PBIO 115: Fall 2011
Lab 5: The Leaf, and Organ Modifications
INTRODUCTION
Leaves are the main photosynthetic organs of most plants. They typically consist of a petiole (stalk) and a
broad, expanded blade. Some leaves also possess stipules, a pair of leaf-like appendages located at the base of the
petiole. Leaves that lack a petiole (the blade arises directly from the stem) are called sessile. Leaves vary extensively
in their morphology and anatomy. In this laboratory you will examine the external morphology and internal anatomy
of leaves, as well as some interesting stem & leaf modifications.
Exercise A: External Morphology of Leaves
Leaf venation may be parallel (the major veins are somewhat parallel to one another), common in monocots,
or reticulate (the veins interconnect with one another, forming a network), typical of dicots. There are two types of
reticulate venation: palmate, with all of the main veins arising from a single point, and pinnate, with one main
midvein and many smaller secondary veins arising from it like a feather.
Leaves may be simple or compound. Simple leaves have a single blade, while compound leaves are divided into
smaller leaflets. Leaves that are just divided into one set of leaflets are called once compound; when the leaflets are
in turn divided into leaflets the leaves are twice compound, etc. Compound leaves may be palmately compound
(the leaflets arising from one point at the top of the petiole) or pinnately compound (the leaflets arranged along the
rachis like a feather).
The margin of a leaf or leaflet may be entire (smooth), toothed, or lobed.

Examine the plants on display and find examples of: simple and compound leaves; parallel and reticulate
venation (both palmate and pinnate); smooth, lobed, and toothed leaf margins; simple, once and twice
compound leaves; pinnately and palmately compound leaves; and leaves with stipules.
Exercise B: Histology of the Leaf
Leaves are internally modified in many ways depending on the type of environment in which the plant
grows. You will look at leaves from plants adapted to grow in moist (mesic) environments (Syringa), dry (xeric)
environments (Nerium), and aquatic (hydric) environments (Nymphaea).
Part 1. Mesomorphic leaf

Examine a prepared slide of a Syringa (lilac) leaf cross section (pp. 562-563).
Identify the cuticle, stomata, upper epidermis, lower epidermis, palisade parenchyma, spongy parenchyma, bundle
sheath, vein, substomatal chamber, intercellular space, xylem, and phloem.

Examine a prepared slide of a Syringa leaf paradermal section (p. 562). A paradermal section is a section
parallel to the surface of the leaf. Many of the tissues are in cross section, but others, such as veins that are
oriented along the leaf, are in longitudinal section.
Identify the same structures/tissues you identified in the cross section. Look for different types of trichomes.
Part 2. Xeromorphic leaf

Examine a prepared slide of a Nerium (oleander) leaf cross section (p. 563). Identify stomatal crypts,
stomata, cuticle, trichomes, palisade parenchyma, upper epidermis, lower epidermis, veins, and spongy
parenchyma. What modifications does this leaf have that might help it survive in a dry environment?
Part 3. Hydromorphic leaf

Examine a prepared slide of a Nymphaea (water lily) leaf cross section (p. 563). Identify stomata, sclereids,
palisade parenchyma, spongy parenchyma, upper epidermis, lower epidermis, trichomes and veins. What
modifications does this leaf have that might help it survive in an aquatic environment? Where are the
stomata located? Compare their distribution to that in Nerium.
Part 4. Grass leaf

Examine a prepared slide of a Zea mays (corn) leaf cross section. Identify parenchyma, stomata, guard cells
(bordering stomata), xylem, phloem, bundle sheath, epidermis, and bulliform cells.
Part 5. Pine leaf

Examine a prepared slide of a Pinus leaf cross section (p. 415). Identify stomata, transfusion tissue,
epidermis, resin ducts, endodermis, phloem, xylem, hypodermis, and mesophyll. The transfusion tissue is
believed to conduct materials between the mesophyll and the vascular bundles.
Part 6. Epidermis

Make epidermal peels of the two plants on display. A good epidermal peel will be just one cell layer thick
(compare to p. 564).
Look at the shape of the epidermal cells, and then look for stomata scattered among the epidermal cells. The stomata
are bordered by a pair of guard cells, which should be easily visible. The guard cells open and close the stomata to
let gases enter and leave the leaf. Do you see any specialized subsidiary cells around the stomata? Subsidiary cells
are epidermal cells that surround the guard cells and look different from the other epidermal cells.
Exercise C: Leaf Abscission
In areas with extended cold or dry seasons, stems may lose some or all of their leaves. In Athens, we see this
period of leaf abscission in the fall. Before the leaves are shed, reusable substances are transported out of the leaves
and stored elsewhere in the plant for future use. Then an abscission zone develops at the base of the leaf, separating it
from the stem and sealing the scar that will be left when it falls off. Leaf scars serve as a reminder of the phyllotaxy
(leaf arrangement) of the stem even when leaves aren't present.

Examine the demo of leaf abscission in a maple (see Fig. 25-33 on p. 570). The pointer is on the abscission
zone, which is located at the base of the petiole and is differentiated into two layers: the separation layer
(also called an abscission layer) along which the break occurs, and the protective layer, which is composed
of cells with suberized walls. The protective layer forms a leaf scar on the surface of the stem after the leaf
has abscised.
Exercise D: Stem & Leaf Modifications

Examine the specialized stem, leaf and root modifications on display.
In each case, what organ has been modified, and what are the functions of each of these modifications? (Fill in the
blanks with these two answers)
Fleshy stems of cacti and succulents _________________________________________
Fleshy leaves of succulents _________________________________________
Spines of cacti _________________________________________
Carrot root _________________________________________
Potato tuber _________________________________________
Stolons (runners) _________________________________________
Rhizomes _________________________________________
Bulbs _________________________________________
Thorns _________________________________________
Tendrils _________________________________________
Prickles _________________________________________
Pitchers of the pitcher plant _________________________________________
Tanks____________________________________________
Bracts____________________________________________
Cladophylls__________________________________________
Some of these specialized structures are easy to identify as modifications of normal plant organs. Others are not.
Use the information about plant organs that we have been studying in the past two weeks to figure out which
vegetative organ or organs is (are) represented by each specialized structure above. Under each category of modified
structure above, name the organ or organs that are represented by the structure.