# 2: The Leaf: Functional relationships

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Cell arrangement within leaves

No matter whether the leaf is from gymnosperm or angiosperm origin, developmental sequences in these groups  are superficially similar.. Clearly, the differentiation of leaves follows a strictly-controlled and sequenced series of events with marginal, submarginal and procambial initials and their derivatives forming the dermal tissue system, the mesophyll and the vascular system within the leaf.  It is very important to have a clear understanding of vein maturation within the leaf itself, and to understand that much of what happens during the maturation sequence, is governed by growth phases and the transition form sink to source. This is discussed in detail elsewhere in this Factfile.

Procambium will differentiate in all cases to form the vascular tissues - xylem and phloem, which is  arranged in vascular bundles of differing size and complexity. In dicotyledons, the veins within the lamina may be classified as being either major or minor. Major veins may be associated with mechanical supporting tissues, and will displace the mesophyll tissues in all C3 leaves. In C4 dicotyledonous plants, of course, the mesophyll is radially-arranged around the bundle sheath, as in the C4 monocotyledons. Minor veins are generally not associated with mechanical supporting tissues, and are usually situated between the palisade and spongy mesophyll in mesomorphic leaves. Position obviously varies in non mesomorphic species. Some have sclerenchyma caps or even complete sclerenchyma sheaths, (this is particularly so in monocots) but they will rarely be associated with girders

In monocotyledons the leaf blade contains large, intermediate and small veins, apart from the marginal bundles. Wide (also described as large in some texts) veins are not supported by mechanical tissues; contain (when mature) at least one protoxylem lacunae; wide metaxylem vessels as well as other tracheary elements, including xylem parenchyma; and phloem, which consists of parenchymatous elements, including companion cells and phloem parenchyma.

The conducting phloem in mature bundles contain functional early (thin-walled) and late (thick-walled) metaphloem sieve tubes. Intermediate veins lack large metaxylem vessels and protoxylem lacunae Hypodermal sclerenchymatous (or collenchymatous) strands or girders whose function clearly is support, may occur on either or both surfaces of the large and intermediate vascular bundles. In addition to lacking wide metaxylem vessels and protoxylem lacunae, the small veins are not associated with hypodermal strands or girders.

The vasculature in dicotyledons is described as a reticulate, anastomosing network, and in monocotyledonous as parallel, although monocotyledonous plants do have large numbers of cross-veins, interconnecting the large, intermediate and small veins. The phloem within the leaves of monocotyledonous plants, contain two distinctly different functional phloem types - early metaphloem and late (thick-walled) metaphloem. The former are associated with companion cells, the latter with vascular parenchyma, and are apparently isolated from the rest of the functional phloem to varying degrees.

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The Interplay of Transport Systems

Fluorescence image of a young Pea leaf, showing strong sink activity and offloading of a phloem-mobile fluorescent dye, 5,6-carboxyfluorescine Image by Dr. O.E. Ade-Ademilua

Maturation of the vascular tissues in leaves results in the tips of leaves being mature before their bases. This may be true but to a lesser extent in dicots. Thus, export of assimilates from the apical regions, and import to the basal regions in the same leaf is not unlikely. That there is an interplay between the xylem and phloem, should be fairly obvious to any anatomy student. Witness the close, spatial arrangement of the xylem and the phloem. In addition, there is a close spatial relationship between the xylem and the apoplast and free space, as well as between the phloem and the interconnected symplast. By definition, the apoplast is that region of the plant, which is outside the plasma membrane system - in other words, the cell wall and the intercellular (free) space.

The symplast in contrast is that region of the plant bounded by the plasmamembrane with interconnected cells often linked by plasmodesmata. In contrast the apoplast forms a continuum which excludes the symplast. Cell walls and intercellular spaces form the apoplast. It may be partly disjointed in places.

Functional phloem is dependent upon functional xylem and transport within the phloem is dependent upon free available water which is input to satisfy the rather high pressure and osmotic potential differentials generated at the phloem loading interfaces. This explains the rather close relationship between the xylem and the phloem in vascular bundles.

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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 mostly with dorsiventral symmetry. The generally flattened shape is 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.


Fig. 1. Ontogenetic relationship between the marginal and submarginal initials during leaf development. Based on the diagram in 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 and what follows is simply a completion of a process.

Fig. 3. Illustrates the acropetal differentiation of the major vein network in a typical dicotyledonous leaf.

It is clear that leaf development is a complex process - one which involves differentiation of epidermal and sub-epidermal layers. It is the sub-epidermal layers, more specifically, the mesophyll and its associated vascular system, which requires careful examination of monocot and dicot foliage leaves, as well as good comparisons to gymnosperm leaves and leaf-like structures, which will allow us to focus on the structure-function relationships that have to be evolved, in order to allow efficient transport of solute and solvent.