Leaves, physical factors, and adaptations Lab.
Today’s lab has four main parts, all of which have to do with a plant’s various
ways of adapting to their environments and the pressures, both biotic and abiotic,
that they encounter.
 Part one is about leaves-types and internal structure.
 Part two is about adaptations to living in wet, salty, and dry habitats.
 Part three is about sun/shade leaves
 Part three is about spices
PART I. LEAVES
In this part of the Lab today, you will be doing the following
• Internal structure of a mature leaf
• External morphology of a leaf
• Leaf types
• Specialized leaves
NOTE the following:
• Vascular bundles with xylem and phloem
• The midrib
• Cuticle (is there one?)
• Epidermis (upper and lower)
• Guard cells and stomata
• Spongy parenchyma (mesophyll) and palisade parenchyma (mesophyll), and air spaces
• Where are the stomata located? Why?
• Collenchyma cells
• Trichomes
• What is different about a conifer (pine) leaf?
• External morphology: know how to classify type of leaf as well as if it is monocot or
eudicot:
o
o
o
o
o
Petiole
Blade
Branch buds
Simple leaves: palmate, pinnate
Compound leaves: pinnate, palmate, double compound
• Types of specialized leaves
Internal structure of a mature leaf
1. Obtain a living Eudicot leaf and look at it under the dissecting microscope.
Where are the stomata? On the upper or lower surface? Or both? Try several
different kinds of plants/leaves.
2. Examine a slide of a cross section of a lilac leaf under the low power. Note the
vascular bundles throughout and the difference in size from the very large
central midrib cut across to lesser sized bundles cut both across and
longitudinally.
3. At a higher power locate the cuticle, upper and lower epidermis, guard cells,
palisade parenchyma (or palisade mesophyll), spongy parenchyma (or spongy
mesophyll) with air spaces. In a vascular bundle locate the xylem and phloem
and in the midrib the surrounding and reinforcing collenchyma. The smaller
bundles also have a small bundle sheath. The epidermis bears mushroom shaped
"hairs" called trichomes. Do not confuse them with guard cells. Some trichomes
are hairs, while others specialize in secretory functions (like salvias – smelly!)
4. Study the slide of a cross section through a pine needle (there are often two
together on the slide) and note the heavily cutinized epidermis, the sunken
stomata, (What are these adaptations for?) the cortex, (equivalent of mesophyll)
and the central vascular bundle of xylem and phloem. The vascular bundles are
surrounded by a layer of heavy–walled cells called endodermis which prevents
excess water loss from the vascular bundle.
External morphology and leaf types
1. Examine the various leaves provided and note the petiole or stalk of each and the
expanded blade or lamina. At the base of the petiole there may be leaf-like
stipules. Some will have branch buds in the axils (between the leaf and the
stem). Note that some are simple, that is, one blade per petiole, others are
compound and have several leaflets per petiole. Each leaflet is attached by a
petiolule. The compound leaves may have the leaflets arranged on either side of
the petiole and are said to be pinnately compound or the leaflets may radiate out
in several directions from the tip of the petiole and be palmately compound. The
vascular tissue in the leaf is visible as veins throughout the leaf. Note the
different patterns of venation. Most monocots have parallel venation and most
eudicots have netted venation which may predominantly be palmate or pinnate.
Note that leaves are arranged differently on the stem in different species. They
may be opposite, alternate or whorled. The arrangement is called phyllotaxis.
2. Specialized Leaves
Note the following:
• "Needles" of Pine trees which are typically borne in bundles on very short branches
• Needles of other kinds of conifers
• Scale-like leaves of the Cypress which overlap
• Bud scales
• Leaves reproducing the plant asexually in the Kalanchoe
• The leaves of the insectivorous plant which digest small insects
• The red "flower" of the Poinsettia are really showy red leaves or bracts around the
small inconspicuous flower. The same is true of the "petals" of the dogwood
"flower". These white structures are leaves.
Part II. PLANT ADAPTATIONS: IDENTIFYING XEROPHYTES,
MESOPHYTES AND HYDROPHYTES
Water is one of the basic raw materials of photosynthesis. It is the major component of
plant tissues, making up 90% of the plant body. Water is the substance in which most
materials enter and leave the cells of plants, and it is the solvent for the biochemical
reactions which occur in living cells.
The amount of water used by plants is much greater than that used by animals because of
transpiration. Over 90% of the water waken in by the root system is evaporated into the
air as water vapor. Consequently, plants have developed extensive and efficient transport
systems and numerous morphological adaptations to conserve water. In this exercise,
you will work with a partner to try to determine the habitat of various "mystery plants."
Many types of habitats can be found in nature with respect to water supply. These can
be divided into xeric, mesic, and hydric habitats. The plants that are adapted for living
in these habitats are called xerophytes, mesophytes, and hydrophytes, respectively.
Xerophytes include a number of species that live in habitats where the supply of water is
deficient. Mesophytes inhabit regions of average water conditions, and include the
majority of wild and cultivated plants. Hydrophytes form an extensive flora living on
the surface of water or submerged at various depths.
