34 



FIELD AND WOODLAND PLANTS 



spirally except round an axis that is either erect or forms a very large 

 angle with the horizontal plane. 



Should the twining stem succeed in reaching a favourable prop, 

 it immediately commences to bend itself round and round, forming 

 a more or less compact spu-al ; and it is probable that the shght 

 pressure, caused by the contact, acts as 

 a stimulus which incites the pecuUar 

 mode of growth. 



The direction which the spiral takes 

 is not always the same. In the Hop, 

 Honeysuckle, and the CUmbing Buck- 

 wheat or Black Bindweed, the direction 

 is always the same as that of the hands 

 of a clock ; while in the Bindweeds the 

 spu-al is invariably contra-clockwise. 

 Further, it is not possible to compel any 

 species to tm-n in a direction opposite to 

 that which it natm'ally follows. Its stem 

 may be forcibly twined in the wrong 

 direction any number of times, but the 

 free end will always follow its natural 

 course as soon as it is left undisturbed. 



Should the stem of a young twining 

 plant fail to reach a suitable support, it 

 bends over, not being sufficiently rigid to 

 support itself, and at last the apex 

 Then, starting afresh from this second 

 position of rest, it begins to ascend, and its upper end again com- 

 mences to revolve as before. The chances are that it will, from 

 this second position, find something round which it can twine ; 

 but faihng this its summit may again and again bend to the 

 ground, thus renewing its attempts from various positions more 

 or less distant from one another, and in each effort so made the 

 revolving uj^per end of the stem gradually lengthens, and describes 

 a larger and larger circle in search for a favom-able prop. 



A twining stem sometimes has the advantage of additional 

 support afforded by the stiff natme of the base of the stem, which 

 is often rendered still more rigid by a twist or torsion resembling 

 that of the strands of a rope. Such advantage is often still fm-ther 

 increased by the presence of longitudinal ridges of the stem, fre- 

 quently bearing rows of hooked prickles or haus that hold on to 



STEM OP THE Bindweed, 

 T^vrN^NG to the left. 



reaches the ground. 



