up the revolving motion, until the final inter- 

 node, or tip, ceases to move. In most plants, Darwin found that 

 three internodes were revolving at the same time, but in every 

 case at least two were at work, " so that by the time the lower one 

 ceased to revolve, the one above was in full action, with a termi- 

 nal internode just commencing to move." A Hop-shoot with 

 three internodes revolving was carefully watched. It was 14 

 inches long, and at such an angle to its support that its tip swept 

 a circle of 4 ft. 9 in. This it did in 2|- hours, giving an average 

 movement of 23 inches an hour. With another plant, one of the 

 AsdepiadacecE^ a shoot consisting of five internodes, measuring 

 altogether 31 inches, described a circle of 16 ft. 6 in. in 6 hours, 

 giving an average speed of 33 inches an hour. 



The rate of revolution varies widely in different plants. The 

 shortest periods for one revolution ranged from i h. 40 m. in the 

 white Convolvulus to 18 J hrs. in an exotic plant i^SpJicej'osteind). 

 The rate by night or by day differs but little. Vigorous health 

 and moderate warmth favour the movement. The twining Poly- 



