TWINING PLANTS. 



405 



of the cells composing it, wliile changes in the position of cotjle- 

 dons devoid of them are due to unequal growth. 



1052. Circumnutation of the young parts of mature plants. By 



methods similar to those described in 1045, it can be shown that 

 the growing extremities of stems, branches, leaves, and their 

 numerous modifications possess the power of movement ; in 

 sume instances exhibiting essentiall}^ the same phenomena as 

 those presented by the parts of the seedling, while in other cases 

 they show differences at an early stage. The most striking of 

 these differences is that observed in twining stems. In this case 

 there is a greatlj' increased amplitude of the orbit through which 

 the tip of the stem passes. Although only a special case under 

 a general class, twining stems ma}' well receive a somewhat 

 detailed description. 



1053. Twiners are distinguished from proper climbers by the 

 absence of au}' special organs, other than the stem itself, for 



grasping sup- 

 ports ; climbers 

 being provided 

 with some sort 

 of tendrils, or 

 other help, by which the plant is held to its sur- 

 roundings. Taking the simplest cases of twiners, 

 such as that of the common Morning Glory, it is 

 to be observed that (1) the revolving movement 

 begins at the earliest moment ; (2) onlj' a few 

 young internodes are concerned in the revolving ; 

 (3) the revolving stem cannot twine around a 

 smooth support (for example, a glass rod), but 

 requires in the sui)port some degree of rough- 

 ness ; (4) there is a limit of size to the support, 

 different for different twiners, beyond which it 

 cannot be grasped by the revolving stem ; (5) 

 the dii-ection of the revolution is not tlie same for all twiners ; 

 (6) the rate differs with the plant and with the surroundings. 



1054. In the early state of a twining plant the movements are 

 in narrow ellipses ; but with even a slight increase in size of the 

 seedling, the transverse axis of the ellipse becomes greater, and 

 soon the orbit is practicallj' a circle. 



1055. The number of internodes concerned in the twining 

 movement is usuallj- not more than three or four, and sometimes 



Fig. 179. Bevolving shoot of Morning Glory. 



