VIIl] TWINING PLANTS 107 



tend to straighten, as may be illustrated by coiling a piece 

 of elastic round a pencil, and then pulling the upper end 

 of the elastic and steepening the coils, and partly by 

 internal actions which are not understood but whose 

 effects are comparable to those in the twisted stems of 

 erect plants. These latter are well seen in the Hop and 

 Convolvulus, where they extend right up to the end of the 

 young internodes, and are formed before the tightening up 

 of the coils begins. 



It is very questionable whether any real insight into 

 the causes of the twining are to be got from what we 

 know so far as to the geotropism of these shoots. The 

 facts are these. 



If a pot-plant of a climber such as the Hop is allowed 

 to slowly rotate on the klinostat, the already twined 

 youngest parts unwind themselves from' the vertical 

 support, and straighten themselves vertically. This and 

 other experiments have led to the assumption that the 

 young growing internodes are geotropic not in the usual 

 sense, where they would grow more rapidly either on the 

 lower side (apogeotropic) or the under side (geotropic), 

 but so that one side grows more quickly than the other, 

 and so drives the nutating portion across the line of the 

 vertical (lateral geotropism) and moves it in the plane of 

 the horizon. This brings it about that the plant can only 

 twine round a vertical or nearly vertical support, as is 

 found to be the case with all twiners. As the young 

 twining internodes get older, but before growth has ceased 

 in them, they become more and more apogeotropic, and so 

 erect themselves more and more, thus tightening the coils 

 as they become steeper. 



In view of the demonstrated irritability of the twining 

 stems of Cuscuta, a parasitic convolvulaceous plant which 

 sends haustoria into the host-plant on which it twines, it 



