INSECTIVOROUS AND CLIMBING PLANTS. 335 



occasion. Most leaves make no regular sweeps ; but 

 when the stalks of a leaf-climbing species come into 

 prolonged contact with any fitting extraneous body, 

 they slowly incurve and make a turn around it, and 

 then commonly thicken and harden until they attain 

 a strength which may equal that of the stem itself. 

 Here we have the faculty of movement to a definite 

 end, upon external irritation, of the same nature with 

 that displayed by Dioncea and J)rosera, although slow- 

 er for the most part than even in the latter. But the 

 movement of the hour-hand of the clock is not differ- 

 ent in nature or cause from that of the second-hand. 



Finally distribution of office being, on the whole, 

 most advantageous and economical, and this, in the 

 vegetable kingdom, being led up to by degrees we 

 reach, through numerous gradations, the highest style 

 of climbing plants in the tendril-climber. A tendril, 

 morphologically, is either a leaf or branch of stem, or 

 a portion of one, specially organized for climbing. 

 Some tendrils simply turn away from light, as do those 

 of grape-vines, thus taking the direction in which some 

 supporting object is likely to be encountered ; most 

 are indifferent to light ; and many revolve in the man- 

 ner of the summit of twining stems. As the stems 

 which bear these highly-endowed tendrils in many 

 cases themselves also revolve more or less, though they 

 seldom twine, their reach is the more extensive ; and 

 to this endowment of automatic movement most ten- 

 drils add the other faculty, that of incurving and coil- 

 ing upon prolonged touch, or even brief contact, in the 

 highest degree. Some long tendrils, when in their 

 best condition, revolve so rapidly that the sweeping 



