Leaves often differ greatly in form on the same plant. Observe the different shapes of leaves on the young growths of mulberries (Fig. 2) and wild grapes; also on vigorous squash and pumpkin vines. In some cases there may be simple and compound leaves on the same plant. This is marked in the so-called Boston ivy or ampelopsis (Fig. 109), a vine that is used to cover brick and stone buildings. Different degrees of compounding, even in the same leaf, may often be found in honey locust and Kentucky coffee tree. Remarkable differences in forms are seen by comparing seed-leaves with mature leaves of any plant (Fig. 30). [Illustration: FIG. 109.--DIFFERENT FORMS OF LEAVES FROM ONE PLANT OF AMPELOPSIS.] =The Leaf and its Environment.=--The form and shape of the leaf often have direct relation to the _place in which the leaf grows. Floating leaves are usually expanded and flat_, and the petiole varies in length with the depth of the water. _Submerged leaves are usually linear or thread-like_, or are cut into very narrow divisions: thereby more surface is exposed, and possibly the leaves are less injured by moving water. Compare the sizes of the leaves on the ends of branches with those at the base of the 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.