On the leaves and stems of many plants you will find a large number of ~hairs~. In some cases there are so many as to make the whole plant quite woolly, like the mouse-ear leaves. These hairs are protective, and keep the leaf warm and dry, and in some cases may shelter it from the sun. Hairs may consist of one cell, or several in a row, or of cells which are branched in a complicated way. Certain hair-cells protect the plant by stinging, as you can see if you watch a nettle-leaf with your magnifying-glass, and then rub your finger along it, only touching the hairs. You will find that it is they which sting you, and not the leaf itself. Now we have found several kinds of tissues in plants, the ~skin~ and ~cork~ covering all over and protecting the rest; the ~central vessels~ or ~water-pipes~, corresponding to the veins and arteries of animals, the soft ~white ground~ tissue, which in some stems may be very loosely packed, and the soft ~green tissue~ in the leaves and young stems, which we found was the food-manufacturing part of the plant. There are also strands of simple ~strengthening tissue~, both by the water-pipes and in separate bundles in the soft tissue; these we may take as representing the bones of animals. We have noticed (Chapter VIII.) that plants are sensitive to light and bend towards it, that they feel heat and cold, and that the stem and root seem to know when they are growing in the right or wrong positions, and bend accordingly. We know that we ourselves and the animals recognize such things by the help of nerves which carry messages to the brain. But where is the brain in plants, and the nerves? No true nerves have been found in plants, and it seems as though different parts of the plant were specially sensitive without there being any “brains.” So that we cannot speak of a central nervous system in even the highest plants as we can in the animals. In this respect they are built on quite a different plan from animals. PART III. SPECIALISATION IN PLANTS CHAPTER XVIII. FOR PROTECTION AGAINST LOSS OF WATER If you go along the lanes and in the gardens in the height of summer when it is hot and dry and the sun beats on the plants all day, you may see them beginning to wither for want of water. The roots are not able to find enough moisture in the soil to supply the leaves, which, being in the hot air, continue to transpire away the water resources of the plant, so that in the end each of its cells must suffer and the whole become limp and droop. This happens because the ordinary green plants of our country make no special preparation for such dry weather. Our hot season is short, and even in the summer we have frequent showers which keep the soil moist enough to provide the plants with water from day to day, so that they have not become accustomed to long periods when there is no prospect of rain. [Illustration: Fig. 97. A Cactus with needle-like spines for leaves, and a thick green stem.]