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Horticulture Principles and Practices

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Upper epidermis<br />

Palisade layer<br />

FIGURE 3–14 A transverse<br />

section of a typical dicot leaf<br />

showing internal structure.<br />

Vascular bundle<br />

Spongy mesophyll<br />

Intercellular chamber<br />

Lower epidermis<br />

Stoma<br />

Guard cells<br />

FIGURE 3–15 Leaf venation<br />

in a (a) monocot, (b) dicoty, (c) an<br />

actual dicot leaf venation. (Source:<br />

George Acquaah)<br />

(a) Parallel venation<br />

(b) Reticulate venation<br />

(c) Reticulate venation<br />

Stem<br />

FIGURE 3–16<br />

leaf.<br />

A typical grass<br />

Lamina<br />

Sheath<br />

more than one row of this tissue if the plant is exposed to intense sunlight. Palisade tissue<br />

cells contain chloroplasts used in photosynthesis. Next to the lower epidermis is a<br />

layer of widely spaced cells called spongy mesophyll. This tissue provides flexibility of<br />

the lamina as it moves in the wind.<br />

The vascular system of the stem extends to the leaf. Dicot leaves generally have a<br />

single, large central vein, or midrib, from which secondary <strong>and</strong> tertiary veins branch out<br />

into the lamina. This vascular system provides the framework of the leaf. The pattern of<br />

veins (called venation) is a basis for classifying plants. Dicots have reticulate venation<br />

(weblike), while monocots have parallel venation (Figure 3–15).<br />

Leaves drop after a period of being on the stem. In dicots, a zone called the abscision<br />

zone, located at the base of the petiole, is responsible for the dropping of leaves as they<br />

age or as a result of adverse environmental conditions. The base of the petiole is a swollen<br />

structure called a pulvinus, enlarged as a result of water imbibed by the parenchyma cells<br />

in that region. Under moisture stress, the pulvinus cells lose water <strong>and</strong> collapse, resulting<br />

in drooping of the petiole. When the pulvinus becomes turgid again, the leaf is lifted up.<br />

Monocot leaves lack a petiole (Figure 3–16). Instead, they have a base (sheath) <strong>and</strong><br />

a lamina with parallel venation. The junction where the lamina attaches to the sheath may<br />

have a structure called a ligule. An auricle or stipule may also be present.<br />

3.4 Leaf 77

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