# 3: The leaf - structure function relationships
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|Fig. 1||Fig. 2||Fig. 3||Fig. 4||Fig. 5||Fig. 6 & 7|
|Fig. 8||Fig. 9||Fig. 10|
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.
|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|
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Each of these is either initiated or takes place directly in the mesophyll of leaves.
|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
It is possible to separate Angiospermous from Gymnospermous leaves, by using some basic diagnostic criteria.
|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
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.
|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.|
]Fig. 1. Illustrates the general mechanical requirements of a typical leaf. Adequate gas exchange, and functional transport pathways are essential.
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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.
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.) .
(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.
Major Vein Differentiation
The Minor Veins
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.).
Fig. 4. Diagram illustrating the differences between major and minor veins in a generalized dicotyledonous foliage leaf.
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:
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.
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
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.
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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. 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 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.
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