[Illustration: Fig. 100. Leaf of the Sand-grass. A, rolled up; B, open. (~a~) and (~b~), sections across the same.] ~You will find that in nature, water is one of the most important things in the surroundings of plants~, and in their struggles to get it and keep it they have changed their forms in many ways, and in some cases have become extraordinary-looking creatures as a result. CHAPTER XIX. SPECIALISATION FOR CLIMBING If you go into a wood, or even a thicket, in summer, you can see how the leaves of the big trees make, what is for us, a delightful shade. But look at the ground under these tall trees, at a place where they are growing thickly together, and you will find that there are very few plants below them, and that the earth is almost bare except for dead leaves, twigs, and a few mosses. In deep pine-woods there are great patches without even the mosses, where are only dead pine-needles and some toadstools. You can well understand, when you remember how very important light is for the plants, that it is too dark for them to grow under the heavy shadow of thick trees. Even in gardens you may see how the tall, quickly growing plants kill off the smaller ones beneath them. When many plants are growing together, it is easy to see that the taller ones get most light, but if a plant grows very tall it requires a strong stem to hold it up right, and that means the building of a large amount of wood which takes a quantity of material, so that the growth must be slow and costly. Some plants, however, have learned to grow up into the light without building a firm stem for themselves, because they use instead the support of other plants, and especially of trees. You must often have noticed in a wood great sprays of honeysuckle sprawling high up over the trees; sometimes one of the festoons of honeysuckle may lie over the branches of several trees, and so get into the best positions for the light. The Travellers’ Joy, or white clematis, grows all over the tall hedges, and may sometimes completely smother a young tree, so that one can see nothing but the leaves and light green and white flowers of the clematis. Then, too, there is the ivy, which you know may sometimes grow up trees to a very great height, covering over the leaves so that the whole looks like a giant ivy bush. These plants all get their support from trees, which have built themselves strong stems. Pull down a big branch of honeysuckle or Travellers’ Joy from the supporting tree-trunks, and you will see that it cannot remain upright but falls limply to the ground. It is true that these plants have some wood in their stems--sometimes clematis and ivy may have woody stems several inches thick, but they are never strong enough to support the weight of the crown of leaves and branches. By clinging to others in this way these plants can economise much building material and reach the light far quicker than they could do otherwise.