Famous Leaves I have Known

Do you recognize the leaves here? Lettuce is obvious. Did you know that onions and the socalled “flower”of the Little Boy Plant (the red one) are actually leaves? Cut open an onion some time and see if you can see how the leaves form.

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LEAVES • Photosynthetic organ of the plant…they produce enough food energy for the whole plant! • Leaf Parts – Blade or lamina: Flat expanded area – Petiole: stalk that connects the

leaf blade to stem BLADE

The principal function of leaves is to absorb sunlight to manufacture plant sugars through a process called photosynthesis. Leaf surfaces are flattened to present a large area for efficient light absorption. The blade is the expanded thin structure on either side of the midrib and usually is the largest, most conspicuous part of a leaf. A leaf is held away from its stem by a stem-like appendage called a petiole, and the base of the petiole is attached to the stem at a node.

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Coleus Shoot Tip

The leaf emerges from a bud (apical meristem) which is at the top of the plant or lower on the stem.

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More parts of a leaf • • • • •

Blade Petiole Midrib (and veins) Leaf Margin Leaf Node

• Used for plant identification! More on this in the text.

There are many kinds of plant leaves. The most common and conspicuous leaves are referred to as foliage and are the primary location of photosynthesis. However, there are many other types of modified leaves.

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Simple versus Compound Leaves

Rachis

Pinnate

A compound leaf may have many leaflets.

The vascular bundles of xylem and phloem extend from the stem, through the petiole, and into the leaf blade as veins. There are two principal types of venation: parallel-veined and net-veined. In parallel-veined leaves, numerous veins run parallel to each other and are connected laterally by minute, straight veinlets. Parallel-veined leaves occur most often on monocotyledonous plants. In net-veined leaves, veins branch from the main rib or ribs and subdivide into finer veinlets. These veinlets then unite in a complicated network. Netveined leaves occur on dicotyledonous plants.

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Leaf Arrangement on the Stem Opposite

Spiral

Whorled

The arrangement of leaves on the stem are useful for plant identification.

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Leaf Venation (the veins are the vascular system)

Parallel venation

Net venation

Most monocots are parallel veined, like corn. Most dicots are netted.

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Leaf Differences between Monocots and Dicots

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There are lots of leaf shapes

This too is used for identification purposes.

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And Different Leaf Margins

And more detail yet if you really like to get into this stuff. I don’t!

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Leaves Work • Leaves capture solar energy and turn it into chemical energy (sugar) the plant can use. • This is the process of photosynthesis. • Leaves are “constructed” for this purpose.

An overview

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Leaf Anatomy • Leaf anatomy has three basic parts; 1) protective cells, 2) internal working cells, and 3) transport cells Epidermis (protection) Internal “working cells” Vascular Transport cells Epidermis (protection)

An overview.

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Leaf Cross Section

Epidermis (protection)

Vascular Transport cells

Internal “working cells”

A cross section.

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Leaf Anatomy • Leaf anatomy has three basic parts; 1) protective cells, 2) internal working cells, and 3) transport cells Epidermis (protection) Internal “working cells” Vascular Transport cells Epidermis (protection)

The epidermis cells are clear and allow light to pass through. Epidermis – Transparent, usually single cell protective layer. Leaves have both an upper and lower epidermis consisting of compactly arranged cells. The epidermis is often covered by a cuticle (waxy layer) to reduce water loss. Most leaves have an epidermis only one cell thick. However, some xerophytes (drought tolerant plants like cacti) may have up to three layers of cells. Most of the stomates are in the lower epidermis.

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Leaf epidermis • Is transparent – so that sun light can go through. • Waxy surface protects against drying out • Has stomates for gas and water exchange

The primary function of the epidermis is to protect the other layers of leaf tissue. The cuticle is part of the epidermis. It produces a waxy layer which protects the leaf from dehydration and disease. The amount of cutin on a leaf increases with increasing light intensity. For this reason, when moving plants from shade into full sunlight, do so gradually over a period of a few weeks. This gradual exposure to sunlight allows the cutin layer to build up and protect the leaves from rapid water loss or sunscald.

