STRUCTURE OF LEAVES. The minute anatomy or histology of the ordinary foliage leaf is quite uniform. The upper and under surfaces are bound ed by a single layer of colorless compact cells, forming the epidermis (Fig. 2). Between these two layers of epidermis, above and below, the working cells of the leaf are found, called collec tively mesophyll. The mesophyll cells contain the green color-bodies (chloroplasts), and in ordinary horizontal leaves are organized into two distinct regions. The cells against the upper epidermis are elongated and stand closely side by side. with the long axis at right angles to the leaf surface. This is called the palisade region of the mesophyll, and it is explained by the fact that this surface is exposed to the direct rays of light. The meso phyll cells in the lower region of the leaf are irregular in form and loosely aggregated, leaving larly known as 'breathing-pores.' They are auto matic gateways in the sense that the calibre of the pore may be enlarged or diminished in response to various conditions, thus regulating the amount of exchange between the air in the mesophyll re gion and that outside. The regulating mecha nism consists in two concentric cells, called guard cells, which face each other, and which may change the size of the opening between them, as the lips may regulate the opening of the mouth. Stomata occur in any epidermis which overlies green cells, and therefore they are naturally found in greatest numbers on the leaves. In the ordinary horizontal (dorsiventral 1 leaves they occur for the most part and sometimes exclusive ly on the under surface, averaging about 60.000 to the square inch, although in seine eases the number may reach over 400,000. Leaves which are exposed to the light on both sides have the stomata equally distributed upon the two sur faces. In floating leaves the stomata are updn the upper surface only. The significance of the
occurrence of stomata chiefly upon the under side of horizontal leaves is found in the fact that the intercellular passageways with which stomata communicate are best developed on the under side of the leaf. Associated with the epidermis of leaves there are also numerous hairs I see Tin cuomr.$), whose occurrence and character form part of the descriptions of systematic botany. For example. if hairs are absent. the leaf is spoken of as glabrous; and if present, the terms pubescent. hirsute, tomentose, woolly, etc., de scribe their character.
•Alonirimi LEAVES. There are numerous struc tures in plants which have long been regarded as modified leaves—that is. leaves which have been diverted from their ordinary work as foliage leaves to serve some other purpose either exclusively or in addition to their ordinary work. It is a matter for serious doubt whether all such structures have actually been derived from foliage leaves. but they all may be grouped as foliar organs. In addition to foliage leaves, therefore, prominent among other foliar organs are the following: Pitchers, as in the various 'pitcher plants' for entrapping various insects; sensitive fly-traps, as in sundews and Dionrea (q.v.) (see CARNIVOROUS PLANTS) ; storage organs. as in bulh•seales, and many seed leaves (cotyledons) ; bud-scalcs, used for pro tecting paing parts; spines. as in the barberry and holly, where every gradation between spiny toothed leaves and true spines is food; ten drils, which are often loaves or leaf-parts adapted for climbing; hrnets, are leaves modified in size and color and associated with flowers; the floral organs. as sepals, petals, stamens, car pels, all of which may be regarded as foliar struc tures, but probably not modified foliage leaves as is commonly stated.