CIRCTJMNUTATION OF TENDRILS. 417 



1078. Sensitiveness of stems and branches. Under ordinary 

 conditions even twining stems are not sensitive to slight mechani- 

 cal irritation. The reactions to moisture, light, gravitation, etc., 

 have been already noticed, and it is now intended to call atten- 

 tion to the extraordinary sensitiveness of certain tendrils, some 

 of which are modified branches, while others are modified leaves 

 or parts of leaves. 



1079. Tendrils circumnutate, and by their revolving movement 

 reach out for a proper support. Moreover, they are produced 



on the young 

 and circum- 

 nutating ex- 

 tremities of 



shoots, so that two modes of revolution 

 are frequently to be observed simulta- 

 neously. But in this revolving move- 

 ment the tendrils are prevented from 

 becoming entangled with the rest of the 

 shoot. The manner in which this is 

 done is thus described : ' ' When a ten- 

 dril, sweeping horizontally, comes round 

 so that its base nears the parent stem 

 rising above it, it stops short, rises stif- 

 fly upright, moves on in this position 

 until it passes by the stem, then rapidly 

 comes down again to the horizontal po- 

 sition, and moves on so until it again 

 approaches and again avoids the im- 

 pending obstacle." * 



1080. When a light thread is placed 

 upon a long revolving tendril of Passiflora, Echinocystis, or 



excited by gravitation the same part bends towards the centre of gravity. In 

 almost every case we can clearly perceive the final purpose or advantage of the 

 several movements. Two, or perhaps more, of the exciting causes often act 

 simultaneously on the tip, and one conquers the other, no doubt in accordance 

 with its importance for the life of the plant. The course pursued by the radi- 

 cle in penetrating the ground must be determined by the tip ; hence it has 

 acquired such diverse kinds of sensitiveness. It is hardly an exaggeration to 

 say that the tip of the radicle thus endowed, and having the power of directing 

 the movements of the adjoining parts, acts like the brain of one of the lower 

 animals ; the brain being seated within the anterior end of the body, receiving 

 impressions from the sense organs, and directing the several movements" 



(p. 572). 



i Gray : How Plants Behave, 1872, p. 18. 



FIG. 187. Shoot of Passiflora, showing tendrils. 



27 



