# 3: The leaf - structure function relationships

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Introduction

As mentioned in Factfile2, leaves are the major lateral organs of the stem and form an integral part of the aerial axis of the plant.

Leaves are typically, organs of determinate growth and of dorsiventral symmetry. The generally flattened shape being ideal for maximising exposure to sunlight for photosynthesis. Leaves may be classified as microphylls or macrophylls.

In phylogenetic terms, a macrophyll is a modified branch system and is therefore cauline in origin.

In contrast, the smaller microphylls are generally enations or outgrowths of the axis, which is not associated with leaf gaps.

 

The vascular system in microphylls is rudimentary and not extensively connected to that of the axis. Both types of leaves originate from a primordium at the shoot apex. Obviously, one may argue that the small size of microphylls represents a failure to undergo the extensive growth and elaboration usually associated with macrophylls. Microphyllous leaves occur in the Psilotales, the club mosses and some pteridophytes.

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Leaf form

There are several examples of plants where the leaf changes form during maturation - this is termed heteroblastic development, whereas in many species leaves do not undergo changes in form during the  plants development from juvenile to adult. In such cases, development is termed homoblastic. Leaves serve several vital functions in the day-to-day life of higher plants and it is of interest that we look at their development, (ontogeny) their structure as well as their numerous functions. There are three major functions of leaves - each of which is inter-related and interdependent to some degree, upon the functioning status of the others. back to index
1. PHOTOSYNTHESIS
2. TRANSLOCATION
3. TRANSPIRATION.
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Each of these is either initiated or takes place directly in the mesophyll of leaves.

1. Photosynthesis.

That leaves have vastly differing internal structure, is demonstrated by the  mesophyll cells which are arranged in different patterns and locations. These patterns are related to the functional processes of the photosynthetic cycle occurring within the leaf, as well as the ecological niche that plants occupy. In addition, there are differences between the leaves of dicots, monocots and gymnosperms.

It is possible to separate Angiospermous from Gymnospermous leaves, by using some basic diagnostic criteria.

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Dicotyledons generally have a mesophyll which is composed of two differing chlorenchyma cell types - palisade and spongy mesophyll cells. Leaves may be isolateral, isobilateral, dorsiventral or even needle-like in cross-section. Whatever the shape of the leaf , chloroplasts are concentrated within the cytoplasmic matrix of these cells and, for the most part, the majority of the chloroplasts are to be found in the upper palisade mesophyll cells. Mitochondrial populations in these obviously-photosynthetic cells may be high as well.
2. Translocation in the vascular tissues
1 The xylem.
The xylem is responsible for the major proportion of apoplasmic transport in vascular plants. Apoplasmic transport is not limited totally to water transport, but in addition, the transport of various macro and micronutrients, amino acids and other important inorganic substances, from the roots to the stem and ultimately, the leaf via the apoplasmic continuum.
2 The phloem.
The phloem is responsible for the transport of the major proportion of soluble carbohydrate as well as other essential products. The phloem forms the major long-distance symplasmic transport pathway in all vascular plants. Translocation usually takes place from a site of synthesis of assimilated material (called  sources) to a site or sites of utilization (called sinks). The assimilated material is translocated in a water-based medium, which emphasizes the essential inter-relationship between the xylem and phloem, more particularly so in the leaf where most of the phloem loading takes place in mature plants.

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3. Transpiration
Transpiration is the main driving force, which facilitates the movement of solutes through the xylem. Transpiration requires that water entry is facilitated via the roots and that it occurs in a conducting system, and that the water is translocated to other regions of the plant where it is utilized in numerous biochemical and growth-related reactions. The xylem is also the principal pathway through which water is translocated from point of entry, to points of exit, which in higher plants are the stomata. This process is termed transpiration. Transpiration itself facilitates leaf cooling by evapotranspirational heat loss to the atmosphere.
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Leaf Structure

In general terms, all leaves are composed of similar features -- an epidermis, made up of cells that include stomata, mesophyll and vascular tissue. However, the arrangement of these three components is, to a large extent, dictated by the physical environment - water availability, light intensity and ecological niche. Thus it is the interplay of these environmental parameters which serve to modify leaf structure. The epidermis may, for example, be simple or compound, there may be either a thick or a thin cuticle, there may be a hypodermis associated with the epidermis, stomatal distribution may be amphistomatous (stomata on both surfaces of the leaf) or hypostomatous (stomata on one surface of the leaf only) and they may be raised above the general leaf surface, flush with the leaf surface, or in some cases, sunken into crypts. The ground tissue (mesophyll) may be specialized or unspecialized.

