LEAF are often compound, i. e., made up of smaller leaves or leaflets, which are articulated with a common petiole. Among trees, the locust is an example of the pinnate and the horse chest- nut of the palmate compound leaves. Some- times a stem appears to pass directly through the blade of a leaf ; to such the name of per- f oliate has been given ; in some cases this ap- pearance is produced by the lobes of a sessile heart-shaped leaf uniting to enclose the stem, and in others by the union of the bases of two opposite leaves. The petiole is essentially of the same structure as the leaf, but its cellular tissue is usually small in proportion to its woody portion ; its size and length, in propor- tion to the leaf, vary greatly ; in aquatic plants with floating leaves it is several feet long, and in some palms the petiole is so large as to serve for making oars ; in our garden rhubarb it takes on an unusual development, and is the useful part of the plant. In some plants the petiole is so short as to be barely perceptible, and in many entirely absent. In the aspen the petiole is flattened at right angle^ to the axis of the leaf, which allows the leaf to move with the slightest mo- tion of the air, and to keep up the con- tinuous fluttering for which the foliage of this tree is proverbial. The petiole may be channelled, or fur- nished with a wing on each side, as in the orange, or it may be 'broadly expanded at 7.-Vetch with Leaf de- t ] ie b ase an ^ gheathe velopedasTendnl. ^ ^ ^ ^ ^ umbelliferce, thus when present affording use- ful characters in describing plants. In some plants the blade of the leaf is wanting, but the petiole expands and becomes leaf -like, and takes upon itself the functions of the leaf; it is then called a phyllodium. The Austra- lian acacias afford numerous examples of phyl- lodia ; several of these, when raised from seed, produce upon the young plants the compound leaves common to the genus; some species, when but a few inches high, show a tendency to suppress the blade of the leaf ; the succes- sive leaves have wider and wider petioles and less and less blade, until the leaf-like petioles or phyllodia constitute the sole foliage ; these have parallel veins and their edges instead of their surfaces are presented to the sky and earth. Stipules, the accessory leaf -like bodies found in many leaves at the base of the petiole, present great variety in size, form, and dura- tion ; they frequently fall away as the leaves expand ; in our tulip tree (liriodendroti), and in the related magnolias, they are only to be found as the leaves are unfolding, and are then very conspicuous ; again, they remain as long as the leaf to which they belong, and often form a large part of it, as in the garden pea, and in a related species the blade of the leaf is wanting, and the whole foliage of the plant consists of stipules. In some cases the stipules are dis- tinct, but in many, as in the rose, they are at- tached to the petiole by one edge; in the docks, rhubarb, and other members of the polygonum family, they are united by both margins, and thus form a sheath which sur- rounds the stem. The appearance of stipules in the form of spines is not rare, and is notice- able in the common locust, the caper, and other plants. Those parasitic plants which, like the dodder, rob other plants of elaborated food, have no leaves, their function being performed by the foliage of the host to which they are attached. The prickly pear and others of the cactus family are usually regarded as leaf- less ; but these have minute leaves upon the young stems, which soon drop, and in the older ones the whole surface of the stem performs the functions of the leaf. Leaves vary greatly in size, but generally what is lacking in size is FIG. 8. Barberry with Spiny Leaves. made up in number ; and thus trees with minute leaves, like the arbor vi- tse, where they are like small green scales cloth- ing the branches, present in the aggregate as large a surface of foliage as trees with much larger leaves. A little plant of our fresh-water ponds has leaves only V of an inch long, while those of the Victoria regia, of the South American lakes, have a diameter of 6 ft., and afford a standing place for aquatic birds while they are watch- ing for their prey. Some palms have leaves of enormous size, and an arad discovered a few years ago in Central America (Godwinia gigas) has leaves over 13 ft. long. Leaves differ greatly as to their duration; some are fuga- cious, falling soon after they appear; those which fall at the close of the season are decid- uous, and when they remain through the year they are persistent, as in the evergreens. In some of the evergreen coniferous trees the leaves of the former year fall as soon as those of the present year are developed, while in some firs they remain 10 or 12 years before they fall. In deciduous trees the fall of the leaf is as well provided for as its development, and is not due, as is supposed by some, to the advent of frost. A distinct line of separation is early visible, and before the leaf separates
