Lecture 20

Leaves – Basics and Development

I.  Introduction

 A.  Leaves are extremely variable in plants, but we generally consider a “typical” photosynthetic leaf that has an expanded blade, petiole, etc. 

B.  The basics of leaf shape and morphology.

C.  Variations on leaves, such as between monocots, dicots and gymnosperms are covered in Esau Chapter 19.

II.  Histology

 A.  Like the stem and root, has the same three systems: dermal, vascular and ground.  Secondary growth is uncommon in leaves, but sometimes seen on petioles. Periderm may develop in bud scales.

B.  Epidermis.  Pavement cells with stomata, cuticle, etc.  Covered in Chapter 7.

C. Basic leaf organization sumarized in photos of Syringa in Figure 18.1.  Nice treatment on Wikipedia HERE.  The mesophyll may:
1. be homogeneous (image of Brassica) or differentiated into palisade and spongy mesophyll (Figure 18.2, image of Solanum, image of Ligustrum). Both have abundant air spaces around the cells.
2.  have palisade mesophyll on upper surface (adaxial, dorsal), spongy layer on lower (abaxial, ventral) surface (in bifacial leaves). [note that Esau, p. 323, has the terms ventral and dorsal reversed]. Unifacial (also called isobilateral) leaves have palisade on both sides. 
3. have spongy parenchyma cells that may be branched, forming a 3-D network (Figure 18.2, image of Ligustrum) with lots of intercellular spaces.  Main direction selection is acting here is to increase surface area.

D.  Vascular system. Distributed throughout the leaf blade – closely associated with mesophyll. 
1. Vascular bundles are called veins, pattern venation (Figure 18.3).
2.  Patterns commonly given are parallel Figure 18.4 and reticulate. These are frequently associated with dicots and monocots, respectively (but exceptions exist).  For example, the leaves of the monocot Trillium do not have parallel venation.
3.  In reticulate pattern, primary, secondary and tertiary veins occur (image), the smallest forming meshes delimiting areoles of mesophyll (Figure 18.5, image of Acer leaf, image of Morella leaf). Some veins have free ends – end blindly and don’t anastomose as shown in Syringa (image). Even parallel venation has some degree of reticulation in smaller veins (Figure 18.4). 
4.  Vascular bundle number and form varies a lot across species of plants (Figure 18.6).  This is reflected in the number of vascular bundle scars present in the leaf scars of deciduous trees and shrubs.
5.  Xylem often extends farther than phloem towards the tips of the minor veins (Figure 18.7A), but not always (Figure 18.7B). 
6.  Structure of Cannabis leaflet (image1, image2, Figure 18.8).  Midvein enclosed by ground tissue (not photosynthetic), forming a rib (costa) on the abaxial side (image).  Smaller veins enclosed in mesophyll in intercostal areas. 
7. Small veins also have a bundle sheath (Figure 18.7) – parenchyma and sclerenchyma that extends to the ends of minor veins, thus preventing water loss from the vasculature.
8. There may be bundle sheath extensions, as in Syringa (Figure 18.1, image) or Castanea (image) connecting smaller veins to the upper epidermis

E.  Minor veins and translocation.  Very important because they connect the transpiration stream to mesophyll cells. 
1.  Figure 18.9 of Beta (beet) shows sieve elements (se), companion cells (cc), parenchyma (pa), xylem (x), immature xylem (ix).
2.  TEM of Senecio and Armeria (Figure 18.10 A and B).  Mesophyll cells have dense cytoplasm in companion cells and parenchyma, called transfer cells (a in 18.10 A&B).  Cells are involved in loading minor veins with photosynthates – intermediary cells (ala Fisher) in Figure 18.9.  Leaf begins as a sink (obtaining photosynthates) to sustain growth and then later converts to a source where it exports photosynthates.  This is when transfer cells form.
3. Diagram showing apoplastic and symplastic movement of solutes and water (Figure 18.11).  As the solute (sugar) is loaded into the sieve element, water enters from apoplastic free space and increases the hydrostatic pressure, thus causing the sap to move in the phloem sieve tubes. Solutes are removed at the sink cells elsewhere via active transport. This is the Bulk Flow model (diagram).

III.  Leaf Development

 A. Initial leaf primordia. 
1. Chapter 18, shows leaf initials forming as protrusions at apical meristem. Result from periclinal cell divisions in subsurface cells followed by anticlinal divisions in protoderm.
2.  Process for Linum (Figure 18.12) and for Hordeum (Figure 18.13). 
3.  Process for Oenothera (Figure 18.14) where leaves have stipules.
4.  Generalized diagram for leaf growth in a dicot (Figure 18.15).  Apical growth (increases length), marginal growth (increases width).

B.  Concepts for ontogenesis
1.  Derivation of leaf cell types from specific initials – two models shown in Figure 18.16.  These are not well supported by empirical data (e.g. chimera studies).
2.  Intercalary growth (vs. just apical and marginal) occurs – cell divisions more random (Figure 18.18, Figure 18.19).

C.  Growth of leaf blade.
1. Leaf composed of several self-perpetuating layers.  Plate meristem – produces parallel layers, cells dividing anticlinally – gives rise to majority of intercalary growth and results in the major increase in leaf size.
2.  Periclinal divisions establish the number of layers (Figure 18.17 B, D), anticlinal divisions extend the layers (Figures 18.17 A, C).
3.  Proportions of the cell division types for marginal and plate meristems (Figure 18.20)

D.  Variation in development patterns
1.  Compound leaves as in Pelargonium alternans (Figure 18.21).  Marginal growth localized in areas that will give rise to leaflets.  Individual leaflet development follows pattern seen in simple leaves.
2.  Unifacial and bifacial leaves (Figure 18.22).
3.  Palm leaves such as Cocos (Figure 18.23). Abaxial and adaxial marginal panels form on leaf primordium. These panels are at first smooth but later become corrugated.  The corrugations composed of interdigitating plications (folds).  Individual leaflets develop and an extension of the rachis separates them. How the folds develop is controversial – composed of two processes, differential growth and separation.

E.  Mesophyll differentiation.  Figure 18.24. 
1. First anticlinal divisions in future palisade cells occurs.  This happens also in the spongy layer, but less frequently. 
2. Epidermal cells expand, end up with several palisade cells connected to them (Figure 18.24 C).
3. Then the palisade cells separate from one another, preceded by spongy mesophyll cells – all generating the intercellular spaces.

F.  Development of the vascular tissue.  Starts with the differentiation of the procambium (Figure 18.14 F, G) while leaf is still very young.  Procambium differentiates acropetally, but the leaf veins mature basipetally.  Phloem differentiates acropetally – from largest to the smallest veins.  In grasses, large veins acropetal, small veins basipetal.

IV.  Abscission

 A.  Definition: active separation of a leaf, flower, fruit from a stem without injury.  The site where this happens is called the abscission zone (Figure 18.25, image1, image2 of Ulmus.

 B.  Two layers involved:
1.  abscission (separation) layer. Weak cells, less sclerified tissue, with vascular bundles towards the middle, not the periphery (as in stems).
2.  protective layer. A cicatrice is formed via deposition of suberin and wound gum in cell walls and intercellular spaces.  Later may be replaced by periderm, connected to periderm below.
3.  Auxin can deter abscission, ethylene can drive it to completion. Peroxidases and phosphatases also involved.  Cell walls are degraded by enzymes.  Middle lamella is reduced or lost, tracheary elements ruptured.
Last updated: 14-Oct-22 / dln