Lecture 20

Leaves – Structural Variation

I.  Leaf Structure and the Environment

A.  Assumption that there is a one-to-one association between plant structure and adaptation is false.  Environmental selection pressures have resulted in many “solutions” (character combinations) for different groups of plants.  Examples for habitats deficient in water:
1.  Succulence; loss of leaves, stems store water, are photosynthetic. Examples: cacti, some Euphorbia, some Apocynaceae such as Stapelia.
2.  Deep roots.  Example: Welwitschia.
3.  Underground storage organs.  Examples: many South African monocots, such as this Homeria (Iridaceae).
4.  Short life spans. Spring ephemerals in the western U.S. These annuals come up from seed, flower and produce fruits and seeds within a few weeks.  Example: Dendromecon (Papaveraceae).

B.  Water availability strongly influences leaf morphology and anatomy
1.  Xerophytes. Plants adapted to dry environments (xeromorphic = structural features of a xerophyte).
2.  Mesophytes. Plants adapted to habitats that are neither too wet or too dry (mesomorphic).
3.  Hydrophytes. Floating, emergent, or submergent plants (hydromorphic).

C.  Xeromorphy (Figure 19.1)
1.  Leaves with high volume / surface ratio (i.e. small and compact)
2.  Palisade mesophyll strongly developed
3.  Small intercellular spaces in mesophyll
4.  Compact veins, with few bundle sheath extensions
5.  Succulence – water-filled cells. But also a response to low N and salt spray (Baccharis, Figure 19.2 A & D).
6.  Trichomes.  Function in reflectance of solar radiation, insulation against heating up, reduce air flow, hence evapotranspiration.
7.  Thick epidermal cells and thick cuticles
8.  Stomata – present mainly on abaxial side of leaf, sunken (cavities, crypts, grooves)
9.  Presence of a hypodermis (with few or no chloroplasts)
10.  Abundant sclerenchyma. Affects leaf reaction to wilting stress.

D.  Light.  Sun and shade leaves.  Figure 19.2 C-E for Acer platanoides.

E.  Leaf morphology and position on plant. Developmental / ontogenetic differences.  Juvenile and adult leaves differ. Examples:
1.  Fragaria and Fraxinus – juvenile leaves simple, adult leaves compound.  Opposite situation for Acacia.
2.  Ulmus – number of marginal teeth on leaf increases with age
3.  Ulex – typical leaves in juveniles, spines in adults
4.  Lathyrus – “normal” leaves in juvenile, tendrils in adults
5.  Hedera – not only is lobing different on juvenile and adult leaves, phyllotaxy different too.

F.  Many leaf modifications
1. Cotyledons - seed leaves (different from vegetative leaves), variable in number. Amaranthus has the typical number of "dicots" (2), but some angiosperms like this Psittacanthus mistletoe have many cotyledons.
2. Spines - sharp-pointed. May be a leaf or stipule modifications. Stipular spines in Acacia.
3. Stipules - leaf-like outgrowths at the base of leaves. Stipules on Prunus.
4. Cataphylls - leaves inserted at low levels on shoot system.  Examples include bud scales that are present in most temperate woody plants, rhizome scales, protective scales around the bud of cycads. Cataphylls on Pontederia, on shoots of mistletoe Phoradendron.
5. Bracts - extremely variable in shape, position, etc. Bract on inflorescence of Medinilla (Melastomataceae).
6. Scales - common in conifers such as Juniperus, dwarf mistletoe (Arceuthobium); condition with scale leaves is called flabellate.
7. Tendrils (may also be modified branches) usually modified leaves or leaflets. Positively thigmotrophic, help plant climb by means of twining. Tendril as modified leaflet in Vicia, as a modified leaf in Cissus.
8.  Cladophylls - enlarged petiole, e.g. Acacia.
9.  Insectivorous plant leaves - pitchers (e.g. Sarracenia and Nepenthes), snap-trap (Dionaea), and sticky via glandular trichomes (Drosera).

II.  Dicot Leaves

A.  Mesophyll variation
1. Undifferentiated mesophyll (mostly spongy). Pisum, Linum, Lactuca, Brassica.
2. Differentiated mesophyll - palisade on adaxial side (bilateral).  Vitis, Syringa, Ligustrum, Solanum, Tilia.
3. Differentiated mesophyll - palisade on adaxial and abaxial sides (isobilateral). A xeromorphic adaptation.  Artemisia, Atriplex, Chrysothamnus, Olea, Sarcobatus and many others. In succulent leaves (Figure 19.1C), palisade forms a continuous layer.
4.  Aquatics.  Submerged leaves are often highly dissected.  Mesophyll with many air spaces. Figure 19.6A for Nymphaea, Potamogeton.

B.  Supporting tissues.  Tissues along the veins may be collenchyma or sclerenchyma; e.g. bundle cap (on vascular bundle in cross section, e.g. Ranunculus), bundle sheath and bundle sheath extension (as in Rosa).

C.  Petiole
1.  Basically has same tissues as the stem
2.  Some plants have a pulvinus on the petiole or petiolule of compound leaves.  Involved in movements of the blade in response to light / dark or touch.
3.  Many Fabaceae have pulvini, e.g. Mimosa pudica (sensitive plant - Wiki page here).  Actually has primary, secondary, and tertiary pulvini. Figure 19.4 shows the anatomy of these. Reaction involves K+ flux across membranes. Cells on adaxial side wrinkle when leaf / leaflet fold. A lot of YouTube videos on the plant, but here's one (other than the anthropomorphic expressions like "how plants learn", it's generally accurate).

