100 



NATURE OF CLIMBING STEMS 



and therefore spoken of as adaptive variations will be considered 

 in the following paragraphs. 



45. The Climbing Type of Stems. — Climbing stems are char- 

 acterized by being rather small and having greatly elongated 

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

 ment. Perhaps this variations 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. 67). 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 

 Fig. 67. Twining contact also acts as a stimulus, causing the 

 habit of wild bean. ^^^^ ^^ ^^^^ ^^^^ energetically. Certainly 

 Note the horizontal . 1 1 • 1 1 



position of the upper ^^ many plants the size and roughness of 



portion of the stem the support are important factors in induc- 

 which results in the jj^g ^^e twining. At first the coils are 

 apex nutating through 11. 11 • 11 



a wider circle. 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 



