If you examine their wood, you will see that it is not quite like that of usual plants. Cut through the stem of a clematis which is about an inch thick, and even before you look at it with a magnifying-glass you will see how very loosely built the wood is, with wide rays of soft tissue and very large water vessels. It is not built for strength and support, but merely to carry supplies of water up to the leaves, for although these plants use trees as supports, they do not get anything more from them, and must supply themselves with all else they need. You may often see that the central part of the wood is not in the true centre of the stem, but is pushed to one side, and the rings of the year’s growth are very irregular, being much more to one side than to the other. This is because they lean against the supporting branches, and so must grow chiefly on the side away from them. Sometimes as the ivy grows right round the support, it will grow more, first on one and then on the opposite side of its stem, and so the centre does not remain in one place, but shifts round. The other parts of these woody climbing plants are but little out of the common. They have merely learnt to economise their own stem-material, and at the same time to reach a good position in the light, so that it is in their stems that we find their chief differences from usual plants. The honeysuckle and clematis have no special climbing organs, but the Ivy has clusters of adventitious roots which come out from the back of its stem, and hold it on to the support (~see~ p. 56 and fig. 101). [Illustration: Fig. 101. Adventitious roots growing out from the stem of Ivy between the leaf stalks.] In climbing plants in which the above-ground parts live only for one year and then die down, we do not get a woody stem. Such soft green plants as the hop and convolvulus, for example, are entirely dependent on others for their support. They have specially sensitive tips to their stems, which feel the support and definitely twine round it in a close spiral, which clings ever closer to the support as they grow (~see~ fig. 102). [Illustration: Fig. 102. Soft twining stem of Convolvulus.] Climbers of this kind have only modified their stems; the rest of their parts are not in any way specially altered by this habit. Some plants which sprawl about on others hold themselves up by the power of clinging and twining in their leaf-stalks, for example, in the nasturtium we find that the plant is held up entirely by the leaf stalks, which catch on to anything in their way (~see~ fig. 103). [Illustration: Fig. 103. Nasturtium stem held up by the support given by the leaf-stalks, which cling around any suitable prop.]