The Leaf: Site of Photosynthesis & Transpiration
The leaf is arguably the most diverse and specialized plant organ across taxa. It was the most recent plant organ to evolve, and is not found in the most primitive plants. The leaf is the main site of photosynthesis and gas exchange in most plants.
External Anatomy of the Leaf:
leaves lacking a petiole, and attached directly to the stem at the base are said to be sessile
In most monocots, the leaf base is expanded to surround the point of stem attachment, forming a sheath. Monocot leaves lack a petiole, yet another characteristic that links this monophyletic group.
Phyllotaxy is arrangement of leaves along the length of the stem at the nodes.
- helical - one leaf per node, with each one slightly offset from the previous so that the leaves form a spiral along the stem. (What do you suppose is the advantage to this arrangement?)
- distichous or alternate - one leaf per node, offset in opposite directions, alternating. Typical of grasses.
- opposite - two leaves per node, set in opposite directions
- opposite and dicussate - two leaves per node, set in opposite directions, with successive leaves at each nodes arranged at right angles from each other. Typical of members of the mint family, like this basil...
- whorled - multiple leaves per node, set in a spiral.
Individual Leaf Morphology
simple leaf - a single blade, not divided into sections. These can come in several shapes, for example...
lobed leaf - the margin of the leaf is indented, but not all the way to the midrib. For example...
compound leaf - leaf blade is divided into leaflets, all the way down to the midrib (which is called the rachis in a compound leaf). These can be...
- pinnately lobed - feather-shaped
- palmately lobed - palm-shaped
- pinnately compound - feather-shaped leaf composed of leaflets
- even - even number of leaflets
- odd - odd number of leaflets
- doubly pinnate - leaflets are divided into leaflets (can be even or odd)
- palmately compound - palm-shaped leaf composed of leaflets
Other diagnostic characters of leaves include
- leaf margin morphology
- leaf venation pattern
Internal anatomy of the leaf:
From top surface downward, note the...
cuticle - a noncellular, waxy sheet of cutin secreted by the epidermis
epidermis - this one-cell thick layer of "skin" tissue lacks chloroplasts.
palisade mesophyll - layer of photosynthetic parenchyma (chlorenchyma) of a tall, columnar shape.
spongy mesophyll - second layer of photosynthetic parenchyma (chlorenchyma), this one with copious air spaces to facilitate transpiration.
vein system (xylem on top; phloem on bottom)
lower epidermis - contains most of the stomates
lower cuticle - usually a thinner layer than the upper layer
Here's what an actual cross section micrograph looks like...
Metabolically active and often composed of diverse cell types, this is the plant's first line of defense against environmental insult.
generalized epidermal cells
The typical epidermal cell is flattened and elongate, though the ratio of length to width may vary across species. Note the lack of chloroplasts in the cells!
The epidermal cells secrete and are covered by a waxy cuticle, shown here from above, and in cross-section:
Because of its tight structure and the waxy cuticle, the epidermis provides strength and form to the leaf.
Gas exchange pores, stomates are located primarily on the underside of leaves in most plant species, but may also occur on the top. In some species, stomates occur only on the upper surface. (What type of plant would you expect to have stomates on the tops of leaves only?)
Opening and closing of stomates occurs in response to light conditions, environmental humidity, and water content of the plant body in general. (More on this when we talk about water movement through plants.)
hydrophyte - plant evolved to thrive in very wet conditions
xerophyte/xeriphyte - plant evolved to thrive in very dry conditions
mesophyte - plant evolved to thrive in moderate conditions (with respect to water)
Would you expect a xerophyte to have relatively few, or relatively many stomates, compared to a mesophyte?
What water-saving epidermal features do xerophytes have?
numerous trichomes (some with secreted resins)
thick waxy cuticle
few upper surface stomates
stomates recessed into grooves on the leaf surface
Mesophyll is the leaf's ground tissue, composed primarily of parenchyma containing a great density of chloroplasts (hence, this is chlorenchyma).
Pallisade Parenchyma - vertical, columnar cells closest to the upper epidermis. These have more chloroplasts than the rest of the mesophyll, and most photosynthesis seems to take place in the pallisade parenchyma.
Spongy Parenchyma - these are typical, box-shaped cells, with much more air/fluid space between them than the pallisade cells. Fewer chloroplasts, but photosynthesis still takes place here. This is the site of water and gas exchange, with the air spaces in the spongy mesophyll connected directly to the stomates.
The Vascular System
The vascular system is distributed throughout the mesophyll, and is visible on the surface of the leaf as the venation pattern.
A cross section of the petiole reveals that the phloem is on the bottom of each vein, and the xylem is on top (picture it as peeling away from the stem as it grows outward from the stem). Both are primary, and do not develop from the vascular cambium.
major veins - readily visible, associated with the "ribs" (protruding ridges on the undersides of the leaf)
minor veins - netted throughout the mesophyll, these are responsible for most of the collection of photosynthate directly from the photosynthetic parenchyma, and join the major veins at various junctions.
Major veins are not in contact with photosynthetic cells or to the intercellular spaces of the mesophyll, as they are surrounded by dense parenchyma.
Minor veins are surrounded by small cylinders of dense parenchyma that form a bundle sheath around each vein. Bundle sheaths complete enclose each minor vein, all the way to the tip, and act much the way the endodermis of the root does: all photosynthates and other materials must pass through the sheath in order to get to the vein for transport, so the sheath cells act as a selectively permeable barrier, controlling the substances that the vascular system transports from the leaf.
In some plants bundle sheath extensions travel from the tips of the veins to the epidermis, and may permit the transport of water from the xylem to the leaf epidermis.
Where Do Baby Leaves Come From?
Leaf primordia are initiated by masses of cells at the edges of the apical meristems, and includes all three primary meristems. These cells are called the founder cells of the leaf, and they give rise to the budding leaf primordia, as shown below.
Leaf Modifications and Specializations
Leaves show the greatest diversity of modification of all the plant organs. An overview of some of these follows, but you will encounter leaves that will amaze and astound you beyond what you see here.
sporophylls - specialized for reproduction
leaves modified for high rainfall - drip tips
leaves modified for very dry environments
leaves modified for extreme environments (wind; cold)
leaves modified for carnivory
Leaf structure and shape is intimately related to the environmental conditions under whch the plant evolved, but also in which it has developed. The genetic plasticity of plant organs is clearly reflected in such things as variable shapes, pigmentation and sizes of leaves and stems on the same plant that grow in sun versus shade, or other variable environments.