Bidlack, Stern's Introductory Plant Biology, 15e-Highlights LEAF

Highlights from
Bidlack, Stern's Introductory Plant Biology, 15e
4.3 Tissues Produced by Meristems 9/13/21

e tissues and where they may be found in the plant.
7.2 Leaf Arrangements and Types 9/13/21
The variety of shapes, sizes, and textures of leaves seems to be almost infinite. The leaves of
some ofthe smaller duckweeds are less than 1 millimeter (0.04 inch) wide. The mature leaves of
the Seychelles Island palm can be 6 meters (20 feet) long, and the floating leaves of a giant
water lily, which reach 2 meters (6.5 feet) in diameter ( Fig. 7.3 ), can support, without sinking,
weights of more than 45 kilograms (100 pounds) distributed over their surface. In lilies, pines,
ferns, and many other plants, different forms of leaves (e.g., tiny, papery scales; colored leaves
called bracts; spines) may be produced, along with typical photosynthetic leaves, on the same
plant.
The variety of shapes, sizes, and textures of leaves seems to be almost infinite. The leaves of
some ofthe smaller duckweeds are less than 1 millimeter (0.04 inch) wide. The mature leaves of
the Seychelles Island palm can be 6 meters (20 feet) long, and the floating leaves of a giant
water lily, which reach 2 meters (6.5 feet) in diameter ( Fig. 7.3 ), can support, without sinking,
weights of more than 45 kilograms (100 pounds) distributed over their surface. In lilies, pines,
ferns, and many other plants, different forms of leaves (e.g., tiny, papery scales; colored leaves
called bracts; spines) may be produced, along with typical photosynthetic leaves, on the same
plant.
The arrangement of veins in a leaf or leaflet blade ( venation ) may also be either pinnate or
palmate. In pinnately veined leaves, there is one primary vein called the midvein , which is
included within an enlarged midrib ; secondary veins branch from the midvein. In palmately
veined leaves, several primary veins fan out from the base of the blade. The primary veins are
parallel to one another in monocots ( Fig. 7.5 ) and diverge from one another in various ways in
dicots. The branching arrangement of veins in dicots is called netted, or reticulate venation (see
Fig. 7.9 ) . In a few leaves (e.g., those of Ginkgo ), no midvein or other large veins are present.
Instead, the veins fork evenly and progressively from the base of the blade to the opposite
margin. This is called dichotomous venation (see Fig. 7.4k ).
7.3 Internal Structure of Leaves 9/13/21
Page 104
If a typical leaf is cut transversely and examined with a microscope, three regions stand out:
epidermis, mesophyll, and veins (referred to as vascular bundles in our discussion of roots and
stems) ( Fig. 7.6 ). The epidermis is a single layer of cells covering the entire surface of the leaf.
The epidermis on the lower surface of the blade often has more stomata, which are discussed in
the next section.
1 of 2 9/14/21
When seen from the top, the wavy, undulating walls of epidermal cells often resemble pieces of
a jigsaw puzzle fitted together. Except for guard cells, the upper epidermal cells for the most
part do not contain chloroplasts.
Their function is to protect the delicate tissues inside the leaf. A coating of waxy cutin (the
cuticle —see Chapter 4 ) is normally present, although it may not be visible with ordinary light
microscopes without being stained. In addition to the cuticle, many plants produce other waxy
substances on their surfaces ( Fig. 7.7 ). Wax production can be impacted by air pollution in as
little as 24 hours after exposure. Presumably, the wax provides added protection to the leaves.
In some plants, waste materials occasionally accumulate and crystallize in epidermal cells.
Different types of glands may also be present in the epidermis. Glands occur in the form of
depressions, protuberances, or appendages either directly on the leaf surface or on the ends of
hairs (see Chapter 4 ). Glands often secrete sticky or smelly substances, often to ward off insect
pests.
7.4 Stomata 9/13/21
The functioning of guard cells is aided by the photosynthesis that takes place within them. The
primary functions include (1) regulating gas exchange between the interior of the leaf and the
atmosphere, and (2) regulating evaporation of the water entering the plant at the roots. Guard
cell walls are distinctly thickened but quite flexible on the side adjacent to the pore. As the guard
cells inflate or deflate with changes in the amount of water within the cells, their unique
construction causes the stomata to open or close. When the guard cells are inflated, the
stomata are open; when the water content of the guard cells decreases, the cells deflate, and
the stomata close. (For more detailed discussions of this stomatal mechanism, see “ Regulation
of Transpiration ” in Chapter 9 and “ Turgor Movements ” in Chapter 11 .)
7.4 Stomata 9/13/21
The lower epidermis of most plants generally resembles the upper epidermis, but it typically has
a thinner layer of cutin and is perforated by numerous tiny pores called stomata (singular:
stoma ) ( Fig. 7.8 ). Some plants (e.g., alfalfa, corn) have stomata in both leaf surfaces, while
others (e.g., water lilies) have them exclusively on the upper epidermis; they are absent
altogether from the submerged leaves of aquatic plants. Stomata are very numerous, ranging
from about 1,000 to more than 1.2 million per square centimeter (6,300 to 8 million per square
inch) of surface. An average-sized sunflower leaf has about 2 million of these pores throughout
its lower epidermis. Each pore is bordered by two sausage- or dumbbell-shaped cells that are
usually smaller than most of the neighboring epidermal cells. These guard cells , which originate
from the same parental cell, are part of the epidermis, but they, unlike most of the other cells of
either epidermis, contain chloroplasts.
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4.3 Tissues Produced by Meristems 9/13/21

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