 

Some Dicotyledonous foliage leaves contain a specialized, longitudinally-orientated mesophyll, called the paraveinal mesophyll, which separates the upper palisade from the lower spongy mesophyll. In most monocotyledonous plants, the mesophyll is not differentiated into spongy and palisade layers. The mechanics of a typical leaf is illustrated in Fig. 1

 

In some species, leaves may have palisade tissue on both sides of the leaf (isobilateral) as is the case in many succulents. The mesophyll may be compact, with few intercellular spaces as in xerophytes, or may contain a large intercellular space volume, as in some mesophytes and hydrophytes.

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]Fig. 1. Illustrates the general mechanical requirements of a typical leaf. Adequate gas exchange, and functional transport pathways are essential.

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The vascular bundles are surrounded by an initially-parenchymatous bundle sheath, which may undergo lignification as the cells mature. There may be a specialized, concentric arrangement of the photosynthetic mesophyll surrounding the bundle sheath cells as in C4 plants.

.Leaf Development

All leaves develop from a foliar buttress, which, in simple terms, is a meristematic projection above the general surface of the protoderm. Foliar buttresses are initiated near the apex, in regular sequence and lead to the formation of mature leaves. The ontogenetic sequence for a typical dorsiventral Dicotyledonous foliage leaf is illustrated in Fig. 2.

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Fig. 2. Ontogenetic relationship between the marginal and submarginal initials during leaf development. (adapted and redrawn from Esau, Anatomy of Seed Plants)

The marginal initials (MI) give rise to the adaxial and abaxial epidermis, whilst the submarginal initials (SI) give rise to all internal leaf tissues, including the procambium, from which all vascular tissues are differentiated. In dicotyledonous plants the transition from photoassimilate sink to source status begins shortly after the leaf has begun to unfold, at which point, the major morphogenetic events that determine leaf shape are to all intents and purposes, over (Fig. 3.) .

 

 

 

 

 

 

 

Fig. 3. (left) Illustrates the acropetal differentiation of the major vein network in a typical dicotyledonous leaf. The image on the right shows the minor vein network in a deciduous leaf in autumnal colours.
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Maturation of the phloem and xylem in the midrib and the higher-order veins, which occurs in an acropetal directions is largely complete before the transition begins . During leaf unfolding, the functional maturation of the minor veins begins in a basipetal direction. There is thus a degree of maturation of the leaf from the base to the tip o the lamina during the sink to source transition. The minor venation network, forms the distribution network of the leaf which provides first an importing and then an exporting network as the leaves continue to expand.

Major Vein Differentiation
The sequence of events which takes place within the dicotyledonous foliage leaf may be summarized as follows. Once the blade or lamina starts to expand, due to anticlinal and periclinal cell division within the marginal and sub-marginal initials, the procambial strands begin to form -- first of these to become evident, is the midrib or main vein. This vein is blocked out acropetally , and vascular tissues differentiate in regular sequence (protophloem followed by protoxylem then metaphloem, followed by metaxylem) towards the tip of the still-immature expanding leaf. The major veins of the lamina follow suite, initiating at the base of the leaf, they too, differentiate acropetally. Thus the first-formed of the major lamina veins matures first, and the last-formed (apical) major veins, differentiate and mature last.

The Minor Veins
In dicotyledonous foliage leaves, the minor veins differentiate basipetally, from the apex and the leaf margin, back towards the major vein network. Thus, it is quite feasible for the tip of the developing leaf to mature with respect to transport, before the base of the leaf. The apex could therefore conceivable, export photo-assimilated material to the still immature basal part of the leaf, during the overall maturation and development process.
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Structural specialization of Companion cells
The term companion cell is used to describe that cell, or group of cells which are derived from the same mother (procambial) cell as the sieve tube member. However, the identification of the 'companion cell' may be problematic is some species, more especially so in monocotyledons. In contrast, transfer cells are relatively easy to identify as they always have prominent wall ingrowths, that are assumed to enhance cell uptake either from associated symplasmic continua, or directly from the apoplast. This is achieved through the increased surface area of the plasmamembrane, associated with the wall ingrowths. In contrast, the term intermediary cell is applied to large parenchymatous cells, with dense cytoplasm, which loosely abut the parenchymatous bundle sheath in several dicotyledonous species. Such intermediary cells have been identified in Cucurbita pepo, Cucumis melo and Coleus blumeii to name three common species. Thus companion cells, transfer cells, and intermediary cells all have similar functions in the plant -- involvement to some degree in the loading of sugars in source leaf veins. Several attempts have been made to correlate companion cell type with plasmodesmatal frequency, or the type of sugar or sugar alcohol transported. However, the data for this is scanty and incomplete.

