NATURE OF PLANTS 107 



note that in the majority of cases the stem twines about the sup- 

 port in left hand spirals or clockwise; less frequently right hand 

 spirals are formed, as in the hop, some honeysuckles and knot 

 weeds. Why the growth of the stem results in left or right hand 

 twining is not known but the fact remains that the plants can not 

 be induced to change their method of twining. 



46. The Tendril Type of Stem. — One of the most interesting 

 variations of climbing plants is seen in those stems that climb 

 by means of tendrils. In this type almost any part of the plant 

 may become modified so as to act as a tendril and bind the plant 

 to a support; e. g., the petioles of nasturtium and clematis, 

 stipule-like outgrowths in smilax, midrib of leaf in many members 

 of the bean family, or modified branches as in the grape vine, etc. 

 The ordinary tendrils as we see them in the gourd, passion flower, 

 etc., are highly specialized stems that are very sensitive to touch. 

 These organs are rapidly nutating, frequently completing two 

 or more circles in an hour, and as they approach maturity they 

 often become slightly curved and hooked (Fig. 68, A). Their 

 sensitiveness to contact, which is often localized on the concave 

 upper third of the tendril, ^Iso becomes so keen in some forms 

 that a thread weighing one-fiftieth of a gram will cause a curva- 

 ture of the tendril when placed upon it. In fact a much smaller 

 weight if caused to vibrate will act as a stimulus, so that we 

 have here an illustration of sensitiveness to weight that is keener 

 than our own. When the sensitive part of a tendril is stroked 

 by a pencil or comes in contact with any hard object, this acts 

 as a stimulus and causes a very rapid growth upon the convex 

 side of the tendril. As a result of this growth the tip of the 

 tendril curves, forming a complete loop in from a few seconds 

 to a few minutes. The contact of the tip of a tendril with any 

 object acts as a continued stimulus and the tendril soon becomes 

 tightly wound around it. As soon as the tip is firmly bound to 

 the support a new impulse is frequently transmitted to the free 

 portions of the tendril which in a day or so begin to assume the 

 form of coils that are often reversed in the middle (Fig. 68, B). 

 By this unique device tendril bearing stems are firmly fastened to 

 their supports but at the same time the coiling of the tendrils. 



