Lab 2: Leaves 1 Name: ______Date/Lab time: ______

Lab 2: Plant Morphology: Leaves Supplies: Carnivorous plant Leaf types Cactus with needles Simple- Succulent leafed plant Sessile- without petiole Monocots and Dicots Compound- Equisetum Pinnate Mother of 1000’s Palmate Ginkgo , pine, fern, etc. Vocabulary to know: Abaxial surface, Adaxial surface, Blade, Compound leaves, Leaflet, Lobed, Midrib, Palmate compound leaf , Petiole, Phyllotaxis, Pinnate compound leaf , Serrated, Sessile leaves, Simple leaves.

Introduction:

A leaf is defined by its formation (during its very early stage, a leaf extends over and protects the shoot tip) (google shoot meristem). Find a live shoot tip on a plant supplied in lab. Note that you have to dig through a lot of very young leaves to get to it. This definition holds true for even modified leaves. Look at the shoot tip of the cactus (ouch! don't touch it!). Note the cactus spines (needles) form over the shoot tip, thus needles are leaves, though highly modified. Another common feature of leaves is the presence of axillary buds (see figure on next page). Locate a leave's axillary bud just above the point where the leaf attaches to the stem. Axillary buds have the potential of forming new branches or flowers.

Leaf Arraignment on Stem The arraignment of leaves on a branch is called phyllotaxis . Note branches with alternate phyllotaxis (leaves arranged alternately one per node), opposite phyllotaxis (leaves arranged 2 per node) or whorled phyllotaxis (more than 2 leaves per node). Identify plants with these types of phyllotaxis. Whorled phyllotaxis is less common. See sweet woodruff and horsetail ( Equisetum ) for examples.

Leaf Forms Figure 1. Phyllotaxis Leaves come in many shapes and forms. Look at the leaves available and recognize simple leaves (with a flat blade and a petiole) (Fig.2) sessile leaves (flat blade without a petiole) and compound leaves (with leaflets ). Compound leaves can be palmate of pinnate: palmate (leaflets arising from a central axis- like your palm) pinnate (leaflets along a row from a central midline- like a bird feather). Leaves can also be fused. Additional feature of leaves are extensive. Leaves can be lobed, serrated, hairy, waxy, succulent , etc.). Before leaving lab you should become familiar with the basic leaf structure.

Lab 2: Leaves 2

Simple Leaves

Leaves of most plants are designed to maximize photosynthesis and minimize water loss. Note the general external form of a simple leaf including the blade (lamina), petiole, midrib (main vein) , adaxial surface (top) and abaxial surface (bottom). Also note the axillary bud (these may be microscopic).

Stipules- small, paired, leaf-like structures, found where the leaf connects to the stem. Stipules are found on the leaves of some plants and vary in form.

Figure 2. Simple Leaf

Leaf venation - veins conduct water and nutrients in the leaf. Can vary between different groups of plants. A few examples: Major types of venation  Branched (Palmate) - like the palm of the hand.  Branched (Pinnate)- like a feather.  Parallel - parallel venation.

Note: Monocots tend to have parallel venation. Dicots tend to have branched venation.

PROCEDURE- Drawing 1. Note the huge variety of leaf forms. Look at the venation of leaves. Is it branching (pinnate)? Branching (palmate)? Non- branching (parallel)? Is there a single midrib? Is the leaf waxy, hairy? What are margins of the leaf like (entire, lobed, serrate, etc.) Do the leaves also have stipules (pared appendages at the bottom of the petiole)?

Lab 2: Leaves 3 2. Sketch a simple leaf (palmate or pinnate) from one of the plants in class. Label the external features (midrib, blade, petiole, axillary bud).

Compound Leaves

Compound leaves are composed of leaflets . Note that each leaflet of the compound leaf does NOT have a bud in its axis. Look at where the compound leaf attaches to the stem. Does the whole leaf have a bud in its axis? The answer should be yes (sometimes axillary buds are microscopic). Leaves may be palmately compound, pinnately compound (multiple deviations) or many other types.

Lab 2: Leaves 4 PROCEDURE- Leaf morphology comparisons

Monocot vs. Dicot leaves One of the easiest ways to tell monocots from dicots is by looking at the leaf venation. Recall monocots tend to have parallel venations while those of dicots tend to branch.

1. Other than venation, how do the external structures of leaves from Monocots differ from those of Dicots? Also note any leaves available from Bryophytes (moss), Pterophytes (ferns, horsetail) and gymnosperms. Note any characteristics as described above. (Put your observation into the Plant Forms table). Note the external venation, shape (compound/simple, pinnate/palmate), etc. (put your observation into the Plant Forms table)

2. Pterophytes and Gymnosperm leaves are highly variable (see ferns vs. Ginkgo vs. pine). Check out the cedar leaves with a dissection microscope. Again note these differences in the Plant Forms table

Leaf modifications As with stems, many leaf modifications are found in the plant kingdom. Note some of the following on the plants available.

Tendrils- leaf or stem (support) (bean, pea) Stipules- photosynthetic, tendrils Spines- protection, shading (cactus) Bud scales (dormant buds)- protection Bracts- at flower base, large ones can be attractants (poinsettia, dogwood) Flowers- parts thought to be modified leaves Storage leaves- onion (has vegetative leaves too), succulent Insect trapping leaves- several types, pitcher plant, Venus fly trap, etc. Cotyledons- made in embryo (storage, photosynthesis)

Though modified stems and roots are often utilized as a mode of asexual reproduction, leaves are rarely so (see the “Mother of 1000’s” plant for a notable exception).

Taxonomy using leaves: Differences in leaf morphology can aid in plant ID. Be observant when examine plants. Leaves can be simple or compound; they can be smooth or hairy; they can have stipules, variations in leaf margins or many other variations.

Questions:

1. If asked on an exam, how would you tell a simple leaf from a leaflet of a compound leaf?

2. How does the venation differ between Monocot and Dicot leaves? This is an important characteristic between these two classes of flowering plants!