LEAF. 45 this type, perhaps, is cUiscd venation, implying tliat the veins do not end freely in the margin. As a result of this, such leaves do not become toothed or lobed. nor do they branch. In a reticulately veined leaf the veins branch freely in various directions, and there is usually evident, especially on the lower surface, a distinct and often conspicuous network of veins. Such leaves may be characterized as cxhiliitinp; open venation, as many of the veins have free ends, especially in the margins, resulting in a tendency to toothing, lobing, and branching. The dicotyledons exhibit such leaves in the greatest profusion. The ferns also have open venation, but the veins fork re- peatedly, that is. they are dichotomous. (See Dichotomy.) Reticulately veined leaves exhibit two prominent types, the palmate and the pin- nate. In the former, three or more main ribs diverge from the base of the blade, resulting in a broad outline. When such leaves are lobed or branched they are said to be palmately lobed or compound. In pinnate leaves there is a main rib (midrib) which runs through the blade from base Fig. 2. CROSS-9ECTIOX of leaf. Showinjr upper and lower epitiermia with stomata (s), air-chamber (;), in palisade parenchyma, and section of a vein in the spongy parenchyma, to apex, and gives rise to lateral ribs of secondary importance, resulting in a comparatively narrow outline. When such leaves are lobed or branched they are said to be pinnately lobed or compound. Fern-leaves ('fronds') are very comnlonly pin- nately compound, and a special terminology' is applied to the branches, the main branches being called pinnae, and the ultimate branches pinnules. Structure of Leaves. The minute anatomy or histology- of the ordinary foliage leaf is quite uniform. The upper and u'.dcr 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 arc found, called collec- tively mesophyll. The mesophyll cells contain the green color-bodies (chloro])lasts) , 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 LEAF. irregular but continuous intercellular passage- ways. This region is called the spongj- meso- phyll. The third histological region of the foliage leaf is the vascular region, represented by the veins which traverse the mesophyll in every direc- tion. The epidermis docs not hermetically seal the mesophyll tissue from the outside air, but in leaves exposed to air it is perforated by numerous very small openings, called stomata " ( Fig. 3 ) , popu- FlG. 3. EPIDERMAL CELLS AND STOMATA FROM A LEAP. 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 cliange the size of the opening between them, as the lips may regulate the opening of the mouth. Stomata occur in any epidermis which overlies gieen cells, and therefore they are naturally found in greatest numbers on the leaves. In the ordinary horizontal (dorsiventral) leaves they occur for the most part and sometimes exclusive- ly on the under surface, averaging about GO.OOO to the square inch, although in some cases 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 ai'e upon 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 (sec Tri- CHOMES), 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. JIoDiFiED Lea-es. 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
