io6 



NATURE OF CLIMBING STEMS 



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 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 varia- 

 tions is seen in twining stems, as the morn- 

 ing 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 cer- 

 tain height has been reached the stems are 

 stimulated by gravity so that their upper 

 portions grow r more or less horizontally 

 (Fig. 67). This position is a decided ad- 

 vantage since the stem is now revolved 

 through a larger circle and has a greater 

 chance of coming into contact with an ob- 

 ject about w r hich it can twine. As soon as 

 the stem comes into contact with any sup- 



t ^ nutat i on w jn cause it to wind or 

 habit of wild bean. Note 



the horizontal position of twine about it. The contact also acts as 

 the upper portion of the a stimulus, causing the stem to bend more 



stem which results in the ener getically. Certainly in many plants the 

 apex nutatine through a e ,, n 



wider circle. slze and rou g hness of thc support as wel 



as other features, are important factors in 



inducing the twining. At first the coils are merely horizontal 

 but owing to the elongation 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 



FIG. 67. Twining 



