TWINING PLANTS. 



405 



of the cells composing it, while changes in the position of cotyle- 

 dons devoid of them are due to unequal growth. 



1052. Circnmnutation of the yonng 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 

 some instances exhibiting essentially 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 greatly increased amplitude of the orbit through which 

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

 a general class, twining stems may well receive a somewhat 

 detailed description. 



1053. Twiners are distinguished from proper climbers by the 

 absence of any 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) only a few 

 j-oung 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 support some degree of rough- 

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

 different for different twiners. be3 T ond which it 

 cannot be grasped by the revolving stem ; (5) 

 the direction of the revolution is not the 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 practical!}- a circle. 



1055. The number of internodes concerned in the twining 

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



FIG. 179. Involving shoot of Morning Glory. 



