86 



PRACTICAL COURSE IN BOTANY 



Fig. 98. — Leaf of common pea, 

 showing upper leaflets reduced to 

 tendrils. 



by coiling, the body of the plant 

 is drawn upward proportionally. 

 It will be observed that the helix 

 is interrupted at one or more 

 points, above and below which 

 the coils turn in opposite direc- 

 tions. This is because the ten- 

 dril is attached at both ends and 

 cannot adjust itself to the oppo- 

 site strains of torsion. Twist 

 with your fingers a piece of tape 

 so attached, and you will see 

 that on one side of your hand it 

 turns from right to left and on 

 the other from left to right. 



98. The cause of twining. — 

 Botanists are not fully agreed 

 on this point. The explanation 

 most generally accepted at present is that the twining of 

 stems is due to the combined action of lateral and negative 



geotropism(51). The first 



^^^^^-^"^^^^ causes one side to grow 



more rapidly than the other, 

 thus forming a succession of coils, while the 

 second, by stimulating the upward growth 

 of the axis, stretches it into a spiral, and in 

 this way draws it more tightly round the 

 support. For this reason twining stems do 

 best on an upright support. 



In tendrils, the twining is thought to be 

 due not to gravity, but to contact with a 

 soHd body, which, by inducing unequal de- 

 velopment on opposite sides of the tendril, 

 of a passion flower causes it to tum about an available object, 

 transformed into ^he coiliug of the free part of the twining 



tendrils. {After . " -^ " 



Gray.) Organ IS in response to the stimulus trans- 



FiG. 99. 



