116 



HISTOLOGY AND PHYSIOLOGY. 



consequence. This is illustrated in twining plants and 

 tendrils. The former generally rotate to the left ; though 

 the Hop, Honeysuckle, and others, rotate to the right. 

 When they touch an upright object they 

 continue their rotation, and thus twine 

 around a support. Tendrils (Fig. 228) 

 grow straight until they have attained 

 about three-fourths of their size. T(;iey 

 are then sensitive to contact, and are 

 continually revolving; when they come 

 in contact with any object, a curva- 

 ture takes place, and a number of revo- 

 lutions are performed around the support 

 (Fig. 228, s) ; whether the coils are few 

 or many, they become attached with con- 

 siderable force ; that portion between the 

 point of contact and base also coils in 

 a cork-screw manner (in two directions, 

 Fig. 228, I and r, since both ends are 

 attached), and this brings the plant 

 nearer the support. Tendrils which do 

 not find an object to encircle, become abortive and fall 

 off (in the Grape- Vine and Virginia Creeper), or roll up 

 slowly from the apex to the base, and form a spiral (in 

 Cardiospermum), or a helix (in Cucnirhita), then dry up 

 and become woody. 



Fig. 228. Tendril of Wild Balsam-apple ; i, z, 3, the three branches ; ten, ten- 

 dril ; s, support ; r, coil to the right ; /, coil to the left ; I/, petiole of leaf; //, stem 

 of a fruit; f/, braach,_, 



