branches or in the interior of the tree top. In dense foliage masses, the petioles of the lowermost or undermost leaves tend to elongate—to push the leaf to the light.
On the approach of winter the leaf usually ceases to work, and dies. It may drop, when it is said to be deciduous; or it may remain on the plant, when it is said to be persistent. If persistent leaves remain green during the winter, the plant is said to be evergreen. Give examples in each class. Most leaves fall by breaking off at the lower end of the petiole with a distinct joint or articulation. There are many leaves, however, that wither and hang on the plant until torn off by the wind; of such are the leaves of grasses, sedges, lilies, orchids, and other plants of the monocotyledons. Most leaves of this character are parallel-veined.
Leaves also die and fall from lack of light. Observe the yellow and weak leaves in a dense tree top or in any thicket. Why do the lower leaves die on house plants? Note the carpet of needles under the pines. All evergreens shed their leaves after a time. Counting back from the tip of a pine or spruce shoot, determine how many years the leaves persist. In some spruces a few leaves may be found on branches ten or more years old.
Arrangement of Leaves.—Most leaves have a regular position or arrangement on the stem. This position or direction is determined largely by exposure to sunlight. In temperate climates they usually hang in such a way that they receive the greatest amount of light. One leaf shades the other to the least possible degree. If the plant were placed in a new position with reference to light, the leaves would make an effort to turn their blades.
When leaves are opposite the pairs usually alternate. That is, if one pair stands north and south, the next pair
