Very many plants which depend on others for support modify their leaves, or parts of leaves, to form sensitive tendrils which twine quickly round any prop they can find, and thus hold up the stem (~see~ fig. 104). The young tip of the stem continues to grow upwards, the next young leaves unfold their soft green tendrils which twist round a support directly they feel it, and so the plant goes on growing higher and higher. You can see the fate of a pea-plant which does not find supports, by growing one in a big pot all by itself. It will grow upright at first, but it will soon have to creep along the earth and fall over the edge of the pot, for its stem is not strong enough to support its own weight. [Illustration: Fig. 104. Sensitive tendrils of the Pea. (~t~) tendril at the end of foliage leaf, (~o~) ordinary leaflets.] In vines and marrows we also get tendrils, but they are not modified leaves, but special branches which have become sensitive. In some plants the sensitive tendrils do not twine, but instead form little sticky suction pads at their tips whenever they come in contact with the support, and these hold the tendril very firmly on, as you can see in the ampelopsis, which grows right up the walls of houses. If you look under the thick covering of leaves, you will find these tiny padded tendrils clinging tightly to the wall (~see~ fig. 105). This is the reason that the ampelopsis grows so well up the walls without being held up artificially. There are many other things you may find out about climbing plants, but you will have seen enough to be able to look for more for yourself, and to understand how it is that the climbing plants can reach such a great height so quickly. ~They have learnt to avoid the trouble and expense of building strong supporting stems for themselves, and by getting their support from others, they are able to grow quickly out into the good positions for the light which they could not otherwise have reached.~ [Illustration: Fig. 105. Ampelopsis, which supports itself by the little suction pads developed at the ends of the tendrils.] CHAPTER XX. PARASITES We call a plant or animal a ~Parasite~ when it does no food-building for itself, but adapts its whole structure to obtain and use the food made by the work of other plants or animals. Plant parasites generally attach themselves to a “Host” plant so closely that they suck their food from it, and sometimes remain with it till they have finally killed it, and so have destroyed their only source of food and means of life.