152 



VEGETABLE LIFE AND WORK. [SECTION 16. 



in Locust-trees, in the Sensitive Plant (Fig. 490), and in Woodsorrel 

 Young seedlings droop or close their leaves at night in plants which are 

 not thus affected in the adult foliage. All this is thought to be a protec- 

 tion against the cold by nocturnal radiation. 



470. Various plants climb by a coiling movement of their leaves or their 

 leaf-stalks. Familiar examples are seen in Clematis, Maurandia, Tropseo- 

 lum, and in a Solanum which is much cultivated in greenhouses (Fig. 172). 

 In the latter, and in other woody plants which climb in this way, the 

 petioles thicken and harden after they have grasped their support, thus 

 securing a very firm hold. 



471- Tendril movements. Tendrils are either leaves or stems (98, 

 16S), specially developed for climbing purposes. Cobaea is a good exam- 

 ple of partial transformation ; some of the leaflets are normal, some of the 

 same leaf are little tendrils, and some intermediate in character. The 

 Passion-flowers give good examples of simple stem-tendrils (Fig. 92) ; 

 Grape-Vines, of branched ones. Most tendrils make revolving sweeps, like 

 those of twining stems. Those of some Passion-flowers, in sultry weather, 

 are apt to move fast enough for the movement actually to be seen for a part 

 of the circuit, as plainly as that of the second-hand of a watch. Two 

 herbaceous species, Passiflora gracilis and P. sicyoides (the first an annual, 

 the second a strong-rooted perennial of the easiest 

 cultivation), are admirable for illustration both of 

 revolving movements and of sensitive coiling. 



472. Movements under Irritation. The most 

 familiar case is that of the Sensitive Plant (Fig. 490). 

 The leaves suddenly take their nocturnal position 

 when roughly touched or when shocked by a jar. 

 The leaflets close in pairs, the four outspread par- 

 tial petioles come closer together, and the common 

 petiole is depressed. 

 The seat of the move- 

 ments is at the base of 

 the leaf-stalk and jtalk- 

 lets. Schrankia, a near 

 relative of the Sensitive 

 Plant, acts in the same 

 way, but is slower. 

 These are not anoma- 

 lous actions, but only 

 extreme manifestations of a faculty more or less common in foliage. In 

 Locust and Honey-Locusts for example, repeated jars will slowly pro- 

 duce similar effects. 



490 



Fig. 490. Piece of stem of Sensitive Plant (Mimosa pudica), with two leaves, 

 the lower open, the upper in the closed state. 