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Leaf architecture:

Classification into major and minor veins.

All leaves have two components to their networks - the so-called major and minor vein system. What makes them different? Simply, the major veins in dicotyledonous foliage leaves, occupy much of the cross-sectional area of the leaf, and are often associated with hypodermal collenchymatous, or sclerenchymatous strands. Viewed in cross section, they may even show signs of a cambial zone. However, this cambial zone displays only limited secondary growth, which is more evident nearer the base of the leaf and (if present) down in the petiole, where the vasculature of the main vascular supply to the leaf, assumes a more cauline appearance (that is, it is more stem-like). In contrast, minor veins lack associated mechanical supporting tissue. Unlike the major network veins, the minor veins are usually embedded within the interface between the palisade and spongy mesophyll layers. As mentioned, minor veins are embedded within a horizontally orientated mesophyll, termed the paraveinal mesophyll in some Dicotyledons, which, judging by the relatively high plasmodesmatal frequencies recorded between adjacent cells, is the principal symplasmic solute conduction pathway from the palisade and spongy layers, into the surrounding parenchymatous bundle-sheath cells, terminating at the sieve tubes within major and minor veins (Fig. 4.).

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Fig. 4. Diagram illustrating the differences between major and minor veins in a generalized dicotyledonous foliage leaf.

The Monocotyledonous foliage leaf
As in dicots, monocot leaves can have C3 or C4 anatomy.  A commonly-mentioned anatomical feature of C4 plants is the orderly arrangement of mesophyll cells with reference to the bundle sheath cells, forming concentric layers around the vascular bundle as seen in transection. Bundle sheath cells of C4 plants have few if any intercellular air spaces between them, which is in direct contrast to the sometimes large intercellular space volume between mesophyll cells in C3 plants. Observations of the concentric arrangement of mesophyll and bundle sheath cells of certain grasses and sedges, prompted Halberland to compare the mesophyll layer to a Kranz ( wreath-like) structure. The vascular bundles in C4 plants are often close together, separated by as few as 2-4 cells.

The structure of monocotyledonous foliage leaves depends to a large extent on the type of photosynthesis (i.e. C3; C4) and on the environmental conditions that the plants grow in (i.e. xerophytic, mesophytic, or hydrophytic). All monocotyledonous foliage leaves are basically parallel-veined, but large numbers of cross-veins serve to interconnect the parallel vein system. The parallel venation may not be as evident in monocot leaves that are strap-shaped.

Classification of leaf-blade vein order size is based upon the following criteria:
(a) Large bundles. These bundles are characterized by the presence of wide metaxylem vessels on either side of the protoxylem, which is often represented by a lacuna. Obliterated protophloem is evident on the abaxial side of the bundle (or to the inner side, if the bundles are arranged in a ring in TS).. Conspicuous girders (either collenchyma or sclerenchyma) generally extend from the bundle to either both ad- and abaxial leaf surfaces., or the abaxial surface only.
(b) Intermediate bundles. These bundles lack wide metaxylem vessels and protoxylem lacunae. Hypodermal strands or girders may occur on both ad- and abaxial, or on the abaxial surface only.
(c) Small bundles. In addition to lacking wide metaxylem vessels and protoxylem lacunae, these bundles are not associated with either hypodermal girders or strands but may have some sclerenchyma associated with them in bundle caps (Fig. 5. ).

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Fig. 5. Illustrates the difference between a hypodermal strand (left) and a girder (right) in a generalized monocotyledonous leaf. Note that the girder is shown penetrating the parenchymatous bundle sheath.

Girders are defined as structures which interrupt the epidermis, as well as the parenchymatous bundle sheath (at least in cereals), whist strands do not interrupt the epidermis or the bundle sheath.

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Leaf Blade Bundle Anatomy
Amongst the grasses, two anatomical variations are noteworthy - that is, the Panicoid and Pooid (Figs. 6&7) groups.

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Figs. 6 and 7. Line drawings based on electron micrographs of typical Panicoid (Fig.6) and Pooid (Fig. 7) leaf blade bundle anatomy. PS=parenchymatous (Kranz) sheath, BS=parenchymatous bundle sheath; MS=mestome sheath; VP=vascular parenchyma cell.(See Fig. 6.30 in Cutler Botha and Stevenson).