In this exercise, you will examine prepared slides of plant tissues and attempt to
determine whether a plant is a xerophyte, a mesophyte, or a hydrophyte based on its
visible characteristics. You should base your decisions on the following list of
adaptations.
The following categories are not always sharply defined….
Your task, in part, is to be able to distinguish these types by their characteristics
Mesophytes are plants that generally grow in regions of moderate water supply.
They are the common plants of temperate forests, grasslands and medows. Sometimes
they are differentiated into ‘sun’ and ‘shade’ forms. The majority of flowering plants are
mesophytes. Examples include bean, tobacco, aster, geranium, tomatoes, lilacs, maple,
most crop plants, fruit trees, and typical flower garden plants.
Procedure: DRAW! examine a prepared slide of a cross section of Syringa (lilac). Start
at low power and then move to higher power. Note the size and position of the veins.
Can you find the vascular cambium in the midvein (secondary xylem towards the upper
leaf surface and secondary phloem towards the lower surface)? Find the upper
epidermis and cuticle, palisade and spongy parenchyma, lower epidermis and cuticle,
and stomata. Look for any hairs (trichomes) too. Is there a difference between the
upper and lower epidermis? Where are the stomata and why? Is there water storage?
Halophytes are plants that grow in salty soils or water where the high salt
concentrations make water osmotically unavailable to them. They live in essentially
physiologically ‘drought’ conditions, and resemble xerophytes.
Procedure: DRAW! If we have any of these, take a look at what we have. Many will
look like succulents. Why? How do halophytes resemble xerophytes and why?
Hydrophytic adaptations (aquatic plants, either floating or submerged): The chief
modifications exhibited by hydrophytes are an increase in leaf surface, the presence of air
chambers (will look like empty spaces), and a reduction in protective, supportive, and
conductive tissues. Stomata, if present, are on upper surfaces only as that is the surface
exposed to air. These are plants that grow in wet habitats.
Procedure: DRAW! Examine a slide of the water lily (Nymphaea) leaf. Notice the upper
and lower epidermis and location of the stomata. Identify the xylem and phloem in the
leaf. Identify the spongy and palisade parenchyma. Look at the size and location of the
intercellular spaces. Identify the thick sclerids in the spongy parenchyma. How is this
leaf adapted for life on the water?
Examine the Elodea plants. Lift one out of the water. What happens to it? What
conclusions can you draw about the supportive (mechanical) tissues in this plant? Are
roots present? Why/why not?
Xerophytic adaptations (desert plants, including cacti and succulents):
The
chief modifications exhibited by xerophytes are sunken stomata, hairy leaves, thick
protective cutin, very well developed supporting tissues. Some have leaves capable of
rolling up tightly and others possess large amounts of water storage tissues. (These are
called fleshy xerophytes.)
Procedure: DRAW! Examine the various examples of xerphytic plants. What external
features are shared by many? Make a list of adaptations you see. Look for trichomes
and their roles here. How many different kinds do you see? Use the dissecting scope.
Look at fresh Salvia under the dissecting microscope. Find the trichomes.
Examine a prepared slide of an Oleander leaf. Notice the upper and lower epidermis and
location of the stomata. Identify the xylem and phloem in the leaf. Identify the spongy
and palisade parenchyma, and the size and location of the stomata. Do you see
trichomes or appendages?
PART III: Sun and shade leaves, and high efficiency leaves
The leaves of a plant, particularly large plants such as trees and shrubs, are not all equally
exposed to sunlight. For example in a tree, leaves in the crown of the tree (top) receive
far more sunlight than those on the lower, shaded, branches. The outer leaves are called
sun leaves, and the shaded ones are called shade leaves. More water is lost by sun
leaves than shade leaves due to the exposure to sun and heat. The amount of water
affects leaf growth – the more water the more growth. The amount of light also
influences the development of the palisade parenchyma.
Procedure: DRAW! Examine examples of sun and shade leaves. Note the differences in
external morphology (compare two leaves from the same plant if possible). Examine a
prepared slide of a comparison of sun and shade leaves of the same species (we have
pear). Compare the width of the leaf, number of palisade parenchyma cell layers,
number and size of veins, and thickness of the cuticle.
High efficiency leaves: Among the most important crop plants are the monocotyledonous
plants in the Poaceae (grass) family (such as sugar cane, corn, and sorghum). These are
known to be very efficient photosynthesizers, especially at high light intensities and high
temperatures. They can achieve photosynthetic rates 50% higher than many other plants!
Many are found (and originated) in the tropics. The leaf morphology of these plants
differs from many you’ve looked closely at. Examine a slide cross section of Zea (corn).
Have a scope with a cross section of Lilac ready to compare). Find the palisade and
spongy parenchyma. Compare the number of leaf veins to the Lilac. Look at the
bundle sheath cells surrounding the vein in the corn, and notice the large chloroplasts in
them. These chloroplasts are specialized in starch storage. Look at the large bubble-like
cells in the epidermal layer. These are called bulliform cells. They lose water more
quickly than other epidermal cells. This causes the leaf to curl and roll up, which reduces
evaporation from the leaf surface – conserving water. All grasses and most monocots
have these.
PART IV: Spices (see separate handout)