II.  Monocot Leaves

A.  General features
1.  Some with blade and petiole, many with blade and sheath (grasses and sedges)
2.  Hydrophytes - abundant aerenchyma.
3.  Carex (sedge) has large air cavities with thin-walled cells inside (Figure 19.5).
4.  Iris - leaves are equitant (= ensiform) image1, image2. The leaves are unifacial above, bifacial below.  Vascular bundle orientation shown in Figure 19.6 D,E.
5.  Allium - leaves are tubular. Image of A. canadense, young leaves emerging from vegetative bulbils.  Figure 19.6 B,C

B.  Grass leaves
1.  Blade and sheath enclosing stem, with auricles and ligules (grasses such as Bromus)
2.  Vascular bundles of different sizes alternate (Figure 19.7, Figure 19.8), connected by commisural veins.
3.  No differentiation into palisade and spongy mesophyll.  Oriented with ends of cells against bundle (Figure 19.7)
4.  Stomata, subsidiary cells, silica cells, cork cells all discussed in Chapter 7.
5.  Bulliform cells and / or hinge cells on epidermis. Involved in rolling the leaf under dessicating conditions or when developing from a bud (Figure 19.9). Image of Ammophila leaf cross section.
6.  Festucoid grasses have two layers of bundle sheath cells - the inner called the mestome (Figure 19.10 B&C) which is equivalent to an endodermis (has suberized lamella on walls).  This inner layer is absent in Panicoid grasses (just one layer of thick walled cells).

C.  Grass leaves and photosynthesis
1.  Two ways to fix carbon in plants: C3 (Calvin-Benson cycle), C4 (Hatch-Slack pathway). Wiki page. CAM metabolism (Wiki page) is similar to C4 in that it concentrates CO2 around Rubisco, increasing its efficiency.  But CAM plants do this temporally, not spatially, by opening their stomata at night, allowing CO2 in.  It is fixed as an organic acid, stored in the vacuole, and released during the day when it enters the Calvin Cycle.
a.  C3 (3-phosphoglycerate) is the first product of photosynthesis. But under warm conditions or low CO2, the C3 cycle is more sensitive to photorespiration. Here Rubico picks up O2 instead of CO2, so the substrate is oxidized instead of carboxylated.
b.  C4 (oxaloacetate) is the first product of photosynthesis. CO2 initially enters the mesophyll cells surrounding the bundle sheath where it is turned into oxaloacetate by PEP carboxylase. The oxaloacetate is transported into the bundle sheath cells and broken down, liberating CO2, which is fixed (turned into glucose) in the Calvin cycle. Thus, the bundle-sheath cells isolate Rubisco from atmospheric oxygen and provide an environment saturated with the CO2.
2.  C4 occurs in plants from warm (tropical) regions; occurs in many families, about half the grasses.
3.  C4 metabolism impacts many areas: physiology, anatomy, ecology, evolution, agriculture.
4.  Anatomy differs between C3 and C4 plants.  C3 bundle sheath with fewer, smaller chloroplasts (relative to mesophyll cells). C4 bundle sheath with many large chloroplasts, also many mitochondria and microbodies, more numerous starch grains, reduced grana. Figure 19.10A , image (Zea), Figure 19.11 (Cynodon).
5.  The tight association of the bundle sheath to the vascular bundle is seen when the latter are isolated - the bundle sheath comes along with it. Figure 19.12.
6.  Degree of concentric layering of mesophyll and bundle sheath varies across grass species, but more pronounced than in the C3 grasses.  This wreath-like arrangement is called Kranz anatomy.

IV. Gymnosperm Leaf Variations

A. Less variable than dicot leaves.  May be broad like angiosperms (Gnetum, Podocarpus), needles (Pinus, Picea), scales (Juniperus).

B.  Most are evergreen, but not all.  Exceptions include Gnetum, Ginkgo and conifers such as Larix and Taxodium.

C.  Leaf structure in Pinus - a plant adapted to dry enviroments (Figure 19.13)
1.  Needles from short shoots in groups (fascicles); may be one or several per fascicle. Pinus strobus (easten white pine) with five needles per fascicle - image.
2.  Shape is either round or triangular. Pinus resinosa leaf X.S. image.
3.  Epidermis thick-walled, with a heavy cuticle, sunken stomata, overarching subsidiary cells (see Figure 7.8). Stomata on all sides, in rows.
4.  Sclerified hypodermis below the epidermis - image.
5.  Mesophyll cells plicate (with folds and ridges) protruding into the lumen of the cell.
6.  No palisade parenchyma - only spongy.
7.  Resin ducts (or canals). Pinus resinosa image. A mechanism for retaining water (present in virtually all tissues of the plant). Also deter pathogens and herbivores, prevents freezing, and help absorb heat.
8.  Vascular bundles may be one or two side by side.  Xylem consists of protoxylem and metaxylem as well as xylem parenchyma alternating with tracheids.  The phloem appears in regular vertical stacks. Pinus resinosa image1, image2.
9.  Vascular bundles surrounded by transfusion tissue composed of tracheids and parenchyma. Figure 19.13 B&C.
10.  Vascular bundle and transfusion tissue surrounded by a thick-walled endodermis with Casparian strip - image.

D.  Other conifers.  Figure 19.14 (Larix, Tsuga and Abies).
1.  Hypodermis absent in conifers like Taxus but five layers thick in Araucaria.
2.  Palisade and spongy mesophyll present in Abies, Cunninghamia, Sequoia, Taxus, and Torreya.  Palisade on both sides in Araucaria, Podocarpus.
3.  Endodermis not as clear as in Pinus for others (just parenchymatous sheath).
4.  Position of transfusion tissue relative to vascular bundle variable
5.  Cycas.  Figure 19.15A.  See text for description.
6.  Ginkgo. Figure 19.15B.  See text for description.
Last updated: 14-Oct-22 / dln