122 CLIMBING PLANTS. 



special movements, and we shall not here consider 

 them. 



The first division is well represented by the Hop 

 or Morning-Glory. If we observe a young shoot 

 which has grown beyond its support (Fig. 34, 

 Morning-Glory), we can see that its tip describes a 

 circle, or rather an ellipse. This is not caused by 

 the twisting of the shoot, for it would soon break 

 if it continued to twist round and round. We can 

 see, by holding a string in one hand and twisting 

 it with the other, how great a strain is thus 

 brought to bear. Such a strain would at once 

 snap a delicate stem in two. 



The movement is caused by the bowing of the 

 shoot successively to every point of the compass. 

 If we hold a stick upright, we can bend it towards 

 the north, then, without twisting the stick, to the 

 northeast, then to the east, and so on, till we bend 

 it to the north again. If we make a mark on the 

 upper side of the stick when it is bent towards the 

 north, the mark will be on the under side when 

 the stick bows towards the south. Dr. Gray thus 

 explains this bowing movement : 1 "To learn how 

 the sweeps are made, one has only to mark a line 



1 "How Plants Behave." By Asa Gray. Ivison, Blakeman, Taylor, 

 & Co. 1872. p. 13. 



