9 o 



NATURE OF CLIMBING STEMS 



acterized by being rather small and having greatly elongated 

 internodes. Stems grown in the dark show a similar develop- 

 ment. Perhaps this variation that we call the climbing type 

 has been brought about by competition with larger plants. The 

 feeble light has stimulated these stems so that they attain a very 

 extended growth and finally are able to reach the light and 

 display their leaves. Many variations in the structure and sensi- 

 tiveness of the stem are associated with this elongation, all of 

 which are designed in one way or another to enable the stem 

 to reach the light. One type of these variations is seen in 

 twining stems, as the morning glory, bean, 

 hop, etc. Young twining plants behave 

 quite like the ordinary plant. The stems 

 are erect and actively nutating, the apex 

 traveling through a rather large circle in 

 one to three hours. When a certain height 

 has been reached the stems are stimulated 

 by gravity so that their upper portions grow 

 more or less horizontally (Fig. 66). This 

 position is a decided advantage since the 

 stem is now revolved through a larger circle 

 and has a greater chance of coming into 

 contact with an object about which it can 

 twine. As soon as the stem comes into con- 

 tact with any support its nutation will cause 

 it to wind or twine about it. Doubtless the 

 contact also acts as a stimulus, causing the 

 stem to bend more energetically. Certainly 

 in many plants the size and roughness of 

 the support are important factors in induc- 

 ing the twining. At first the coils are 

 merely horizontal but owing to the elonga- 

 tion of the stem these coils are gradually 

 pushed upwards and become steep and very firmly bound around 

 the support. Twisting of the stems and reflexed bristles often 

 assist in anchoring the plant to its support. It is interesting to 

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



FIG. 66. Twining 

 habit of wild bean. 

 Note the horizontal 

 position of the up- 

 per portion of the 

 stem which results 

 in the apex mutating 

 through a wider circle. 