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Stomate Open stomate

Closed stomate

Stomates (pores for gas exchange) may occur on both leaf surfaces, but they are usually more numerous on the lower surface Special epidermal cells called guard cells open and close in response to environmental stimuli, such as changes in weather and light. They regulate the passage of water, oxygen, and carbon dioxide into and out of the leaf through tiny openings called stomates. Conditions that would cause plants to lose a lot of water (high temperature, low humidity) stimulate guard cells to close. In mild weather, they remain open. Guard cells also close in the absence of light (like at night).

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Leaf epidermis may have leaf hairs • gives fuzzy texture • Reduces water loss from inside the leaf

(“Panda plant”)

Hairy leaves lose less water.

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Leaf Anatomy • Leaf anatomy has three basic parts; 1) protective cells, 2) internal working cells, and 3) transport cells Epidermis (protection) Internal “working cells” Vascular Transport cells Epidermis (protection)

The vascular system provides for transport of water, nutrients and sugars.

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Veins are the vascular tissue

Parallel venation

Net venation

The veins we see are the vascular bundles.

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Leaf vascular tissue Veins are composed of xylem (water transport) phloem (food transport) and bundle sheaths, cells surrounding the xylem/phloem for strength & support

1. Vascular bundles contain xylem (usually towards the upper surface) and phloem (usually towards lower surface) 2. In dicots veins are usually arranged in a net-like fashion, whereas in monocots they are arranged in parallel rows 3. Much like the human vascular system, leaf veins spread out in progressively smaller units, usually from a large, central midvein 4. Water moves through plants through the vascular system. Both root pressure and evaporation from the leaves “power” the water through the plant. Approximately 90% of the water taken up by the roots escape through the leaves within a few hours. 20

Leaf Anatomy • Leaf anatomy has three basic parts; 1) protective cells, 2) internal working cells, and 3) transport cells Epidermis (protection) Internal “working cells” Vascular Transport cells Epidermis (protection)

The “internal working cells” are both palisade cells and spongy mesophyl. .

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Leaf Mesophyll • Middle of the leaf (meso-phyll) • Composed of photosynthetic cells: • Palisade cells (long columns below epidermis; have lots chloroplasts for photosynthesis) • Spongy cells with air spaces around, (for gas exchange)

Palisade layer - the cells are arranged in a compact, columnar fashion. Most of the photosynthetic activity takes place in the palisade layer. Therefore its cells have many more chloroplasts than the spongy layer Spongy mesophyl layer - the cells are irregularly shaped and not as compact as the palisade layer, giving them a "spongy" appearance. The "spongy" appearance is particularly evident in floating aquatic leaves. Cells of the spongy layer have fewer chloroplasts than the palisade layer and photosynthetic activity is lower than in the palisade layer. They are in close contact with the air to allow gas exchange.

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The whole enchilada

An overview.

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Important Economic Leaves •Lettuce •Cabbage •Onions •Marijuana •Garlic •Tobacco •Celery (petiole)

Notice these common plants that we use which the usable portion is a leaf. Did you know that the celery stalk that we eat is technically part of a leaf, because it is mostly petiole (remember, the leaf has two parts – blade and petiole).

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Lettuce

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Onions

Cut one open and see how the compact leaves fold in on each other.

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Onions

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Cabbage

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Anthurium

Some leaves have become highly modified. For example, the “petals” of these anthuriums are actually modified leaves.

The Anthurium or so-called “Little Boy Plant” (named for obvious reasons).

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Anyone know what this is?

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Tobacco has big leaves!

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Cigar Leaves Need Shade

The shade cloth results in tobacco growing thin leaves with fewer layers of palisade cells. This is preferred for cigar wrappers. Leaves grown in low light, tend to be larger and thinner.

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Celery (eat the petiole)

Yup, technically a leaf (or at least the petiole). Notice the small leaf blades.

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So, what are your favorite leaves?

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