If one of these parts were held in front of a looking-glass, the reflected image of this part would represent the part from which it had been separated. Many leaves, however, can not thus be divided. When this is the case they are said to be unsymmetrical. The tropical plant begonia affords an excellent example of an unsymmetrical leaf. The leaves of the spruce are not flat but needle-shaped. In rushes and many species of stone-crops the leaves are cylindrical or round. The leaf consists of three parts, viz., the sheath, the stalk or petiole, and the lamina or blade. The sheath incloses the stem at the insertion of the leaf, and has a tubular or sheath-like form. It is well seen in grasses and such plants as celery, corn, parsnip, carrot, and other plants belonging to the _Umbelliferæ_ [Lat. _umbella_ (_umbra_, shade), little shade, and _ferre_, to bear]. The leaf-stalk is narrow, and has a semi-cylindrical or prismatic form, bearing at its end the expanded leaf. When the stalk is flattened and resembles a leaf, as in the case of the Australian acacias, it is termed a phyllode (Greek _phyllon_, a leaf, and _eidos_, form). Many leaves have no sheath, but only the stalk and the blade. This is the case in the maple and gourd. The leaves of the grasses have no stalk, but only sheath and blade. The blade is often the only part present, as in the tobacco plant and tiger-lily. Small appendages, looked upon as belonging to the sheath, are frequently present, and are termed stipules (from Lat. _stipula_, blade). Leaves having these appendages are called stipulate, and leaves devoid of them are exstipulate (from Lat. _ex_, privative, without, and _stipula_, blade). A few plants, such as grasses, have a small outgrowth from the inner upper surface of the leaf at the part where the sheath and the blade are joined. This outgrowth is named a ligule (from Lat. _ligula_, a little tongue). If a leaf is carefully examined it will be found that the internal tissues differ in character. The fundamental tissue is generally green, and is named the messophyll (Greek, _mesos_, or _messos_, middle, and _phyllon_, leaf). It will be seen that bands run through the fundamental tissue called the veins of the leaf. These veins consist of what are termed fibro-vascular bundles. They endure longer than the fundamental tissue, and may frequently be seen after the leaf is withered and dead, forming the skeleton of the leaf. The arrangement of the veins or fibro-vascular bundles is characteristic of large groups of plants.