In the Panicoid grasses, the mesophyll is radially-arranged and surrounds a parenchymatous bundle sheath. Panicoid grasses contain dimorphic chloroplasts, with granal chloroplasts within the radiating (Kranz) mesophyll and generally agranal chloroplasts within the parenchymatous bundle sheath cells. Bundle sheath chloroplasts are much larger than the Kranz chloroplasts and lack Rubisco - the Calvin cycle is thus not supported within Kranz mesophyll cells.

Instead, these cells are associated with the initial incorporation of CO2 into aspartate, which is transported to the bundle sheath cells via numerous plasmodesmata. Malate or aspartate is decarboxylated in the bundle sheath and the liberated CO2 is immediately incorporated via Rubisco, into the Calvin cycle. Panicoid grasses are thus C4 photosynthetic species. In many Panicoid species, an additional cell layer exists between the bundle sheath and the vascular tissues below. This layer, which consists of thick-walled lignified cells, is termed the mestome sheath. Ontogenetically, the mestome sheath is derived from the procambium. Mestome sheath cells may either completely surround the vascular tissue, or surround the phloem tissue only within the vascular bundles. The middle lamella between the bundle sheath cells and, in some species that between the mestome sheath cells, contains a suberized layer, termed the suberin lamella. The compound middle lamella has been shown to restrict the movement of solutes, forcing transport (i.e. photoassimilate inwards and water outwards) to take a symplasmic route, via plasmodesmata. The suberin lamella may have important ecological consequences, preventing the excessive movement of water from the apoplast, under conditions of water stress. Pooid grasses, like the Panicoid species, may be associated with a mestome sheath (Fig. 7). Unlike the Panicoid grasses however, the Pooid species do not exhibit chloroplast polymorphism, do not have compartmentalized Rubisco activity and all follow the C3 photosynthetic pathway (Fig. 8.).

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Fig. 8. Electron micrograph, showing a small transverse vein of Saccharum officinarum in transverse view. This vein is surrounded by a suberized bundle sheath and consists of a solitary treachery element (above,) a companion cell (lower right) one sieve tube (center) and a parenchymatous element (lower right). Note hydrolyses tracheary element cell wall bordering the vascular parenchyma cell. back to index

Whilst monocotyledonous foliage leaves are described as being parallel-veined, large numbers of transverse veins exist within the leaf blade. Fig. 8 is an electron micrograph of a transverse vein in such sites, the wall of the parenchymatous element (reported here in Saccharum officinarum) may contain a well-developed suberin lamella, which may have a regulatory role in solute loading and water loss from the xylem to the mesophyll. Transverse veins such as that illustrated here, join adjacent parallel veins, forming effective, interconnected transpiration and translocation pathways. Each transverse vein usually contains a single tracheary  element and a solitary sieve tube, with their associated parenchymatous elements. Although there are some reports in the literature that transverse veins may lack functional sieve tubes, most, as illustrated by S. officinarum, do. The single file of sieve tubes are in direct contact with the tracheary elements. Several files of parenchymatic elements are common, and again, these are in direct contact with the tracheary and phloem elements in these cross veins. Examination of the vein in Fig. 8, reveals that portions of the walls of the tracheary element contiguous to the parenchymatous elements, lacks secondary wall thickening. At such sites, the walls of tracheary and parenchymatous elements appear swollen and loosely-fibrillar. In Zea mays, the distance between transverse veins varies greatly (0.06-1.9mm) but is much greater than the corresponding distance between the longitudinal veins.

The presence of a bundle sheath, which contains a suberized compound middle lamella, ensures that these cross veins retain water and, like the parallel veins, that apoplasmic transport is limited, forcing water and solutes to take a symplasmic route to the surrounding mesophyll. Occasionally however, the transverse veins are discontinuous, and mesophyll or intercellular spaces may be in direct contact with a parenchymatous element of the vein. At such sites, the wall of the parenchymatous element (illustrated here for S. officinarum) may contain a suberin lamella, which as stated, may have a regulatory role in solute loading and controlling influence with respect to water loss from the xylem to the mesophyll.

Leaf Anatomy in the Cyperaceae

(Fig. 9.)

