Fig. 99. Gorse, with green stem which does the work of leaves.
Many plants roll up their leaves when it is dry, so that the surface with the transpiring pores is on the inside, and protected by the outer side with its hard skin (see fig. 100). In damp weather these leaves unroll, and do all the work they can. Leaves like this are to be seen in many of the grasses, particularly those growing on sand dunes and moorland; while a number of the heaths and heather do the same thing to protect their transpiring surfaces.
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.