In the narrow linear leaves of grasses the stronger veins run almost parallel. In broad leaves, such as those of the lily-of-the-valley, the veins curve, but do not form a network of tracery as in oaks and other Dicotyledons. The margin of leaves is frequently divided, but the technical terms used in describing such leaves can be found in any text-book of botany. They may either be simple or compound. A simple leaf consists of a single lamina, however much it may be divided, provided the divisions do not extend to the central vein or midrib. A leaf is compound when, besides the principal leaf-stalks, a number of lateral leaf-stalks exist bearing at their ends laminæ. The leaves of many plants are compound. The sensitive plant (_Mimosa pudica_) furnishes an excellent example of the compound leaf. The characteristic color of foliage leaves is green, and they are so arranged as to receive as much sunlight as possible. The importance of the plant receiving a good supply of light will be referred to when treating of the growth of plants. It is as true of plants as of animals that the organs most suitable for their surroundings are so arranged as to be most advantageous to the individual. Had leaves been placed vertically they would only have received diffused sunlight instead of the direct rays of the sun. No vegetable life could exist but for the sun, as plants not only require light but heat as well. When the foliage leaves are small they are very numerous, as may be seen in conifers; and when these leaves are large they are not nearly so numerous as, for example, in the sunflower. Sometimes leaves may consist of scales. These scales are always found on stems growing underground, as in the onion; but they sometimes occur on stems growing above-ground. Such plants as _Orobanche_ and _Neottia_ have no other kind of leaves except scales. The leaves are developed very near the apex of the growing stem. The portions of the stem which lie between the leaves are termed the internodes, and the parts where the leaves are inserted are termed the nodes. Leaves are arranged in various ways, intimately connected with the order of their development. They may be developed so that three or more are at the same level on the stem; this arrangement is termed a _whorl_. Or they may be developed singly; this arrangement is termed _scattered_. For a full account of the various leaf-arrangements any text-book on botany may be consulted. We have here merely referred to some of the more obvious arrangements of the leaves.