Leaf and Internode Introductory article Andrew Hudson, University of Edinburgh, Edinburgh, UK Article Contents Christopher Jeffree, University of Edinburgh, Edinburgh, UK . Introduction . Parts of the Monocot and Dicot Leaf Leaves of different species show wide variation in morphology and anatomy, usually . Physiology and Function associated with specialized roles in photosynthesis. Formation of leaves, from naive . Control of Leaf Initiation meristematic cells at the growing shoot tip, differs subtly in monocotyledonous and . Growth and Patterning of the Leaf dicotyledonous plants, although it appears to involve conserved gene functions. Cell Type Specification in the Leaf . Heteroblasty Introduction dicot leaf usually has a net-like vascular system, in which Leaves are the most variable of plant organs. They differ veins branch and rejoin. Major veins are usually thicker widely in shape, size and anatomy between different than the surrounding blade tissue. The blade is often species, and even within individual plants. Most leaves similar in composition along its length and width, although are specialized photosynthetic organs, but others are its edge may form specialized structures, such as spines. In adapted to different roles as, for example, spines or scales contrast, many leaves show an asymmetric distribution of for protection, tendrils for support, or the traps of tissues along the depth of the blade, with palisade insectivorous plants. Although differing structurally, mesophyll cells towards the upper (adaxial) surface and leaves share a number of characters that distinguish them spongy mesophyll cells below. Further differences are often from other organs of the plant. seen between epidermal cells of the adaxial and abaxial surfaces. 1. They occur on the sides of stems and (together with Many dicots produce compound leaves with a number of leaf-like parts of the flower) are therefore termed individual leaflets on a common stalk (rachis) (Figure 1b). lateral organs. Each leaflet resembles a simple leaf in structure and 2. Unlike the shoot, they have a limited capacity for development (although it has no axillary meristem growth. associated with it). In addition, both simple and compound 3. They are associated with secondary meristems (ax- leaves can form blade-like outgrowths (stipules) from the illary meristems) that form at the junction between the base of the petiole (Figure 1b). Compound leaves have upper (adaxial) part of the leaf and the stem and allow probably evolved from simple leaves on a number of branching of the shoot. occasions, and simple leaves may also have arisen by 4. Most leaves show dorsoventral asymmetry. They are reduction of compound leaves. Some species are able to usually flattened and may also have different tissues in produce both simple and compound leaves during their their upper (adaxial) and lower (abaxial) parts. lifetimes. Leaves probably evolved in a common ancestor of The leaves of monocotyledonous plants (monocots) euphyllous plants (e.g. flowering plants, conifers and differ from dicots in several respects. ferns). Although the leaf-like organs of more primitive plants (e.g. mosses) have similar photosynthetic functions, 1. Many monocot leaves are sword shaped and lack a they probably arose independently on more than one narrower petiole (Figure 1c). The basal part (the sheath) occasion. tightly encircles the stem and may overlap at its margins or form a tube, but the blade is usually free. Specialized structures are found at the sheath–blade boundary: the ligule, a membrane or fringe of hairs Parts of the Monocot and Dicot Leaf that form a seal between the adaxial leaf and stem, and the auricle, which can act as a hinge between sheath The leaf of a dicotyledonous plant (dicot) typically consists and blade (Figure 1d). of a flattened leaf blade joined to the stem by a narrower 2. All but the most minor veins run parallel to each other petiole (Figure 1a). The petiole is usually continuous with along the long axis of the leaf and rejoin only near its the major central vein of the leaf (the midrib) and no tip. Other tissues (e.g. epidermal hairs or stomata) may distinct boundary may be apparent between petiole and be arranged in similar longitudinal stripes. blade, or between the lower (abaxial) petiole and the stem. 3. Monocot leaves tend to show less differentiation However, specialized structures may form at the base of the between adaxial and abaxial tissues. petiole allowing leaf movement or loss under unfavourable conditions (as occurs in deciduous trees). The blade of a ENCYCLOPEDIA OF LIFE SCIENCES © 2001, John Wiley & Sons, Ltd. www.els.net 1 Leaf and Internode 2 Leaf and Internode Although these generalizations are valid for the leaves of are branched, hooked or produce sticky or toxic com- grass-like monocots (e.g. maize), other monocots have pounds as a defence against pests (particularly insects) dicot-like leaves with broader blades, veins that branch (Figure 2c). They may also protect against damage by UV laterally from a central midrib, and