14 NATURAL HISTORY READER. 



swung, so that, if it moves round in a horizontal plane at 

 a uniform height above ground, it will curl round the stick 

 at that level, and thus will not climb up the stick it strikes 

 against. But the climbing plant, although it may swing 

 round, when searching for a stick, at a fairly uniform level, 

 yet, when it curls round a stick, does not retain a uniform 

 distance from the ground, but by winding round like a 

 corkscrew it gets higher and higher at each turn. 



7. As plants have no muscles, all their movements are 

 produced by unequal growth ; that is, by one half of an 

 organ growing in length quicker than the opposite half. 

 Now, the difference between the growth of a twining plant 

 which hends over to one side and an ordinary plant which 

 grows straight up in the air lies in this, that m the upright 

 shoot the growth is nearly ecptal on all sides at once, where- 

 as the twining plant is always growing much quicker on 

 one side than the other. 



8. It may be shown by means of a simple model how 

 unequal growth can be converted into revolving movement. 

 The stem of a young hop is represented by a flexible rod, of 

 which the lower end is fixed, the upper one being free to 

 move. At first the rod is supposed to be growing vertically 

 upward, but when it begins to twine one side hegins to 

 grow quicker than any of the others : suppose the right 

 side to do so, the result will be that the rod will bend over 

 toward the left side. Now, let the region of quickest 

 growth change, and let the left side begin to grow quicker 

 than all the others, then the rod will be forced to bend back 

 over to the other side. Thus, by an alternation of growth, 

 the rod will bend backward and forward from right to left. 



9. But now imagine that the growth of the rod on the 

 sides nearest to and farthest from us enters into the com- 

 bination, and that, after the right side has been growing 

 quickest for a time, the far side takes it . up, then the rod 

 will not bend straight back toward the right, as it did be- 



