260 



BOTANY 



ports. This is, it is true, a limitation to their power of climbing, but one which 

 is not without advantage, for the plants are thus constrained to ascend to freer 

 light and air. 



When an upright support occurs anywhere in the immediate neigh- 

 bourhood of the apex of a climbing shoot it is sure to be discovered. The 

 apical extremity, of which the movement is but little disturbed by the 

 leaves, which remain for a long time undeveloped, 

 is forced by its lateral geotropism against the 

 support, and by its next revolutions twines around 

 it. If the support be thin, the coils, at first 

 almost horizontal, are only loosely wound about 

 it. Later they become more spiral, and so wind 

 more tightly. This is accomplished by the ulti- 

 mate predominance of negative geotropism in 

 the coiled portions of the stem, which tends con- 

 tinually to draw out the coils and make the 

 stems upright. This action of negative geotro- 

 pism is well shown in the case of shoots which 

 have formed free coils without a support (Fig. 

 204). By the resistance offered by the supports' 

 to the complete elongation of the spiral stems, 

 the shoots are held firmly in position. In many 

 twining plants the roughness of their surfaces 

 (due to hairs, bristles, hooks, furrows) also 

 assists in preventing the shoots sliding down 

 their supports. The autonomic torsion arising 

 in the older portions of the stems is also of 

 assistance in holding climbing plants, especially those with furrowed 

 stems, tightly wound about their supports. The twining of stem- 

 climbers, as well as the attachment to their supports, is due to geo- 

 tropic processes of growth, and not, as in tendril-climbers, to contact 

 stimuli. 



In addition to the autonomic torsions, a torsion from purely mechanical causes 

 is necessarily manifested in the elongation of the coils of a twining stem which are 

 at first nearly horizontal, so far at least as it is not equalised by the free movement 

 of the apex. (To make this form of torsion apparent, it is only necessary to hold 

 firmly the inner end of a horizontal coil of rubber tubing, and draw out the other 

 end until the tube is straight. If a mark has previously been drawn along one side, 

 say the convex side of the tube, its position, after the tube has been extended, 

 will show clearly the actual torsion that has taken place.) 



From their manner of winding, stem - climbers can twine only 

 around slender or, at the most, moderately thick supports. Here 

 again is a limitation to their powers of climbing ; but in this instance 

 also the limitation has its advantages, for by climbing the trunks of 

 large shade trees, these plants, which require the unobstructed light, 

 would be placed in an unfavourable position. 



Fig. 204. — Free coils formed 

 by a shoot of Iptymoea imr- 

 furea. (From Detmer's 

 Physiol. Pract.) 