more obvious light or reduce water loss by trapping a layer of still air differentiation of adaxial and abaxial tissues (Figure 1d). around the leaf surface. This has led to the suggestion that ancestral monocots had The vascular tissues of the leaf resembles those in the rest leaves similar to dicots, and that grass-like leaves arose by of the plant, consisting of xylem, phloem and associated reduction of the dicot-like blade and increased growth of a cells. Xylem is responsible for supplying the leaf with water region closer to the stem. Compound leaves in monocots and dissolved inorganic compounds. Phloem supplies the are found only in palms. Unlike compound dicot leaves, developing leaf with organic compounds, and exports these form from a single primordium, which becomes excess products of photosynthesis from mature leaves compound as cells between leaflets die late in development. (usually in the form of sucrose). The vascular cells of minor veins are surrounded by a single layer of photosynthetic bundle-sheath cells. In C4 plants (e.g. maize) bundle-sheath cells are responsible for fixation of carbon dioxide that is Physiology and Function produced from organic acids (usually malic) imported from neighbouring mesophyll cells. The bundle-sheath Most leaves are specialized photosynthetic organs and cells of C4 plants are large, have large chloroplasts and are show adaptations to light harvesting and gas exchange in intimate contact with neighbouring mesophyll cells – a (uptake of carbon dioxide, loss of oxygen). Their flattened characteristic arrangement termed Krantz anatomy shape presents a large area to incident light. Palisade (Figure 2d). mesophyll cells are responsible for most of the photosyn- thetic activity of the leaf. They are located adaxially (and therefore usually towards the light), contain numerous chloroplasts and have a large proportion of their surface ControlofLeafInitiation area exposed for gas exchange (Figure 2a). Spongy mesophyll cells, although also photosynthetic, have fewer The position at which leaves occur on a stem is termed a chloroplasts, but are separated by more extensive air node and the stem tissue separating neighbouring nodes an spaces. The exposed surface area of mesophyll cells may internode. Leaves occur either singly or in groups at each therefore exceed the external surface area of the leaf by node. Because the rate of leaf initiation and growth is almost 20 times. affected by environmental conditions, leaf age is conve- Exchange of gases between the internal air spaces and niently measured in plastochrons – one plastochron being the external atmosphere occurs through pores (stomata) in the time between initiation of leaves at successive nodes. the epidermis (Figure 2b). In many plants, stomata are more Each leaf arises from a group of initial cells within the frequent in the abaxial epidermis. Each pore is bounded by flank of the shoot apical meristem. The initials form a a pair of specialized epidermal cells (stomatal guard cells) primordium growing in a new axis (Figure 3a), while that regulate its aperture. An increase in guard cell turgor surrounding cells form either stem tissues or axillary pressure occurs in conditions favourable for photosynth- meristems. Leaf initials are present in at least four cell esis (light or depletion of internal carbon dioxide by layers of the dicot meristem and may differ in number photosynthesis) causing stomata to open. Water stress or between species (e.g. 100 cells at primordium initiation high internal carbon dioxide cause guard cells to lose in Arabidopsis thaliana, 150 in tobacco). Surgical turgor and stomata to close. Therefore the plant can experiments suggest that the identity of leaf initials is balance the requirement for photosynthetic gas exchange specified at least one plastochron before they form a with water loss by transpiration. Gas exchange and water primordium and that existing primordia may produce an loss through other epidermal cells is limited by a thickened inhibitory signal that prevents adjacent meristem cells external cell wall impregnated with the fatty polymer, assuming leaf fate (thus explaining the regular spacing of cutin, and a hydrophobic surface layer (the cuticle) leaves on stems – termed phyllotaxy). Repression of leaf containing cutin and waxes (Figure 2b). The cuticle also fate in the meristem requires homeobox transcription reduces wetting of the leaf (e.g. by rain) and forms a barrier factor genes of the knotted1 family, which are expressed in against attack by pathogens. the meristem and stem initials, but excluded from leaf Many leaves produce epidermal hairs (trichomes) initials before primordium initiation. Conversely, MYB consisting of one or more specialized cells. Many trichomes transcription factor genes of the phantastica family repress Figure 1 Parts of the monocot and dicot leaf. (a) A simple leaf of the dicot, Antirrhinum majus (snapdragon). (b) A compound leaf of the dicot Pisum sativum (garden pea). (c) Part of the grass-like monocot leaf of Zea mays (maize).