344 



BOTANY 



PART I 





The revolving movement is regarded by some authors as purely autonomic 

 (p. 336) ; on the other hand, it is held that gravity has a determining influence 

 upon it. This disputable question is still unsettled. 



The commencement of the revolving movement does not by itself 

 determine a twining movement. This only begins when the shoot 



meets a more or less vertical and 

 not too thick support. This is 

 enclosed in loose and at first very 

 horizontal spirals which gradually 

 become more erect and steeper. The 

 straightening results from negative 

 geotropism and leads under other- 

 wise favourable conditions when the 

 support is subsequently removed to 

 a complete obliteration of the spiral 

 coils, the straightened stem appearing 

 twisted. If the support is not re- 

 moved it leads to tightening of the 

 spiral and increased pressure on the 

 support. The twining movement 

 thus comes about by the revolving 

 movement together with negative 

 geotropism. The support plays a 

 part in that it prevents the other- 

 wise inevitable straightening. It 

 must stand more or less vertically, 

 because otherwise it would not be 

 continually grasped by the overhang- 

 ing tip of the shoot. 



FIG 



The twining is further assisted by the 

 shoots of the twining plant having to begin 

 with elongated internodes while the leaves 

 remain small. In this respect these shoots 

 resemble those of etiolated plants ; the 



. 281.-I, Sinistrorse shoot of Pharbitis. d } d unfoldi O f the leaves allows of 

 II, Dextrorse shoot of Myrsiphylhim J 



asparagoides. (After NOLL.) the regular circling of the tip which might 



otherwise be interfered with by the leaves 



encountering the support. The firm hold on the support is frequently increased 

 by the roughness of the surface of the stem owing to hairs, prickles, ridges, etc. 

 Torsions also, the causes of which cannot be entered into here, have a similar 

 effect. 



Although STARK has recently shown that twining plants are not insensitive to 

 contact with the support (cf. p. 354), it still holds good that the result of this 

 contact does not determine the twining movement. 



Alteration of the Geotropie Position of Rest. The position 

 assumed by an organ as a result of a definite geotropic stimulation 



