In the case of large forest trees, the far-spreading roots have an immense power of resistance. The large surface of a giant tree in full leaf has to endure an enormous lateral pressure during a high wind, and even hurricanes may fail to uproot a large tree, which they may snap asunder. Not only does the root by penetrating the soil attach the plant to the earth, but it absorbs nourishment from the soil for the support of the plant. The root, therefore, fulfils a double function. The root is at first furnished with a conical hood of cellular tissue, _i. e._, tissue consisting entirely of cells or little closed bags made up of an outside wall and contents. The root cup is well seen in some kinds of water-plants, such as duckweed. There are plants whose roots do not descend. Certain plants hang from the branches of trees, and though they have roots these roots never penetrate the soil. Plants of this kind are called Epiphytes (Greek _epi_, upon, and _phyton_, plant). Aerial orchids, which grow in warm and moist parts of India and other countries, are attached to branches of trees or other kinds of support, and their roots hang down from the peculiar stems and are very soft and delicate at the tips. It must be borne in mind that there is no absolute distinction between root and stem; for some trees have roots which form lateral buds, viz., _Pyrus japonica_, _Maclura aurantiaca_, and many others. This is quite in accordance with the fact that in the organic world different organs frequently shade into one another. The true root of the plant in its earliest state of existence, that is, as it exists in the seed prior to germination, is the downward prolongation of the axis. In the case of the division of flowering plants called Monocotyledons (Greek _monos_, single, and _kotyledon_, seed-leaf), and in such so-called flowerless plants as ferns, the lower end of the axis soon ceases to grow and the roots which supply these plants with nourishment are really lateral growths. The roots of plants are variously named. Sometimes the branches of the roots are small, and the central axis thick and of considerable length. This kind of root is named a tap-root, and may be well seen in the carrot. In the turnip, beet, and other plants, where this organ is developed in such a manner as to serve as a reservoir of nutriment, the root is tuberous. Many roots are fibrous; this may be well seen in grasses. The perennial woody forms of fibrous roots are very characteristic of shrubby Dicotyledons (plants with two seed-leaves). Leaves are of two kinds, namely, foliage-leaves and flower-leaves. A leaf is generally a broad, flat, horizontal surface. It is usually thin, and can be divided by a perpendicular plane, the median plane, into two similar halves. When the leaves are what is called symmetrical, the parts into which they are divided are counterparts.