Page:The American Cyclopædia (1879) Volume X.djvu/270

264 LEAF greater part of the silver with it ; and in the second, the lead which liquates from the copper likewise contains . nearly all the silver. For an extended account of the alloys of lead the reader is referred to an article by Dr. Kich- ardson in Watts's " Dictionary of Chemistry." LEAF. The parts of a plant concerned in its nutrition are the root, stem, and leaf, which are termed the organs of vegetation. The leaf, as is the case with the others, is subject to many modifications, and is indeed sometimes made to serve other purposes than those of foliage ; but its chief function is that of elab- orating the crude material supplied by the roots and absorbed from the air into sub- stances which will nourish and continue the growth of the plant. This work requires the light of the sun, and one of the processes inci- dental to it is evaporation ; hence the leaf is generally constructed and arranged upon the FIG. 1. Pinnately veined Leaf, with Petiole and Stipules. stem in such a manner as to expose the largest surface to the influence of light, and usually pre- sents a broad evaporating and absorbing sur- face to the atmosphere. In its most complete form the leaf consists of an expanded portion, the blade or limb, which is attached to the stem by means of a leaf stalk or petiole, and at the base of this there are two f oliaceous appendages or stipules. The stipules are characteristic of some families of plants and are always present in them, but in other families they are entirely wanting, and hence cannot be regarded as es- sential to the leaf ; so with the petiole, which is frequently absent, the blade being attached directly to the stem, or sessile. The blade is regarded as the only essential part of the leaf, and though this presents itself in a vast variety of forms, the same general structure is mani- fest in all. In all ordinary leaves two distinct structures are visible : a framework or skeleton of fibres, and a green pulpy portion which fills the spaces in this. When a principal bundle of fibres runs from the base to the apex of the leaf, it is termed the midrib ; the branches from this are called veins, and the smaller subdivi- sions veinlets. In many leaves the smaller vein- lets anastomose and thus form a complete net- FIG. 2. Palmately veined Leaf (Maple). work ; in others the veins run parallel and do not anastomose ; as a general rule netted- veined leaves are found in dicotyledonous, and paral- lel-veined leaves in monocotyledonous plants. The form of the leaf largely depends upon the disposition of the veins ; if there is a midrib with smaller branches or veins from each side, the leaf is said to be pinnately veined, and is usually longer than broad ; but if there are sev- eral principal ribs starting from the base of the leaf, it is palmately veined, and its outline will be more or less or- bicular. The skele- ton or framework of the leaf consists of proper wood, and the microscope shows the various vessels, ducts, and fibres found in the stem itself; and this portion of the leaf is regarded as an expansion of the woody system of the stem, or rather of the inner bark. Its structure is beautifully shown in what are called skeletonized leaves, often seen as parlor orna- ments ; these are prepared by macerating the leaves in water until the softer parts have de- cayed, and arresting the process while the fibres still remain intact. The pulpy portion of the leaf, cellular tissue or parenchyma, consists of FIG. 3. Parallel-veined Leaf.