Fig. 9. Line drawings showing the basic anatomical features of leaf blade bundle structure in the Cyperaceae. Notable, are the variation in thickness of the cell walls of the endodermis, and the distribution of chloroplasts in the border parenchyma and the presence of large, agranal chloroplasts in the border parenchyma. Left: C. fastigiatus; C. esculentus; Mariscus congestus. Centre: C. sexangularis; C. pulcher and C. accutiformis. Right: C. albostriatus; C. textilis and C. papyrus. See Fig. 6.32 in Cutler Botha and Stevenson). Click here to go  back to index
The accompanying line drawings (Fig. 9 above) illustrate the variation that has been found amongst local Cyperaceae to date. Like the Poaceae, the Cyperaceae are photosynthetically either C3 or C4 . Like the Poaceae, the phloem within the leaf blade vascular bundles in the Cyperaceae contain two types of sieve tube -- early, thin-walled sieve tubes and late-formed thick-walled metaphloem, the latter generally in close spatial association with the metaxylem, and lacking obvious companion cells. Several anatomical variations are evident when Cyperaceae leaf blade bundles are examined at the electron microscope level. The major differences in Cyperaceae compared to Poaceae, lies in the distribution of the chloroplast-containing parenchyma and in the shape of and thickness of the wall of what has been equated to the mestome sheath of grasses. The leaf blade bundle of C. albostriatus is surrounded by a thickened and lignified mestome sheath, which is devoid of obvious organelles and generally, this layer lacks chloroplasts. The surrounding parenchymatous sheath too, contains few chloroplasts. Suberin lamellae may be present in either outer tangential and/or inner radial tangential walls of this mestome sheath layer. Closer examination of the phloem endarch to the mestome sheath, reveals the presence of chloroplast-containing parenchymatous cells. This layer has been referred to in the literature as 'border parenchyma' (BP, Fig. 10). In contrast the C4 species contain a similar ring of chloroplast-containing parenchyma cells endarch to the thickened, lignified ring (again referred to in the literature as the 'mestome sheath'). However, these chloroplasts are large and obviously agranal. Chloroplast dimorphism , lack of grana in these primary carbon reduction (PCR) cells are indicative of the C4 syndrome. Experimental evidence exists for the positive localization of Rubisco in these large agranal chloroplasts only, which is additive support for the plants being C4.

A possible explanation of the origin of the C4 syndrome in Cyperaceae is illustrated in Fig.10. Fig.10a shows the typical C3 anatomy of many Cyperaceae species, in which the parenchymatous bundle sheath encloses a lignified mestome sheath layer, which, in turn, surrounds the xylem and phloem.
 

The border parenchyma zone (Fig.10b) commonly encircles either both xylem and phloem, or only the phloem. Possibly, the C4 syndrome evolved in Cyperaceae with leaf blade anatomies similar to that depicted in Fig.10b, which would have required the loss of Rubisco activity in the mesophyll (PCA) cells, concentration of Rubisco activity within the parenchymatous bundle sheath (PBS), loss of grana from border parenchyma cells and hence, division of the photosynthetic process into primary carbon assimilation (PCA, producing malate or aspartate) the export of malate or aspartate through the mestome sheath-like cell layer, to the internally located parenchymatous sheath. Anatomically, there are distinct similarities between the C4 Cyperaceae (Fig.10c) and C4 Panicoid (Fig.10d) Poaceae.

One major problem which still requires resolution is the terminology used to describe the layers of lignified cells separating the PCA from the PCR cells - do we call this layer a mestome sheath or not? Its position between PCA and PCR cells seems to dictate that the layer should not be termed a 'mestome' sheath - rather an endodermoid sheath, as it completely encases the underlying PCR and vascular tissue. However, this will not be resolved without a detailed ontogenetic study of leaf blade development (Fig. 10.).

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Fig. 10. Diagrams showing possible evolutionary pathways, leading to the development of the unique arrangement of the C4 subtypes in the Cyperaceae. In C3 Cyperaceae, the vascular bundles are surrounded mesophyll and a Cyperaceae, the vascular bundles are surrounded mesophyll and a parenchymatous bundle sheath, beneath which, is a lignified mestome sheath. Anatomically, C3 Cyperaceae are similar to C3 Poaceae. Many of the Cyperaceae have an additional parenchymatous layer beneath the mestome sheath, which contains granal chloroplasts, forming border parenchyma., thus separating the mestome sheath from the underlying vascular tissues. C4 The evolutionary step to Cyperaceae from the C3, border-parenchyma type, may simply have required the loss of grana, coupled with the compartmentalization of Rubisco in these now agranal chloroplasts. The question which remains unanswered, is , is the lignified layer a mestome sheath or is it an endodermoid sheath? Typical C4 Poaceae bundle anatomy is illustrated above right for comparative purposes.


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