INSECTIYOEOUS 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 Drosera^ 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 oflSce 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 devolve 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 



