The Structure and Appearance of Plant Leaves
Investigating the Internal and External Anatomy of Leaves
Aug 13, 2009
The original solar collectors botanists call leaves come in an astonishing variety of sizes, shapes, and configurations externally. In fact, leaves are so species-specific that they can be used to identify an unknown plant. Internally, however, all leaves have nearly the same arrangement and types of tissues.
The External Anatomy of a Leaf
Regardless of how complex a leaf appears, any leaf is composed of only two parts: the flat, expanded blade containing the veins (vascular tissue) and photosynthetic cells and the petiole, a stalk-like portion that connects the blade to the stem and transports materials into and out of the blade.
As with stems, there are differences between the leaves of dicots and the leaves of monocots. Dicot leaves have net or reticulate venation (veins form a network within the leave) while monocot leaves have parallel venation (Veins running in straight lines parallel to each other).
The leaves of dicots are either simple (a one-piece blade) or compound (blade subdivided into small leaflets). When the leaflets occur in a featherlike pattern, the leaf is said to be pinnately compound but palmately compound when the leaflets have a common attachment point.
The Internal Anatomy of a Leaf
The internal tissues of a leaf may be categorized as epidermis, mesophyll, and vascular bundles.
Epidermis. The epidermis is composed of a single layer of interlocked cells that is continuous over both surfaces of a leaf and therefore is distinguished into upper and lower epidermis. Epidermal cells are clear so that light may pass through them to the photosynthetic cells beneath them.
The epidermis is covered by the cuticle, a varnish-like layer or film that retards the movement of water and gases into and out of the leaf. Plants, such as cactuses, which live in dry or desert conditions, will produce a layer of wax beneath the cuticle, reducing the loss of water through the epidermis to essentially nothing.
The majority of water loss is normally through the stomates Stomates are lens-shaped holes that perforate the epidermis and lead into intercellular spaces within the leaf. Carbon dioxide from the air enters the leaf through the stomates; oxygen and, unfortunately for the plant, water exit the leaf through the stomates. Surrounding each stomate are two crescent-shaped cells known as guard cells that function to open and close the stomate.
Mesophyll. The mesophyll, with the exception of the vascular bundles, includes all the tissue between the upper and lower epidermis and it is here that photosynthesis is localized within the leaf.
The mesophyll is usually divided into two parts. The cells toward the upper side of the leaf are elongated at right angles to the surface and form one to three compact layers. These cells are called the palisade layer because of their resemblance to a palisade, or picket of stakes forming a wall. All the mesophyll cells contain chloroplasts (organelles where photosynthesis occurs) but they are the most numerous in the palisade cells.
Below the palisade layer, and extending to the lower epidermis, is a zone of large irregularly shaped cells with large intercellular spaces. These cells form the spongy layer. The intercellular spaces here form a system of air passage throughout the leaf.
Vascular Bundle. Vascular bundles are usually located about halfway between the upper and lower epidermis and are composed of two kinds of tissue: xylem and phloem. As in roots and stems, xylem tissue (tubes) conducts water and minerals upward from the roots, up the stem, and into the leaf. Phloem tissue conducts organic molecules (food) produced by photosynthesis in the leaf downward to the stem and roots. The vascular bundles of a leaf function not only for transport but also for support.
Cooler Temperatures Trigger Color Changes in Leaves
The green-appearing chemical chlorophyll is found in abundance in the chloroplasts of mesophyll cells. Other pigmented chemicals are found in plant cells as well, including yellow, orange, red, and brown carotenoids.
Chlorophylls is much more abundant and therefore masks the colors of the other pigments. However, in nonphotosynthetic (nongreen) parts of a plant, such as fruits and flowers, the colors of the other pigments show through vividly.
Chlorophyll is sensitive to cool temperatures but carotenoids are not. With cooler fall temperatures, the chlorophyll breaks down and disappears leaving the leaves, especially tree leaves, glowing with the rich hues of yellow, red, orange, and brown we so often associate with fall.
All the plant clans adorn their stem scaffoldings with a finery of leaves, but in the verdant carpet that covers much of the planet, each clan holds high its own uniquely-structured leaf.
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