202 HOW PLANTS CLIMB. 



it move round a quarter of a circle, say to the East : the dotted 

 Hne will be on the side of the shoot facing the North \ move the 

 shoot another 90 degrees, />., to the North : the dots are on the 

 under or concave surface ; when the shoot points West, the dots 

 again appear on the side ; bring it round South again, and the 

 dots appear once more on the u}jper or convex surface. No 

 twisting has taken place, but the shoot has completed its circle of 

 sweeping, and this by successive bowings over of itself in the 

 direction of its revolving movement. 



Now, let us substitute for the dotted line on the convex 

 surface, a very much more rapid growth of the cells on this 

 siu'face than on the other three, preceded by turgescence of the 

 cells. This unequal increase of growth would cause the shoot to 

 bend down in the opposite direction, making the Southern side 

 concave- — in other words, it effects the bending of the shoot to 

 the South. Now, let this turgescence and unequal increase of 

 growth creep round the shoot (just as we made the dotted line to 

 twist round it) in successive stages, until it lias gone the entire 

 round of the shoot. As it travels, it causes each part of the 

 circumference to bend or bow to the opposite side, the result 

 being that the shoot gradually sweeps in an entire circle round the 

 support, the circumference of the circle being dependent on the 

 length of the shoot and its inclination to the support. 



In this, we have the true explanation of the revolving move- 

 ment, or, as it is now termed, circuvijuttation. I referred to this 

 same process of turgescence, succeeded by unequal growth on one 

 side, when describing the phenomenon of Heliotropism. A similar 

 process explains the folding of young leaves over the end of their 

 stem, and their subsequent unfolding, the under surface suffering 

 increase of growth in the former case, the upper surface growing 

 more rapidly in the latter. 



The circumnutation of twining-plants is simply the ordinary 

 circumnutation of the stems and roots of seedlings, and of leaves 

 in general, modified by bei/ig increased in aiiiplitiide. 



The power is innate, and is not excited by external agencies, 

 beyond those necessary for growth and vigour. The process itself 

 once clearly understood, the revolving movement of climbing- 

 plants is no longer the mystery it was before. 



