GROWTH AND MOVEMENT 471 



While the tendril may be sensitive throughout, the responses evoked by excita- 

 tion differ sometimes according to the region stimulated. Thus, a stimulus applied 

 to the " under " side, which at the time of greatest sensitiveness has usually grown 

 near the apex a little less than the other, so that at the tip it is slightly concave, 

 results in a curvature. So also does stimulation of the flanks, and in some tendrils 

 that of the upper side too. But there are some others which give no sign if rubbed 

 on the upper side, except that stimulation there will inhibit a simultaneous stimula- 

 tion on the under side, which ordinarily would result in a curvature. 



Primary response. — The first result of slight rubbing contact with a 

 suitable support (that is, one that is small enough for the tendril to en- 

 circle, no matter in what position it stands) is a prompt curvature. In 

 sensitive tendrils under favorable conditions this follows in the course 

 of a few seconds (5-30), but in others in a few minutes. The facts ob- 

 served are that the cells on the convex side become suddenly consider- 

 ably elongated, while those on the concave side become somewhat short- 

 ened. This and the promptness of the end reaction suggest a turgor 

 change, and many observers have concluded that such is the mechanism 

 of the primary curvature, and that it becomes fixed later by growth. 

 Others attribute these results to a very rapid and extraordinarily sudden 

 growth of the cells of the convex side, and to the consequent compression 

 of those on the concave side. It is not improbable that the truth in this, 

 as in many similar recondite and much controverted matters, will prove 

 to lie between the contentions. So it may very well be that a turgor varia- 

 tion begins the movement, whereupon growth follows it up more promptly 

 than usual, and extends and completes the encircling of the support. 



Secondary response. — After the tendril has become firmly attached, 

 the excitation extends toward the base of the tendril, producing an in- 

 equality of growth on the opposite sides (in this case the " upper " side 

 becomes the convex one) that throws this part of the tendril into coils 

 (see Part III, fig. 958). 



This coiling may be rudely but essentially imitated by placing in a pan of water 

 a narrow strip, slit from the scape of a fruiting dandelion which has not attained its 

 full height, and by pinching each end in a short folded piece of sheet lead to prevent 

 twisting. After a few hours the strip will be found coiled into a spiral, with one or 

 more reversals of direction just as in the tendril, though more irregularly. Here 

 the tissue next the pith cavity grows and becomes more turgid than the epidermal 

 and cortical tissues. The reversal of coil is a mechanical necessity if the ends are 

 not free to rotate. 



These coils are not merely the result of continued growth of the tendril; 

 for if one not full grown becomes attached, it does not reach its possible 



