86 



PRACTICAL COURSE IN BOTANY 



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. 



FIG. 98. -Leaf of common pea, 9 8 ' The CaUSC f twining. - 



showing upper leaflets reduced to 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 



^^>s^ 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 

 solid body, which, by inducing unequal de- 

 velopment on opposite sides of the tendril, 



FIG. 99. Stems . , ., ,, , . 



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

 transformed into T ne co ii mg o f the free part of the twining 



tendrils. (After 



GRAY.) organ is in response to the stimulus trans- 



