. Nature and development of plants. Botany. io6 NATURE OF CLIMBING STEMS 45. The Climbing Type of Steins.—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 associat
. Nature and development of plants. Botany. io6 NATURE OF CLIMBING STEMS 45. The Climbing Type of Steins.—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 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 which it can twine. As soon as the stem comes into contact with any sup- 1. ^'^'r ^I'ju ^"It"/ port its nutation will 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 energetically. Certainly in many plants the apex nutating through a . , , . , wider circle. ^^^^ ^^'^ roughness of the support as well 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 a
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